Fix version info (#1202)

Just tagging as 4.2.0 is not enough, the version number must be incremented for cmake and in the readme. Compare https://github.com/microsoft/GSL/pull/1163.

Co-authored-by: Werner Henze <w.henze@avm.de>

.gitattributes

@@ -0,0 +1 @@
+include/gsl/* linguist-language=C++

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,29 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: 'Status: Open, Type: Bug'
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+```c++
+#include <gsl>
+
+// your repro here: ...
+```
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Spec (please complete the following information):**
+ - OS: [e.g. Windows]
+ - Compiler: [e.g. MSVC]
+ - C++ Version: [e.g. C++20]
+
+**Additional context**
+Add any other context about the problem here.

.github/workflows/android.yml

@@ -1,4 +1,9 @@
 name: CI_Android
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 on:
   push:
     branches: [ main ]
@@ -7,39 +12,43 @@ on:
 
 jobs:
   Android:
-    runs-on: macos-latest
+    runs-on: macos-latest-large
     defaults:
       run:
         working-directory: build
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Create build directory
         run: mkdir -p build
         working-directory: .
 
-      - name: Start emulator
+      - uses: actions/setup-java@v4
+        with:
+          java-version: 8
+          distribution: zulu
+
+      - name: Start Emulator
         run: |
           echo "y" | $ANDROID_HOME/tools/bin/sdkmanager --install 'system-images;android-24;default;x86_64'
           echo "no" | $ANDROID_HOME/tools/bin/avdmanager create avd -n xamarin_android_emulator -k 'system-images;android-24;default;x86_64' --force
           $ANDROID_HOME/emulator/emulator -list-avds
-          echo "Starting emulator"
-          # Start emulator in background
-          nohup $ANDROID_HOME/emulator/emulator -avd xamarin_android_emulator -no-snapshot > /dev/null 2>&1 &
-          echo "Emulator starting"
+          echo "Starting emulator..."
+          nohup $ANDROID_HOME/emulator/emulator -no-audio -no-snapshot -avd xamarin_android_emulator &> /dev/null &
+          echo "Emulator starting in background"
 
       - name: Configure
-        run: cmake -Werror=dev -DCMAKE_TOOLCHAIN_FILE=$ANDROID_HOME/ndk-bundle/build/cmake/android.toolchain.cmake -DANDROID_PLATFORM=16 -DANDROID_ABI=x86_64 -DCMAKE_BUILD_TYPE=Debug ..
+        run: cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK_LATEST_HOME/build/cmake/android.toolchain.cmake -DANDROID_PLATFORM=16 -DANDROID_ABI=x86_64 -DCMAKE_BUILD_TYPE=Debug ..
 
       - name: Build
         run: cmake --build . --parallel
 
       - name: Wait for emulator ready
+        timeout-minutes: 2
         run: |
           $ANDROID_HOME/platform-tools/adb wait-for-device shell 'while [[ -z $(getprop sys.boot_completed | tr -d '\r') ]]; do sleep 10; done; input keyevent 82'
           $ANDROID_HOME/platform-tools/adb devices
           $ANDROID_HOME/platform-tools/adb shell getprop ro.product.cpu.abi
-          echo "Emulator started"
 
       - name: Deploy tests
         run: |

.github/workflows/cmake/action.yml

@@ -0,0 +1,56 @@
+name: Composite CMake
+inputs:
+  cmake_generator:
+    required: false
+    type: string
+    default: 'Unix Makefiles'
+  cmake_build_type:
+    required: true
+    type: string
+    default: ''
+  cmake_cxx_compiler:
+    required: false 
+    type: string 
+  gsl_cxx_standard:
+    required: true
+    type: number
+  extra_cmake_args:
+    required: false
+    type: string
+    default: ''
+  build_cmd:
+    required: true 
+    type: string 
+    default: 'make'
+  test_cmd:
+    required: false
+    type: string 
+    default: 'make test'
+  shell:
+    required: false 
+    type: string
+    default: 'bash'
+
+
+runs:
+  using: composite
+  steps:
+    - name: Create build directory
+      run: mkdir build
+      shell: ${{ inputs.shell }}
+
+    - name: Configure CMake
+      working-directory: build
+      run: cmake -G "${{ inputs.cmake_generator }}" -DCMAKE_BUILD_TYPE=${{ inputs.cmake_build_type }} -DCMAKE_CXX_COMPILER=${{ inputs.cmake_cxx_compiler }} -DGSL_CXX_STANDARD=${{ inputs.gsl_cxx_standard }} -DCI_TESTING:BOOL=ON -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -Werror=dev ${{ inputs.extra_cmake_args }} ..
+      shell: ${{ inputs.shell }}
+
+    - name: Build
+      working-directory: build
+      run: ${{ inputs.build_cmd }}
+      shell: ${{ inputs.shell }}
+
+    - name: Test
+      working-directory: build
+      run: ${{ inputs.test_cmd }}
+      shell: ${{ inputs.shell }} 
+

.github/workflows/cmake_find_package.yml

@@ -0,0 +1,25 @@
+name: cmake_find_package
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  cmake-find-package:
+    name: Build ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ ubuntu-latest, macos-latest ]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: lukka/get-cmake@latest
+        with:
+          cmakeVersion: 3.14.0
+      - name: Configure GSL
+        run: cmake -S . -B build -G "Ninja" -D GSL_TEST=OFF -D CMAKE_INSTALL_PREFIX=${GITHUB_WORKSPACE}/build/install
+      - name: Install GSL
+        run: cmake --build build --target install
+      - name: Test GSL find_package support
+        run: cmake -S tests/ -B build/tests_find_package -G "Ninja" -D CMAKE_PREFIX_PATH=${GITHUB_WORKSPACE}/build/install -D CMAKE_BUILD_TYPE=Release

.github/workflows/compilers.yml

@@ -0,0 +1,115 @@
+name: Compiler Integration Tests
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+# These jobs are correlated with the officially supported compilers
+# and toolsets. If you change any versions, please update README.md.
+
+jobs:
+  gcc:
+    strategy:
+      matrix:
+        gcc_version: [ 12, 13, 14 ]
+        build_type: [ Debug, Release ]
+        cxx_version: [ 14, 17, 20, 23 ]
+        exclude:
+          # https://github.com/google/googletest/issues/4232
+          # Looks like GoogleTest is not interested in making version 1.14
+          # work with gcc-12.
+          - gcc_version: 12
+            cxx_version: 20
+          - gcc_version: 12
+            cxx_version: 23
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Run CMake (configure, build, test)
+        uses: ./.github/workflows/cmake
+        with:
+          cmake_build_type: ${{ matrix.build_type }}
+          cmake_cxx_compiler: g++-${{ matrix.gcc_version }}
+          gsl_cxx_standard: ${{ matrix.cxx_version }}
+          
+  clang:
+    strategy:
+      matrix:
+        clang_version: [ 16, 17, 18 ]
+        build_type: [ Debug, Release ]
+        cxx_version: [ 14, 17, 20, 23 ]
+        exclude:
+          # https://github.com/llvm/llvm-project/issues/93734
+          # Looks like clang fixed this issue in clang-18, but won't backport
+          # the fix.
+          - clang_version: 17
+            cxx_version: 23
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+     
+      - name: Run CMake (configure, build, test)
+        uses: ./.github/workflows/cmake
+        with:
+          cmake_build_type: ${{ matrix.build_type }}
+          cmake_cxx_compiler: clang++-${{ matrix.clang_version }}
+          gsl_cxx_standard: ${{ matrix.cxx_version }}
+
+  xcode:
+    strategy:
+      matrix:
+        xcode_version: [ '15.4' ]
+        build_type: [ Debug, Release ]
+        cxx_version: [ 14, 17, 20, 23 ]
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: select xcode version
+        run: sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode_version }}.app
+
+      - name: Run CMake (configure, build, test)
+        uses: ./.github/workflows/cmake
+        with:
+          cmake_build_type: ${{ matrix.build_type }}
+          cmake_cxx_compiler: clang++
+          gsl_cxx_standard: ${{ matrix.cxx_version }}
+
+  VisualStudio:
+    strategy:
+      matrix:
+        generator: [ 'Visual Studio 16 2019', 'Visual Studio 17 2022' ]
+        image: [ windows-2019, windows-2022 ]
+        build_type: [ Debug, Release ]
+        extra_args: [ '', '-T ClangCL' ]
+        cxx_version: [ 14, 17, 20, 23 ]
+        exclude:
+          - generator: 'Visual Studio 17 2022'
+            image: windows-2019
+          - generator: 'Visual Studio 16 2019'
+            image: windows-2022
+          - generator: 'Visual Studio 16 2019'
+            cxx_version: 23
+    runs-on: ${{ matrix.image }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: microsoft/setup-msbuild@v2
+
+      - name: Run CMake (configure, build, test)
+        uses: ./.github/workflows/cmake
+        with:
+          cmake_generator: ${{ matrix.generator }}
+          cmake_build_type: ${{ matrix.build_type }}
+          gsl_cxx_standard: ${{ matrix.cxx_version }}
+          extra_cmake_args: ${{ matrix.extra_args }}
+          build_cmd: msbuild GSL.sln
+          test_cmd: ctest . --output-on-failure --no-compress-output
+          shell: pwsh
+

.github/workflows/ios.yml

@@ -12,7 +12,7 @@ jobs:
       run:
         working-directory: build
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Create build directory
         run: mkdir -p build

CMakeLists.txt

@@ -1,61 +1,48 @@
-cmake_minimum_required(VERSION 3.1.3...3.16)
+cmake_minimum_required(VERSION 3.14...3.16)
 
-list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/")
-include(guidelineSupportLibrary)
+project(GSL VERSION 4.2.0 LANGUAGES CXX)
 
-project(GSL
-    VERSION 4.0.0
-    LANGUAGES CXX
-)
-
-# Must include after the project call due to GNUInstallDirs requiring a language be enabled (IE. CXX)
-include(GNUInstallDirs)
-
-# Creates a library GSL which is an interface (header files only)
 add_library(GSL INTERFACE)
-
-# NOTE: If you want to use GSL prefer to link against GSL using this alias target
-# EX:
-#   target_link_libraries(foobar PRIVATE Microsoft.GSL::GSL)
-#
-# Add Microsoft.GSL::GSL alias for GSL so that dependents can be agnostic about
-# whether GSL was added via `add_subdirectory` or `find_package`
 add_library(Microsoft.GSL::GSL ALIAS GSL)
 
-# Determine whether this is a standalone project or included by other projects
-set(GSL_STANDALONE_PROJECT OFF)
-if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
-    set(GSL_STANDALONE_PROJECT ON)
-endif()
+# https://cmake.org/cmake/help/latest/variable/PROJECT_IS_TOP_LEVEL.html
+string(COMPARE EQUAL ${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_SOURCE_DIR} PROJECT_IS_TOP_LEVEL)
 
-### Project options
-option(GSL_INSTALL "Generate and install GSL target" ${GSL_STANDALONE_PROJECT})
-option(GSL_TEST "Build and perform GSL tests" ${GSL_STANDALONE_PROJECT})
+option(GSL_INSTALL "Generate and install GSL target" ${PROJECT_IS_TOP_LEVEL})
+option(GSL_TEST "Build and perform GSL tests" ${PROJECT_IS_TOP_LEVEL})
 
-# This GSL implementation generally assumes a platform that implements C++14 support.
-set(gsl_min_cxx_standard "14")
-
-if (GSL_STANDALONE_PROJECT)
-    gsl_set_default_cxx_standard(${gsl_min_cxx_standard})
-else()
-    gsl_client_set_cxx_standard(${gsl_min_cxx_standard})
-endif()
+# The implementation generally assumes a platform that implements C++14 support
+target_compile_features(GSL INTERFACE "cxx_std_14")
 
 # Setup include directory
 add_subdirectory(include)
 
-# Add natvis file
-gsl_add_native_visualizer_support()
-
-# Add packaging support
-gsl_create_packaging_file()
-
-if (GSL_INSTALL)
-    # Setup install/export logic
-    gsl_install_logic()
-endif()
+target_sources(GSL INTERFACE $<BUILD_INTERFACE:${GSL_SOURCE_DIR}/GSL.natvis>)
 
 if (GSL_TEST)
     enable_testing()
     add_subdirectory(tests)
 endif()
+
+if (GSL_INSTALL)
+    include(GNUInstallDirs)
+    include(CMakePackageConfigHelpers)
+
+    install(DIRECTORY "${PROJECT_SOURCE_DIR}/include/gsl" DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+
+    set(export_name "Microsoft.GSLConfig")
+    set(namespace "Microsoft.GSL::")
+    set(cmake_files_install_dir ${CMAKE_INSTALL_DATADIR}/cmake/Microsoft.GSL)
+
+    install(TARGETS GSL EXPORT ${export_name} INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+    install(EXPORT ${export_name} NAMESPACE ${namespace} DESTINATION ${cmake_files_install_dir})
+    export(TARGETS GSL NAMESPACE ${namespace} FILE ${export_name}.cmake)
+
+    set(gls_config_version "${CMAKE_CURRENT_BINARY_DIR}/Microsoft.GSLConfigVersion.cmake")
+
+    write_basic_package_version_file(${gls_config_version} COMPATIBILITY SameMajorVersion ARCH_INDEPENDENT)
+
+    install(FILES ${gls_config_version} DESTINATION ${cmake_files_install_dir})
+
+    install(FILES GSL.natvis DESTINATION ${cmake_files_install_dir})
+endif()

GSL.natvis

@@ -4,13 +4,7 @@
     vim: syntax=xml
 -->
 <AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
-    <!-- These types are from the gsl_assert header. -->
-    <Type Name="gsl::fail_fast">
-        <!-- na hides the address, otherwise it would appear as 0x.... "Message" -->
-        <DisplayString>{_Data._What,nasb}</DisplayString>
-    </Type>
-
-    <!-- These types are from the gsl_util header. -->
+    <!-- These types are from the util header. -->
     <Type Name="gsl::final_action&lt;*&gt;">
         <DisplayString>{{ invoke = {invoke_}, action = {f_} }}</DisplayString>
         <Expand>
@@ -19,7 +13,8 @@
         </Expand>
     </Type>
 
-    <Type Name="gsl::span&lt;*, *&gt;">
+  <!-- These types are from the span header. -->
+  <Type Name="gsl::span&lt;*, *&gt;">
         <DisplayString>{{ extent = {storage_.size_} }}</DisplayString>
         <Expand>
             <ArrayItems>
@@ -29,29 +24,7 @@
         </Expand>
     </Type>
 
-    <Type Name="gsl::basic_string_span&lt;*, *&gt;">
-        <DisplayString>{span_.storage_.data_,[span_.storage_.size_]na}</DisplayString>
-        <Expand>
-            <Item Name="[size]">span_.storage_.size_</Item>
-            <ArrayItems>
-                <Size>span_.storage_.size_</Size>
-                <ValuePointer>span_.storage_.data_</ValuePointer>
-            </ArrayItems>
-        </Expand>
-    </Type>
-
-    <Type Name="gsl::basic_zstring_span&lt;*, *&gt;">
-        <DisplayString>{span_.storage_.data_,[span_.storage_.size_]na}</DisplayString>
-        <Expand>
-            <Item Name="[size]">span_.storage_.size_</Item>
-            <ArrayItems>
-                <Size>span_.storage_.size_</Size>
-                <ValuePointer>span_.storage_.data_</ValuePointer>
-            </ArrayItems>
-        </Expand>
-    </Type>
-
-    <!-- These types are from the gsl header. -->
+    <!-- These types are from the pointers header. -->
     <Type Name="gsl::not_null&lt;*&gt;">
         <!-- We can always dereference this since it's an invariant. -->
         <DisplayString>value = {*ptr_}</DisplayString>

README.md

@@ -1,5 +1,6 @@
 # GSL: Guidelines Support Library
-[![Build Status](https://dev.azure.com/cppstat/GSL/_apis/build/status/microsoft.GSL?branchName=main)](https://dev.azure.com/cppstat/GSL/_build/latest?definitionId=1&branchName=main)
+[![CI](https://github.com/Microsoft/GSL/actions/workflows/compilers.yml/badge.svg)](https://github.com/microsoft/GSL/actions/workflows/compilers.yml?query=branch%3Amain)
+[![vcpkg](https://img.shields.io/vcpkg/v/ms-gsl)](https://vcpkg.io/en/package/ms-gsl)
 
 The Guidelines Support Library (GSL) contains functions and types that are suggested for use by the
 [C++ Core Guidelines](https://github.com/isocpp/CppCoreGuidelines) maintained by the [Standard C++ Foundation](https://isocpp.org).
@@ -22,59 +23,63 @@ This project makes use of the [Google Test](https://github.com/google/googletest
 # Supported features
 ## Microsoft GSL implements the following from the C++ Core Guidelines:
 
-Feature                            | Supported? | Description
------------------------------------|:----------:|-------------
-[**1. Views**][cg-views]           |            |
-owner                              | ☑   | an alias for a raw pointer
-not_null                           | ☑   | restricts a pointer / smart pointer to hold non-null values
-span                               | ☑   | a view over a contiguous sequence of memory. Based on the standardized verison of `std::span`, however `gsl::span` enforces bounds checking. See the [wiki](https://github.com/microsoft/GSL/wiki/gsl::span-and-std::span) for additional information.
-span_p                             | ☐   | spans a range starting from a pointer to the first place for which the predicate is true
-basic_zstring                      | ☑   | A pointer to a C-string (zero-terminated array) with a templated char type
-zstring                            | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of char
-czstring                           | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of const char
-wzstring                           | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of wchar_t
-cwzstring                          | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of const wchar_t
-u16zstring                         | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of char16_t
-cu16zstring                        | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of const char16_t
-u32zstring                         | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of char32_t
-cu32zstring                        | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of const char32_t
-[**2. Owners**][cg-owners]         |            |
-unique_ptr                         | ☑   | an alias to `std::unique_ptr`
-shared_ptr                         | ☑   | an alias to `std::shared_ptr`
-stack_array                        | ☐   | a stack-allocated array
-dyn_array                          | ☐   | a heap-allocated array
-[**3. Assertions**][cg-assertions] |            |
-Expects                            | ☑   | a precondition assertion; on failure it terminates
-Ensures                            | ☑   | a postcondition assertion; on failure it terminates
-[**4. Utilities**][cg-utilities]   |            |
-move_owner                         | ☐   | a helper function that moves one `owner` to the other
-byte                               | ☑   | either an alias to std::byte or a byte type
-final_action                       | ☑   | a RAII style class that invokes a functor on its destruction
-finally                            | ☑   | a helper function instantiating `final_action`
-GSL_SUPPRESS                       | ☑   | a macro that takes an argument and turns it into `[[gsl::suppress(x)]]` or `[[gsl::suppress("x")]]`
-[[implicit]]                       | ☐   | a "marker" to put on single-argument constructors to explicitly make them non-explicit
-index                              | ☑   | a type to use for all container and array indexing (currently an alias for std::ptrdiff_t)
-joining_thread                     | ☐   | a RAII style version of `std::thread` that joins
-narrow                             | ☑   | a checked version of narrow_cast; it can throw `narrowing_error`
-narrow_cast                        | ☑   | a narrowing cast for values and a synonym for static_cast
-narrowing_error                    | ☑   | a custom exception type thrown by `narrow()`
-[**5. Concepts**][cg-concepts]     | ☐   |
+Feature                                                                  | Supported? | Description
+-------------------------------------------------------------------------|:----------:|-------------
+[**1. Views**][cg-views]                                                 |            |
+[owner](docs/headers.md#user-content-H-pointers-owner)                   | ☑   | An alias for a raw pointer
+[not_null](docs/headers.md#user-content-H-pointers-not_null)             | ☑   | Restricts a pointer/smart pointer to hold non-null values
+[span](docs/headers.md#user-content-H-span-span)                         | ☑   | A view over a contiguous sequence of memory. Based on the standardized version of `std::span`, however `gsl::span` enforces bounds checking.
+span_p                                                                   | ☐   | Spans a range starting from a pointer to the first place for which the predicate is true
+[basic_zstring](docs/headers.md#user-content-H-zstring)                  | ☑   | A pointer to a C-string (zero-terminated array) with a templated char type
+[zstring](docs/headers.md#user-content-H-zstring)                        | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `char`
+[czstring](docs/headers.md#user-content-H-zstring)                       | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const char`
+[wzstring](docs/headers.md#user-content-H-zstring)                       | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `wchar_t`
+[cwzstring](docs/headers.md#user-content-H-zstring)                      | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const wchar_t`
+[u16zstring](docs/headers.md#user-content-H-zstring)                     | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `char16_t`
+[cu16zstring](docs/headers.md#user-content-H-zstring)                    | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const char16_t`
+[u32zstring](docs/headers.md#user-content-H-zstring)                     | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `char32_t`
+[cu32zstring](docs/headers.md#user-content-H-zstring)                    | ☑   | An alias to `basic_zstring` with dynamic extent and a char type of `const char32_t`
+[**2. Owners**][cg-owners]                                               |            |
+stack_array                                                              | ☐   | A stack-allocated array
+dyn_array                                                                | ☐   | A heap-allocated array
+[**3. Assertions**][cg-assertions]                                       |            |
+[Expects](docs/headers.md#user-content-H-assert-expects)                 | ☑   | A precondition assertion; on failure it terminates
+[Ensures](docs/headers.md#user-content-H-assert-ensures)                 | ☑   | A postcondition assertion; on failure it terminates
+[**4. Utilities**][cg-utilities]                                         |            |
+move_owner                                                               | ☐   | A helper function that moves one `owner` to the other
+[final_action](docs/headers.md#user-content-H-util-final_action)         | ☑   | A RAII style class that invokes a functor on its destruction
+[finally](docs/headers.md#user-content-H-util-finally)                   | ☑   | A helper function instantiating [final_action](docs/headers.md#user-content-H-util-final_action)
+[GSL_SUPPRESS](docs/headers.md#user-content-H-assert-gsl_suppress)       | ☑   | A macro that takes an argument and turns it into `[[gsl::suppress(x)]]` or `[[gsl::suppress("x")]]`
+[[implicit]]                                                             | ☐   | A "marker" to put on single-argument constructors to explicitly make them non-explicit
+[index](docs/headers.md#user-content-H-util-index)                       | ☑   | A type to use for all container and array indexing (currently an alias for `std::ptrdiff_t`)
+[narrow](docs/headers.md#user-content-H-narrow-narrow)                   | ☑   | A checked version of `narrow_cast`; it can throw [narrowing_error](docs/headers.md#user-content-H-narrow-narrowing_error)
+[narrow_cast](docs/headers.md#user-content-H-util-narrow_cast)           | ☑   | A narrowing cast for values and a synonym for `static_cast`
+[narrowing_error](docs/headers.md#user-content-H-narrow-narrowing_error) | ☑   | A custom exception type thrown by [narrow](docs/headers.md#user-content-H-narrow-narrow)
+[**5. Concepts**][cg-concepts]                                           | ☐   |
 
 ## The following features do not exist in or have been removed from the C++ Core Guidelines:
 Feature                            | Supported? | Description
 -----------------------------------|:----------:|-------------
-strict_not_null                    | ☑   | A stricter version of `not_null` with explicit constructors
+[strict_not_null](docs/headers.md#user-content-H-pointers-strict_not_null) | ☑   | A stricter version of [not_null](docs/headers.md#user-content-H-pointers-not_null) with explicit constructors
 multi_span                         | ☐   | Deprecated. Multi-dimensional span.
 strided_span                       | ☐   | Deprecated. Support for this type has been discontinued.
 basic_string_span                  | ☐   | Deprecated. Like `span` but for strings with a templated char type
-string_span                        | ☐   | Deprecated. An alias to `basic_string_span` with a char type of char
-cstring_span                       | ☐   | Deprecated. An alias to `basic_string_span` with a char type of const char
-wstring_span                       | ☐   | Deprecated. An alias to `basic_string_span` with a char type of wchar_t
-cwstring_span                      | ☐   | Deprecated. An alias to `basic_string_span` with a char type of const wchar_t
-u16string_span                     | ☐   | Deprecated. An alias to `basic_string_span` with a char type of char16_t
-cu16string_span                    | ☐   | Deprecated. An alias to `basic_string_span` with a char type of const char16_t
-u32string_span                     | ☐   | Deprecated. An alias to `basic_string_span` with a char type of char32_t
-cu32string_span                    | ☐   | Deprecated. An alias to `basic_string_span` with a char type of const char32_t
+string_span                        | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `char`
+cstring_span                       | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `const char`
+wstring_span                       | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `wchar_t`
+cwstring_span                      | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `const wchar_t`
+u16string_span                     | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `char16_t`
+cu16string_span                    | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `const char16_t`
+u32string_span                     | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `char32_t`
+cu32string_span                    | ☐   | Deprecated. An alias to `basic_string_span` with a char type of `const char32_t`
+
+## The following features have been adopted by WG21. They are deprecated in GSL.
+Feature                                                           | Deprecated Since | Notes
+------------------------------------------------------------------|------------------|------
+[unique_ptr](docs/headers.md#user-content-H-pointers-unique_ptr)  | C++11            | Use std::unique_ptr instead.
+[shared_ptr](docs/headers.md#user-content-H-pointers-shared_ptr)  | C++11            | Use std::shared_ptr instead.
+[byte](docs/headers.md#user-content-H-byte-byte)                  | C++17            | Use std::byte instead.
+joining_thread                                                    | C++20 (Note: Not yet implemented in GSL) | Use std::jthread instead.
 
 This is based on [CppCoreGuidelines semi-specification](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#gsl-guidelines-support-library).
 
@@ -86,24 +91,17 @@ This is based on [CppCoreGuidelines semi-specification](https://github.com/isocp
 
 # Quick Start
 ## Supported Compilers / Toolsets
-The GSL officially supports the latest and previous major versions of VS with MSVC & LLVM, GCC, Clang, and XCode with Apple-Clang.
-Within these two major versions, we try to target the latest minor updates / revisions (although this may be affected by
-delays between a toolchain's release and when it becomes widely available for use).
-Below is a table showing the versions currently being tested.
+The GSL officially supports recent major versions of Visual Studio with both MSVC and LLVM, GCC, Clang, and XCode with Apple-Clang.
+For each of these major versions, the GSL officially supports C++14, C++17, C++20, and C++23 (when supported by the compiler).
+Below is a table showing the versions currently being tested (also see [.github/workflows/compilers.yml](the workflow).)
 
 Compiler |Toolset Versions Currently Tested
 :------- |--:
- XCode | 13.2.1 & 12.5.1
- GCC | 11[^1] & 10[^2]
- Clang | 12[^2] & 11[^2]
- Visual Studio with MSVC | VS2022[^3] & VS2019[^4]
- Visual Studio with LLVM | VS2022[^3] & VS2019[^4]
-
-
-[^1]: Precise version may be found in the [latest CI results](https://dev.azure.com/cppstat/GSL/_build?definitionId=1&branchFilter=26).
-[^2]: Precise version may be found in the [latest CI results](https://dev.azure.com/cppstat/GSL/_build?definitionId=1&branchFilter=26). Should be the version specified [here](https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-Readme.md#language-and-runtime).
-[^3]: Precise version may be found in the [latest CI results](https://dev.azure.com/cppstat/GSL/_build?definitionId=1&branchFilter=26). Should be the version specified [here](https://github.com/actions/virtual-environments/blob/main/images/win/Windows2022-Readme.md#visual-studio-enterprise-2022).
-[^4]: Precise version may be found in the [latest CI results](https://dev.azure.com/cppstat/GSL/_build?definitionId=1&branchFilter=26). Should be the version specified [here](https://github.com/actions/virtual-environments/blob/main/images/win/Windows2019-Readme.md#visual-studio-enterprise-2019).
+ GCC | 12, 13, 14
+ XCode | 14.3.1, 15.4
+ Clang | 16, 17, 18
+ Visual Studio with MSVC | VS2019, VS2022 
+ Visual Studio with LLVM | VS2019, VS2022
 
 ---
 If you successfully port GSL to another platform, we would love to hear from you!
@@ -113,19 +111,19 @@ If you successfully port GSL to another platform, we would love to hear from you
 
 Target | CI/CD Status
 :------- | -----------:
-iOS | ![CI_iOS](https://github.com/microsoft/GSL/workflows/CI_iOS/badge.svg)
-Android | ![CI_Android](https://github.com/microsoft/GSL/workflows/CI_Android/badge.svg)
+iOS | [![CI_iOS](https://github.com/microsoft/GSL/workflows/CI_iOS/badge.svg?branch=main)](https://github.com/microsoft/GSL/actions/workflows/ios.yml?query=branch%3Amain)
+Android | [![CI_Android](https://github.com/microsoft/GSL/workflows/CI_Android/badge.svg?branch=main)](https://github.com/microsoft/GSL/actions/workflows/android.yml?query=branch%3Amain)
 
 Note: These CI/CD steps are run with each pull request, however failures in them are non-blocking.
 
 ## Building the tests
 To build the tests, you will require the following:
 
-* [CMake](http://cmake.org), version 3.1.3 (3.2.3 for AppleClang) or later to be installed and in your PATH.
+* [CMake](http://cmake.org), version 3.14 or later to be installed and in your PATH.
 
 These steps assume the source code of this repository has been cloned into a directory named `c:\GSL`.
 
-1. Create a directory to contain the build outputs for a particular architecture (we name it c:\GSL\build-x86 in this example).
+1. Create a directory to contain the build outputs for a particular architecture (we name it `c:\GSL\build-x86` in this example).
 
         cd GSL
         md build-x86
@@ -179,43 +177,44 @@ Include the library using:
 
 ## Usage in CMake
 
-The library provides a Config file for CMake, once installed it can be found via
-
-    find_package(Microsoft.GSL CONFIG)
+The library provides a Config file for CMake, once installed it can be found via `find_package`.
 
 Which, when successful, will add library target called `Microsoft.GSL::GSL` which you can use via the usual
 `target_link_libraries` mechanism.
 
+```cmake
+find_package(Microsoft.GSL CONFIG REQUIRED)
+
+target_link_libraries(foobar PRIVATE Microsoft.GSL::GSL)
+```
+
 ### FetchContent
 
-If you are using cmake version 3.11+ you can use the offical FetchContent module.
+If you are using CMake version 3.11+ you can use the official [FetchContent module](https://cmake.org/cmake/help/latest/module/FetchContent.html).
 This allows you to easily incorporate GSL into your project.
 
 ```cmake
-# NOTE: This example uses cmake version 3.14 (FetchContent_MakeAvailable).
+# NOTE: This example uses CMake version 3.14 (FetchContent_MakeAvailable).
 # Since it streamlines the FetchContent process
 cmake_minimum_required(VERSION 3.14)
 
 include(FetchContent)
 
-# In this example we are picking a specific tag.
-# You can also pick a specific commit, if you need to.
 FetchContent_Declare(GSL
     GIT_REPOSITORY "https://github.com/microsoft/GSL"
-    GIT_TAG "v3.1.0"
+    GIT_TAG "v4.2.0"
+    GIT_SHALLOW ON
 )
 
 FetchContent_MakeAvailable(GSL)
 
-# Now you can link against the GSL interface library
-add_executable(foobar)
-
-# Link against the interface library (IE header only library)
-target_link_libraries(foobar PRIVATE GSL)
+target_link_libraries(foobar PRIVATE Microsoft.GSL::GSL)
 ```
 
 ## Debugging visualization support
+
 For Visual Studio users, the file [GSL.natvis](./GSL.natvis) in the root directory of the repository can be added to your project if you would like more helpful visualization of GSL types in the Visual Studio debugger than would be offered by default.
 
-If you are using cmake this will be done automatically for you.
-See 'GSL_VS_ADD_NATIVE_VISUALIZERS'
+## See Also
+
+For information on [Microsoft Gray Systems Lab (GSL)](https://aka.ms/gsl) of applied data management and system research see <https://aka.ms/gsl>.

SECURITY.md

@@ -0,0 +1,41 @@
+<!-- BEGIN MICROSOFT SECURITY.MD V0.0.7 BLOCK -->
+
+## Security
+
+Microsoft takes the security of our software products and services seriously, which includes all source code repositories managed through our GitHub organizations, which include [Microsoft](https://github.com/Microsoft), [Azure](https://github.com/Azure), [DotNet](https://github.com/dotnet), [AspNet](https://github.com/aspnet), [Xamarin](https://github.com/xamarin), and [our GitHub organizations](https://opensource.microsoft.com/).
+
+If you believe you have found a security vulnerability in any Microsoft-owned repository that meets [Microsoft's definition of a security vulnerability](https://aka.ms/opensource/security/definition), please report it to us as described below.
+
+## Reporting Security Issues
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+Instead, please report them to the Microsoft Security Response Center (MSRC) at [https://msrc.microsoft.com/create-report](https://aka.ms/opensource/security/create-report).
+
+If you prefer to submit without logging in, send email to [secure@microsoft.com](mailto:secure@microsoft.com).  If possible, encrypt your message with our PGP key; please download it from the [Microsoft Security Response Center PGP Key page](https://aka.ms/opensource/security/pgpkey).
+
+You should receive a response within 24 hours. If for some reason you do not, please follow up via email to ensure we received your original message. Additional information can be found at [microsoft.com/msrc](https://aka.ms/opensource/security/msrc). 
+
+Please include the requested information listed below (as much as you can provide) to help us better understand the nature and scope of the possible issue:
+
+  * Type of issue (e.g. buffer overflow, SQL injection, cross-site scripting, etc.)
+  * Full paths of source file(s) related to the manifestation of the issue
+  * The location of the affected source code (tag/branch/commit or direct URL)
+  * Any special configuration required to reproduce the issue
+  * Step-by-step instructions to reproduce the issue
+  * Proof-of-concept or exploit code (if possible)
+  * Impact of the issue, including how an attacker might exploit the issue
+
+This information will help us triage your report more quickly.
+
+If you are reporting for a bug bounty, more complete reports can contribute to a higher bounty award. Please visit our [Microsoft Bug Bounty Program](https://aka.ms/opensource/security/bounty) page for more details about our active programs.
+
+## Preferred Languages
+
+We prefer all communications to be in English.
+
+## Policy
+
+Microsoft follows the principle of [Coordinated Vulnerability Disclosure](https://aka.ms/opensource/security/cvd).
+
+<!-- END MICROSOFT SECURITY.MD BLOCK -->

azure-pipelines.yml

@@ -1,66 +0,0 @@
-trigger:
-  - main
-
-pr:
-  autoCancel: true
-
-stages:
-- stage: GCC
-  dependsOn: []
-  jobs:
-  - template: ./pipelines/jobs.yml
-    parameters:
-      compiler: gcc
-      image: ubuntu-20.04
-      compilerVersions: [ 11, 10 ]
-      setupfile: 'setup_gcc.yml'
-
-- stage: Clang
-  dependsOn: []
-  jobs:
-  - template: ./pipelines/jobs.yml
-    parameters:
-      compiler: clang
-      image: ubuntu-20.04
-      compilerVersions: [ 12, 11 ]
-      setupfile: 'setup_clang.yml'
-
-- stage: Xcode
-  dependsOn: []
-  jobs:
-  - template: ./pipelines/jobs.yml
-    parameters:
-      compiler: 'Xcode'
-      image: macOS-11
-      compilerVersions: [ '12.5.1', '13.2.1' ]
-      setupfile: 'setup_apple.yml'
-
-- stage: VS_MSVC
-  dependsOn: []
-  jobs:
-  - template: ./pipelines/jobs.yml
-    parameters:
-      compiler: 'VS2019 (MSVC)'
-      compilerVersions: [ 'default' ]
-      image: windows-2019
-  - template: ./pipelines/jobs.yml
-    parameters:
-      compiler: 'VS2022 (MSVC)'
-      compilerVersions: [ 'default' ]
-      image: windows-2022
-
-- stage: VS_LLVM
-  dependsOn: []
-  jobs:
-  - template: ./pipelines/jobs.yml
-    parameters:
-      compiler: 'VS2019 (LLVM)'
-      compilerVersions: [ 'default' ]
-      image: windows-2019
-      extraCmakeArgs: '-T ClangCL'
-  - template: ./pipelines/jobs.yml
-    parameters:
-      compiler: 'VS2022 (LLVM)'
-      compilerVersions: [ 'default' ]
-      image: windows-2022
-      extraCmakeArgs: '-T ClangCL'

cmake/guidelineSupportLibrary.cmake

@@ -1,116 +0,0 @@
-# This cmake module is meant to hold helper functions/macros
-# that make maintaining the cmake build system much easier.
-# This is especially helpful since gsl needs to provide coverage
-# for multiple versions of cmake.
-#
-# Any functions/macros should have a gsl_* prefix to avoid problems
-if (CMAKE_VERSION VERSION_GREATER 3.10 OR CMAKE_VERSION VERSION_EQUAL 3.10)
-    include_guard()
-else()
-    if (DEFINED guideline_support_library_include_guard)
-        return()
-    endif()
-    set(guideline_support_library_include_guard ON)
-endif()
-
-# Necessary for 'write_basic_package_version_file'
-include(CMakePackageConfigHelpers)
-
-function(gsl_set_default_cxx_standard min_cxx_standard)
-    set(GSL_CXX_STANDARD "${min_cxx_standard}" CACHE STRING "Use c++ standard")
-
-    set(GSL_CXX_STD "cxx_std_${GSL_CXX_STANDARD}")
-
-    if (MSVC)
-        set(GSL_CXX_STD_OPT "-std:c++${GSL_CXX_STANDARD}")
-    else()
-        set(GSL_CXX_STD_OPT "-std=c++${GSL_CXX_STANDARD}")
-    endif()
-
-    # when minimum version required is 3.8.0 remove if below
-    # both branches do exactly the same thing
-    if (CMAKE_VERSION VERSION_LESS 3.7.9)
-        include(CheckCXXCompilerFlag)
-        CHECK_CXX_COMPILER_FLAG("${GSL_CXX_STD_OPT}" COMPILER_SUPPORTS_CXX_STANDARD)
-
-        if(COMPILER_SUPPORTS_CXX_STANDARD)
-            target_compile_options(GSL INTERFACE "${GSL_CXX_STD_OPT}")
-        else()
-            message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no c++${GSL_CXX_STANDARD} support. Please use a different C++ compiler.")
-        endif()
-    else()
-        target_compile_features(GSL INTERFACE "${GSL_CXX_STD}")
-        # on *nix systems force the use of -std=c++XX instead of -std=gnu++XX (default)
-        set(CMAKE_CXX_EXTENSIONS OFF)
-    endif()
-endfunction()
-
-# The best way for a project to specify the GSL's C++ standard is by the client specifying
-# the CMAKE_CXX_STANDARD. However, this isn't always ideal. Since the CMAKE_CXX_STANDARD is
-# tied to the cmake version. And many projects have low cmake minimums.
-#
-# So provide an alternative approach in case that doesn't work.
-function(gsl_client_set_cxx_standard min_cxx_standard)
-    if (DEFINED CMAKE_CXX_STANDARD)
-        if (${CMAKE_CXX_STANDARD} VERSION_LESS ${min_cxx_standard})
-            message(FATAL_ERROR "GSL: Requires at least CXX standard ${min_cxx_standard}, user provided ${CMAKE_CXX_STANDARD}")
-        endif()
-
-        # Set the GSL standard to what the client desires
-        set(GSL_CXX_STANDARD "${CMAKE_CXX_STANDARD}" PARENT_SCOPE)
-
-        # Exit out early to avoid extra unneccessary work
-        return()
-    endif()
-
-    # Otherwise pick a reasonable default
-    gsl_set_default_cxx_standard(${min_cxx_standard})
-endfunction()
-
-# Adding the GSL.natvis files improves the debugging experience for users of this library.
-function(gsl_add_native_visualizer_support)
-    if (CMAKE_VERSION VERSION_GREATER 3.7.8)
-        if (MSVC_IDE)
-            option(GSL_VS_ADD_NATIVE_VISUALIZERS "Configure project to use Visual Studio native visualizers" TRUE)
-        else()
-            set(GSL_VS_ADD_NATIVE_VISUALIZERS FALSE CACHE INTERNAL "Native visualizers are Visual Studio extension" FORCE)
-        endif()
-
-        # add natvis file to the library so it will automatically be loaded into Visual Studio
-        if(GSL_VS_ADD_NATIVE_VISUALIZERS)
-            target_sources(GSL INTERFACE $<BUILD_INTERFACE:${GSL_SOURCE_DIR}/GSL.natvis>)
-        endif()
-    endif()
-endfunction()
-
-function(gsl_install_logic)
-    install(TARGETS GSL EXPORT Microsoft.GSLConfig)
-    install(
-        DIRECTORY include/gsl
-        DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
-    )
-    # Make library importable by other projects
-    install(EXPORT Microsoft.GSLConfig NAMESPACE Microsoft.GSL:: DESTINATION ${CMAKE_INSTALL_DATADIR}/cmake/Microsoft.GSL)
-    export(TARGETS GSL NAMESPACE Microsoft.GSL:: FILE Microsoft.GSLConfig.cmake)
-
-    install(FILES ${CMAKE_CURRENT_BINARY_DIR}/Microsoft.GSLConfigVersion.cmake DESTINATION ${CMAKE_INSTALL_DATADIR}/cmake/Microsoft.GSL)
-endfunction()
-
-# Add find_package() versioning support. The version for
-# generated Microsoft.GSLConfigVersion.cmake will be used from
-# last project() command. The version's compatibility is set between all
-# minor versions (as it was in prev. GSL releases).
-function(gsl_create_packaging_file)
-    if(${CMAKE_VERSION} VERSION_LESS "3.14.0")
-        write_basic_package_version_file(
-            ${CMAKE_CURRENT_BINARY_DIR}/Microsoft.GSLConfigVersion.cmake
-            COMPATIBILITY SameMajorVersion
-        )
-    else()
-        write_basic_package_version_file(
-            ${CMAKE_CURRENT_BINARY_DIR}/Microsoft.GSLConfigVersion.cmake
-            COMPATIBILITY SameMajorVersion
-            ARCH_INDEPENDENT
-        )
-    endif()
-endfunction()

docs/headers.md

@@ -0,0 +1,884 @@
+The Guidelines Support Library (GSL) interface is very lightweight and exposed via a header-only library. This document attempts to document all of the headers and their exposed classes and functions.
+
+Types and functions are exported in the namespace `gsl`.
+
+See [GSL: Guidelines support library](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#S-gsl)
+
+# <a name="H" />Headers
+
+- [`<algorithms>`](#user-content-H-algorithms)
+- [`<assert>`](#user-content-H-assert)
+- [`<byte>`](#user-content-H-byte)
+- [`<gsl>`](#user-content-H-gsl)
+- [`<narrow>`](#user-content-H-narrow)
+- [`<pointers>`](#user-content-H-pointers)
+- [`<span>`](#user-content-H-span)
+- [`<span_ext>`](#user-content-H-span_ext)
+- [`<zstring>`](#user-content-H-zstring)
+- [`<util>`](#user-content-H-util)
+
+## <a name="H-algorithms" />`<algorithms>`
+
+This header contains some common algorithms that have been wrapped in GSL safety features.
+
+- [`gsl::copy`](#user-content-H-algorithms-copy)
+
+### <a name="H-algorithms-copy" />`gsl::copy`
+
+```cpp
+template <class SrcElementType, std::size_t SrcExtent, class DestElementType,
+          std::size_t DestExtent>
+void copy(span<SrcElementType, SrcExtent> src, span<DestElementType, DestExtent> dest);
+```
+
+This function copies the content from the `src` [`span`](#user-content-H-span-span) to the `dest` [`span`](#user-content-H-span-span). It [`Expects`](#user-content-H-assert-expects)
+that the destination `span` is at least as large as the source `span`.
+
+## <a name="H-assert" />`<assert>`
+
+This header contains some macros used for contract checking and suppressing code analysis warnings.
+
+See [GSL.assert: Assertions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-assertions)
+
+- [`GSL_SUPPRESS`](#user-content-H-assert-gsl_suppress)
+- [`Expects`](#user-content-H-assert-expects)
+- [`Ensures`](#user-content-H-assert-ensures)
+
+### <a name="H-assert-gsl_suppress" />`GSL_SUPPRESS`
+
+This macro can be used to suppress a code analysis warning.
+
+The core guidelines request tools that check for the rules to respect suppressing a rule by writing
+`[[gsl::suppress(tag)]]` or `[[gsl::suppress(tag, justification: "message")]]`.
+
+Clang does not use exactly that syntax, but requires `tag` to be put in double quotes `[[gsl::suppress("tag")]]`.
+
+For portable code you can use `GSL_SUPPRESS(tag)`.
+
+See [In.force: Enforcement](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#inforce-enforcement).
+
+### <a name="H-assert-expects" />`Expects`
+
+This macro can be used for expressing a precondition. If the precondition is not held, then `std::terminate` will be called.
+
+See [I.6: Prefer `Expects()` for expressing preconditions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#i6-prefer-expects-for-expressing-preconditions)
+
+### <a name="H-assert-ensures" />`Ensures`
+
+This macro can be used for expressing a postcondition. If the postcondition is not held, then `std::terminate` will be called.
+
+See [I.8: Prefer `Ensures()` for expressing postconditions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#i8-prefer-ensures-for-expressing-postconditions)
+
+## <a name="H-byte" />`<byte>`
+
+This header contains the definition of a byte type, implementing `std::byte` before it was standardized into C++17.
+
+- [`gsl::byte`](#user-content-H-byte-byte)
+
+### <a name="H-byte-byte" />`gsl::byte`
+
+If `GSL_USE_STD_BYTE` is defined to be `1`, then `gsl::byte` will be an alias to `std::byte`.  
+If `GSL_USE_STD_BYTE` is defined to be `0`, then `gsl::byte` will be a distinct type that implements the concept of byte.  
+If `GSL_USE_STD_BYTE` is not defined, then the header file will check if `std::byte` is available (C\+\+17 or higher). If yes,
+`gsl::byte` will be an alias to `std::byte`, otherwise `gsl::byte` will be a distinct type that implements the concept of byte.
+
+&#x26a0; Take care when linking projects that were compiled with different language standards (before C\+\+17 and C\+\+17 or higher).
+If you do so, you might want to `#define GSL_USE_STD_BYTE 0` to a fixed value to be sure that both projects use exactly
+the same type. Otherwise you might get linker errors.
+
+See [SL.str.5: Use `std::byte` to refer to byte values that do not necessarily represent characters](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rstr-byte)
+
+### Non-member functions
+
+```cpp
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
+constexpr byte& operator<<=(byte& b, IntegerType shift) noexcept;
+
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
+constexpr byte operator<<(byte b, IntegerType shift) noexcept;
+
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
+constexpr byte& operator>>=(byte& b, IntegerType shift) noexcept;
+
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
+constexpr byte operator>>(byte b, IntegerType shift) noexcept;
+```
+
+Left or right shift a `byte` by a given number of bits.
+
+```cpp
+constexpr byte& operator|=(byte& l, byte r) noexcept;
+constexpr byte operator|(byte l, byte r) noexcept;
+```
+
+Bitwise "or" of two `byte`s.
+
+```cpp
+constexpr byte& operator&=(byte& l, byte r) noexcept;
+constexpr byte operator&(byte l, byte r) noexcept;
+```
+
+Bitwise "and" of two `byte`s.
+
+```cpp
+constexpr byte& operator^=(byte& l, byte r) noexcept;
+constexpr byte operator^(byte l, byte r) noexcept;
+```
+
+Bitwise xor of two `byte`s.
+
+```cpp
+constexpr byte operator~(byte b) noexcept;
+```
+
+Bitwise negation of a `byte`. Flips all bits. Zeroes become ones, ones become zeroes.
+
+```cpp
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
+constexpr IntegerType to_integer(byte b) noexcept;
+```
+
+Convert the given `byte` value to an integral type.
+
+```cpp
+template <typename T>
+constexpr byte to_byte(T t) noexcept;
+```
+
+Convert the given value to a `byte`. The template requires `T` to be an `unsigned char` so that no data loss can occur.
+If you want to convert an integer constant to a `byte` you probably want to call `to_byte<integer constant>()`.
+
+```cpp
+template <int I>
+constexpr byte to_byte() noexcept;
+```
+
+Convert the given value `I` to a `byte`. The template requires `I` to be in the valid range 0..255 for a `gsl::byte`.
+
+## <a name="H-gsl" />`<gsl>`
+
+This header is a convenience header that includes all other [GSL headers](#user-content-H).
+Since `<narrow>` requires exceptions, it will only be included if exceptions are enabled.
+
+## <a name="H-narrow" />`<narrow>`
+
+This header contains utility functions and classes, for narrowing casts, which require exceptions. The narrowing-related utilities that don't require exceptions are found inside [util](#user-content-H-util).
+
+See [GSL.util: Utilities](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-utilities)
+
+- [`gsl::narrowing_error`](#user-content-H-narrow-narrowing_error)
+- [`gsl::narrow`](#user-content-H-narrow-narrow)
+
+### <a name="H-narrow-narrowing_error" />`gsl::narrowing_error`
+
+`gsl::narrowing_error` is the exception thrown by [`gsl::narrow`](#user-content-H-narrow-narrow) when a narrowing conversion fails. It is derived from `std::exception`.
+
+### <a name="H-narrow-narrow" />`gsl::narrow`
+
+`gsl::narrow<T>(x)` is a named cast that does a `static_cast<T>(x)` for narrowing conversions with no signedness promotions.
+If the argument `x` cannot be represented in the target type `T`, then the function throws a [`gsl::narrowing_error`](#user-content-H-narrow-narrowing_error) (e.g., `narrow<unsigned>(-42)` and `narrow<char>(300)` throw).
+
+Note: compare [`gsl::narrow_cast`](#user-content-H-util-narrow_cast) in header [util](#user-content-H-util).
+
+See [ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-narrowing) and [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts-named)
+
+## <a name="H-pointers" />`<pointers>`
+
+This header contains some pointer types.
+
+See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-views)
+
+- [`gsl::unique_ptr`](#user-content-H-pointers-unique_ptr)
+- [`gsl::shared_ptr`](#user-content-H-pointers-shared_ptr)
+- [`gsl::owner`](#user-content-H-pointers-owner)
+- [`gsl::not_null`](#user-content-H-pointers-not_null)
+- [`gsl::strict_not_null`](#user-content-H-pointers-strict_not_null)
+
+### <a name="H-pointers-unique_ptr" />`gsl::unique_ptr`
+
+`gsl::unique_ptr` is an alias to `std::unique_ptr`.
+
+See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-ownership)
+
+### <a name="H-pointers-shared_ptr" />`gsl::shared_ptr`
+
+`gsl::shared_ptr` is an alias to `std::shared_ptr`.
+
+See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-ownership)
+
+### <a name="H-pointers-owner" />`gsl::owner`
+
+`gsl::owner<T>` is designed as a safety mechanism for code that must deal directly with raw pointers that own memory. Ideally such code should be restricted to the implementation of low-level abstractions. `gsl::owner` can also be used as a stepping point in converting legacy code to use more modern RAII constructs such as smart pointers.
+`T` must be a pointer type (`std::is_pointer<T>`).
+
+A `gsl::owner<T>` is a typedef to `T`. It adds no runtime overhead whatsoever, as it is purely syntactic and does not add any runtime checks.  Instead, it serves as an annotation for static analysis tools which check for memory safety, and as a code comprehension guide for human readers.
+
+See Enforcement section of [C.31: All resources acquired by a class must be released by the class’s destructor](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-dtor-release).
+
+### <a name="H-pointers-not_null" />`gsl::not_null`
+
+`gsl::not_null<T>` restricts a pointer or smart pointer to only hold non-null values. It has no size overhead over `T`.
+
+The checks for ensuring that the pointer is not null are done in the constructor. There is no overhead when retrieving or dereferencing the checked pointer.
+When a nullptr check fails, `std::terminate` is called.
+
+See [F.23: Use a `not_null<T>` to indicate that “null” is not a valid value](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-nullptr)
+
+#### Member Types
+
+```cpp
+using element_type = T;
+```
+
+The type of the pointer or smart pointer that is managed by this object.
+
+#### Member functions
+
+##### Construct/Copy
+
+```cpp
+template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
+constexpr not_null(U&& u);
+
+template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
+constexpr not_null(T u);
+```
+
+Constructs a `gsl_owner<T>` from a pointer that is convertible to `T` or that is a `T`. It [`Expects`](#user-content-H-assert-expects) that the provided pointer is not `== nullptr`.
+
+```cpp
+template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
+constexpr not_null(const not_null<U>& other);
+```
+
+Constructs a `gsl_owner<T>` from another `gsl_owner` where the other pointer is convertible to `T`. It [`Expects`](#user-content-H-assert-expects) that the provided pointer is not `== nullptr`.
+
+```cpp
+not_null(const not_null& other) = default;
+not_null& operator=(const not_null& other) = default;
+```
+
+Copy construction and assignment.
+
+```cpp
+not_null(std::nullptr_t) = delete;
+not_null& operator=(std::nullptr_t) = delete;
+```
+
+Construction from `std::nullptr_t`  and assignment of `std::nullptr_t` are explicitly deleted.
+
+##### Modifiers
+
+```cpp
+not_null& operator++() = delete;
+not_null& operator--() = delete;
+not_null operator++(int) = delete;
+not_null operator--(int) = delete;
+not_null& operator+=(std::ptrdiff_t) = delete;
+not_null& operator-=(std::ptrdiff_t) = delete;
+```
+
+Explicitly deleted operators. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow these operators.
+
+##### Observers
+
+```cpp
+constexpr details::value_or_reference_return_t<T> get() const;
+constexpr operator T() const { return get(); }
+```
+
+Get the underlying pointer.
+
+```cpp
+constexpr decltype(auto) operator->() const { return get(); }
+constexpr decltype(auto) operator*() const { return *get(); }
+```
+
+Dereference the underlying pointer.
+
+```cpp
+void operator[](std::ptrdiff_t) const = delete;
+```
+
+Array index operator is explicitly deleted. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow treating them as an array.
+
+```cpp
+void swap(not_null<T>& other) { std::swap(ptr_, other.ptr_); }
+```
+
+Swaps contents with another `gsl::not_null` object.
+
+#### Non-member functions
+
+```cpp
+template <class T>
+auto make_not_null(T&& t) noexcept;
+```
+
+Creates a `gsl::not_null` object, deducing the target type from the type of the argument.
+
+```cpp
+template <typename T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value, bool> = true>
+void swap(not_null<T>& a, not_null<T>& b);
+```
+
+Swaps the contents of two `gsl::not_null` objects.
+
+```cpp
+template <class T, class U>
+auto operator==(const not_null<T>& lhs,
+                const not_null<U>& rhs) noexcept(noexcept(lhs.get() == rhs.get()))
+    -> decltype(lhs.get() == rhs.get());
+template <class T, class U>
+auto operator!=(const not_null<T>& lhs,
+                const not_null<U>& rhs) noexcept(noexcept(lhs.get() != rhs.get()))
+    -> decltype(lhs.get() != rhs.get());
+template <class T, class U>
+auto operator<(const not_null<T>& lhs,
+               const not_null<U>& rhs) noexcept(noexcept(lhs.get() < rhs.get()))
+    -> decltype(lhs.get() < rhs.get());
+template <class T, class U>
+auto operator<=(const not_null<T>& lhs,
+                const not_null<U>& rhs) noexcept(noexcept(lhs.get() <= rhs.get()))
+    -> decltype(lhs.get() <= rhs.get());
+template <class T, class U>
+auto operator>(const not_null<T>& lhs,
+               const not_null<U>& rhs) noexcept(noexcept(lhs.get() > rhs.get()))
+    -> decltype(lhs.get() > rhs.get());
+template <class T, class U>
+auto operator>=(const not_null<T>& lhs,
+                const not_null<U>& rhs) noexcept(noexcept(lhs.get() >= rhs.get()))
+    -> decltype(lhs.get() >= rhs.get());
+```
+
+Comparison of pointers that are convertible to each other.
+
+##### Input/Output
+
+```cpp
+template <class T>
+std::ostream& operator<<(std::ostream& os, const not_null<T>& val);
+```
+
+Performs stream output on a `not_null` pointer, invoking `os << val.get()`. This function is only available when `GSL_NO_IOSTREAMS` is not defined.
+
+##### Modifiers
+
+```cpp
+template <class T, class U>
+std::ptrdiff_t operator-(const not_null<T>&, const not_null<U>&) = delete;
+template <class T>
+not_null<T> operator-(const not_null<T>&, std::ptrdiff_t) = delete;
+template <class T>
+not_null<T> operator+(const not_null<T>&, std::ptrdiff_t) = delete;
+template <class T>
+not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
+```
+
+Addition and subtraction are explicitly deleted. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow these operators.
+
+##### STL integration
+
+```cpp
+template <class T>
+struct std::hash<gsl::not_null<T>> { ... };
+```
+
+Specialization of `std::hash` for `gsl::not_null`.
+
+### <a name="H-pointers-strict_not_null" />`gsl::strict_not_null`
+
+`strict_not_null` is the same as [`not_null`](#user-content-H-pointers-not_null) except that the constructors are `explicit`.
+
+The free function that deduces the target type from the type of the argument and creates a `gsl::strict_not_null` object is `gsl::make_strict_not_null`.
+
+## <a name="H-span" />`<span>`
+
+This header file exports the class `gsl::span`, a bounds-checked implementation of `std::span`.
+
+- [`gsl::span`](#user-content-H-span-span)
+
+### <a name="H-span-span" />`gsl::span`
+
+```cpp
+template <class ElementType, std::size_t Extent>
+class span;
+```
+
+`gsl::span` is a view over memory. It does not own the memory and is only a way to access contiguous sequences of objects.
+The extent can be either a fixed size or [`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent).
+
+The `gsl::span` is based on the standardized version of `std::span` which was added to C++20. Originally, the plan was to
+deprecate `gsl::span` when `std::span` finished standardization, however that plan changed when the runtime bounds checking
+was removed from `std::span`'s design.
+
+The only difference between `gsl::span` and `std::span` is that `gsl::span` strictly enforces runtime bounds checking.
+Any violations of the bounds check results in termination of the program.
+Like `gsl::span`, `gsl::span`'s iterators also differ from `std::span`'s iterator in that all access operations are bounds checked.
+
+#### Which version of span should I use?
+
+##### Use `gsl::span` if
+
+- you want to guarantee bounds safety in your project.
+  - All data accessing operations use bounds checking to ensure you are only accessing valid memory.
+- your project uses C++14 or C++17.
+  - `std::span` is not available as it was not introduced into the STL until C++20.
+
+##### Use `std::span` if
+
+- your project is C++20 and you need the performance offered by `std::span`.
+
+#### Types
+
+```cpp
+using element_type = ElementType;
+using value_type = std::remove_cv_t<ElementType>;
+using size_type = std::size_t;
+using pointer = element_type*;
+using const_pointer = const element_type*;
+using reference = element_type&;
+using const_reference = const element_type&;
+using difference_type = std::ptrdiff_t;
+
+using iterator = details::span_iterator<ElementType>;
+using reverse_iterator = std::reverse_iterator<iterator>;
+```
+
+#### Member functions
+
+```cpp
+constexpr span() noexcept;
+```
+
+Constructs an empty `span`. This constructor is only available if `Extent` is 0 or [`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent).
+`span::data()` will return `nullptr`.
+
+```cpp
+constexpr explicit(Extent != gsl::dynamic_extent) span(pointer ptr, size_type count) noexcept;
+```
+
+Constructs a `span` from a pointer and a size. If `Extent` is not [`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent),
+then the constructor [`Expects`](#user-content-H-assert-expects) that `count == Extent`.
+
+```cpp
+constexpr explicit(Extent != gsl::dynamic_extent) span(pointer firstElem, pointer lastElem) noexcept;
+```
+
+Constructs a `span` from a pointer to the begin and the end of the data. If `Extent` is not [`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent),
+then the constructor [`Expects`](#user-content-H-assert-expects) that `lastElem - firstElem == Extent`.
+
+```cpp
+template <std::size_t N>
+constexpr span(element_type (&arr)[N]) noexcept;
+```
+
+Constructs a `span` from a C style array. This overload is available if `Extent ==`[`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent)
+or `N == Extent`.
+
+```cpp
+template <class T, std::size_t N>
+constexpr span(std::array<T, N>& arr) noexcept;
+
+template <class T, std::size_t N>
+constexpr span(const std::array<T, N>& arr) noexcept;
+```
+
+Constructs a `span` from a `std::array`. These overloads are available if `Extent ==`[`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent)
+or `N == Extent`, and if the array can be interpreted as a `ElementType` array.
+
+```cpp
+template <class Container>
+constexpr explicit(Extent != gsl::dynamic_extent) span(Container& cont) noexcept;
+
+template <class Container>
+constexpr explicit(Extent != gsl::dynamic_extent) span(const Container& cont) noexcept;
+```
+
+Constructs a `span` from a container. These overloads are available if `Extent ==`[`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent)
+or `N == Extent`, and if the container can be interpreted as a contiguous `ElementType` array.
+
+```cpp
+constexpr span(const span& other) noexcept = default;
+```
+
+Copy constructor.
+
+```cpp
+template <class OtherElementType, std::size_t OtherExtent>
+explicit(Extent != gsl::dynamic_extent && OtherExtent == dynamic_extent)
+constexpr span(const span<OtherElementType, OtherExtent>& other) noexcept;
+```
+
+Constructs a `span` from another `span`. This constructor is available if `OtherExtent == Extent || Extent ==`[`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent)` || OtherExtent ==`[`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent)
+and if `ElementType` and `OtherElementType` are compatible.
+
+If `Extent !=`[`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent) and `OtherExtent ==`[`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent),
+then the constructor [`Expects`](#user-content-H-assert-expects) that `other.size() == Extent`.
+
+```cpp
+constexpr span& operator=(const span& other) noexcept = default;
+```
+
+Copy assignment
+
+```cpp
+template <std::size_t Count>
+constexpr span<element_type, Count> first() const noexcept;
+
+constexpr span<element_type, dynamic_extent> first(size_type count) const noexcept;
+
+template <std::size_t Count>
+constexpr span<element_type, Count> last() const noexcept;
+
+constexpr span<element_type, dynamic_extent> last(size_type count) const noexcept;
+```
+
+Return a subspan of the first/last `Count` elements. [`Expects`](#user-content-H-assert-expects) that `Count` does not exceed the `span`'s size.
+
+```cpp
+template <std::size_t offset, std::size_t count = dynamic_extent>
+constexpr auto subspan() const noexcept;
+
+constexpr span<element_type, dynamic_extent>
+subspan(size_type offset, size_type count = dynamic_extent) const noexcept;
+```
+
+Return a subspan starting at `offset` and having size `count`. [`Expects`](#user-content-H-assert-expects) that `offset` does not exceed the `span`'s size,
+and that `offset == `[`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent) or `offset + count` does not exceed the `span`'s size.
+If `count` is `gsl::dynamic_extent`, the number of elements in the subspan is `size() - offset`.
+
+```cpp
+constexpr size_type size() const noexcept;
+
+constexpr size_type size_bytes() const noexcept;
+```
+
+Returns the size respective the size in bytes of the `span`.
+
+```cpp
+constexpr bool empty() const noexcept;
+```
+
+Is the `span` empty?
+
+```cpp
+constexpr reference operator[](size_type idx) const noexcept;
+```
+
+Returns a reference to the element at the given index. [`Expects`](#user-content-H-assert-expects) that `idx` is less than the `span`'s size.
+
+```cpp
+constexpr reference front() const noexcept;
+constexpr reference back() const noexcept;
+```
+
+Returns a reference to the first/last element in the `span`. [`Expects`](#user-content-H-assert-expects) that the `span` is not empty.
+
+```cpp
+constexpr pointer data() const noexcept;
+```
+
+Returns a pointer to the beginning of the contained data.
+
+```cpp
+constexpr iterator begin() const noexcept;
+constexpr iterator end() const noexcept;
+constexpr reverse_iterator rbegin() const noexcept;
+constexpr reverse_iterator rend() const noexcept;
+```
+
+Returns an iterator to the first/last normal/reverse iterator.
+
+```cpp
+template <class Type, std::size_t Extent>
+span(Type (&)[Extent]) -> span<Type, Extent>;
+
+template <class Type, std::size_t Size>
+span(std::array<Type, Size>&) -> span<Type, Size>;
+
+template <class Type, std::size_t Size>
+span(const std::array<Type, Size>&) -> span<const Type, Size>;
+
+template <class Container,
+          class Element = std::remove_pointer_t<decltype(std::declval<Container&>().data())>>
+span(Container&) -> span<Element>;
+
+template <class Container,
+          class Element = std::remove_pointer_t<decltype(std::declval<const Container&>().data())>>
+span(const Container&) -> span<Element>;
+```
+
+Deduction guides.
+
+```cpp
+template <class ElementType, std::size_t Extent>
+span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
+as_bytes(span<ElementType, Extent> s) noexcept;
+
+template <class ElementType, std::size_t Extent>
+span<byte, details::calculate_byte_size<ElementType, Extent>::value>
+as_writable_bytes(span<ElementType, Extent> s) noexcept;
+```
+
+Converts a `span` into a `span` of `byte`s.
+
+`as_writable_bytes` will only be available for non-const `ElementType`s.
+
+## <a name="H-span_ext" />`<span_ext>`
+
+This file is a companion for and included by [`<gsl/span>`](#user-content-H-span), and should not be used on its own. It contains useful features that aren't part of the `std::span` API as found inside the STL `<span>` header (with the exception of [`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent), which is included here due to implementation constraints).
+
+- [`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent)
+- [`gsl::span`](#user-content-H-span_ext-span)
+- [`gsl::span` comparison operators](#user-content-H-span_ext-span_comparison_operators)
+- [`gsl::make_span`](#user-content-H-span_ext-make_span)
+- [`gsl::at`](#user-content-H-span_ext-at)
+- [`gsl::ssize`](#user-content-H-span_ext-ssize)
+- [`gsl::span` iterator functions](#user-content-H-span_ext-span_iterator_functions)
+
+### <a name="H-span_ext-dynamic_extent" />`gsl::dynamic_extent`
+
+Defines the extent value to be used by all `gsl::span` with dynamic extent. 
+
+Note: `std::dynamic_extent` is exposed by the STL `<span>` header and so ideally `gsl::dynamic_extent` would be under [`<gsl/span>`](#user-content-H-span), but to avoid cyclic dependency issues it is under `<span_ext>` instead.
+
+### <a name="H-span_ext-span" />`gsl::span`
+
+```cpp
+template <class ElementType, std::size_t Extent = dynamic_extent>
+class span;
+```
+
+Forward declaration of `gsl::span`.
+
+### <a name="H-span_ext-span_comparison_operators" />`gsl::span` comparison operators
+
+```cpp
+template <class ElementType, std::size_t FirstExtent, std::size_t SecondExtent>
+constexpr bool operator==(span<ElementType, FirstExtent> l, span<ElementType, SecondExtent> r);
+template <class ElementType, std::size_t FirstExtent, std::size_t SecondExtent>
+constexpr bool operator!=(span<ElementType, FirstExtent> l, span<ElementType, SecondExtent> r);
+template <class ElementType, std::size_t Extent>
+constexpr bool operator<(span<ElementType, Extent> l, span<ElementType, Extent> r);
+template <class ElementType, std::size_t Extent>
+constexpr bool operator<=(span<ElementType, Extent> l, span<ElementType, Extent> r);
+template <class ElementType, std::size_t Extent>
+constexpr bool operator>(span<ElementType, Extent> l, span<ElementType, Extent> r);
+template <class ElementType, std::size_t Extent>
+constexpr bool operator>=(span<ElementType, Extent> l, span<ElementType, Extent> r);
+```
+
+The comparison operators for two `span`s lexicographically compare the elements in the `span`s.
+
+### <a name="H-span_ext-make_span" />`gsl::make_span`
+
+```cpp
+template <class ElementType>
+constexpr span<ElementType> make_span(ElementType* ptr, typename span<ElementType>::size_type count);
+template <class ElementType>
+constexpr span<ElementType> make_span(ElementType* firstElem, ElementType* lastElem);
+template <class ElementType, std::size_t N>
+constexpr span<ElementType, N> make_span(ElementType (&arr)[N]) noexcept;
+template <class Container>
+constexpr span<typename Container::value_type> make_span(Container& cont);
+template <class Container>
+constexpr span<const typename Container::value_type> make_span(const Container& cont);
+```
+
+Utility function for creating a `span` with [`gsl::dynamic_extent`](#user-content-H-span_ext-dynamic_extent) from
+- pointer and length,
+- pointer to start and pointer to end,
+- a C style array, or
+- a container.
+
+### <a name="H-span_ext-at" />`gsl::at`
+
+```cpp
+template <class ElementType, std::size_t Extent>
+constexpr ElementType& at(span<ElementType, Extent> s, index i);
+```
+
+The function `gsl::at` offers a safe way to access data with index bounds checking.
+
+This is the specialization of [`gsl::at`](#user-content-H-util-at) for [`span`](#user-content-H-span-span). It returns a reference to the `i`th element and
+[`Expects`](#user-content-H-assert-expects) that the provided index is within the bounds of the `span`.
+
+Note: `gsl::at` supports indexes up to `PTRDIFF_MAX`.
+
+### <a name="H-span_ext-ssize" />`gsl::ssize`
+
+```cpp
+template <class ElementType, std::size_t Extent>
+constexpr std::ptrdiff_t ssize(const span<ElementType, Extent>& s) noexcept;
+```
+
+Return the size of a [`span`](#user-content-H-span-span) as a `ptrdiff_t`.
+
+### <a name="H-span_ext-span_iterator_functions" />`gsl::span` iterator functions
+
+```cpp
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::iterator
+begin(const span<ElementType, Extent>& s) noexcept;
+
+template <class ElementType, std::size_t Extent = dynamic_extent>
+constexpr typename span<ElementType, Extent>::iterator
+end(const span<ElementType, Extent>& s) noexcept;
+
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::reverse_iterator
+rbegin(const span<ElementType, Extent>& s) noexcept;
+
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::reverse_iterator
+rend(const span<ElementType, Extent>& s) noexcept;
+
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::iterator
+cbegin(const span<ElementType, Extent>& s) noexcept;
+
+template <class ElementType, std::size_t Extent = dynamic_extent>
+constexpr typename span<ElementType, Extent>::iterator
+cend(const span<ElementType, Extent>& s) noexcept;
+
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::reverse_iterator
+crbegin(const span<ElementType, Extent>& s) noexcept;
+
+template <class ElementType, std::size_t Extent>
+constexpr typename span<ElementType, Extent>::reverse_iterator
+crend(const span<ElementType, Extent>& s) noexcept;
+```
+
+Free functions for getting a non-const/const begin/end normal/reverse iterator for a [`span`](#user-content-H-span-span).
+
+## <a name="H-zstring" />`<zstring>`
+
+This header exports a family of `*zstring` types.
+
+A `gsl::XXzstring<T>` is a typedef to `T`. It adds no checks whatsoever, it is just for having a syntax to describe
+that a pointer points to a zero terminated C style string. This helps static code analysis, and it helps human readers.
+
+`basic_zstring` is a pointer to a C-string (zero-terminated array) with a templated char type. Used to implement the rest of the `*zstring` family.  
+`zstring` is a zero terminated `char` string.  
+`czstring` is a const zero terminated `char` string.  
+`wzstring` is a zero terminated `wchar_t` string.  
+`cwzstring` is a const zero terminated `wchar_t` string.  
+`u16zstring` is a zero terminated `char16_t` string.  
+`cu16zstring` is a const zero terminated `char16_t` string.  
+`u32zstring` is a zero terminated `char32_t` string.  
+`cu32zstring` is a const zero terminated `char32_t` string.  
+
+See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-views) and [SL.str.3: Use zstring or czstring to refer to a C-style, zero-terminated, sequence of characters](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rstr-zstring).
+
+## <a name="H-util" />`<util>`
+
+This header contains utility functions and classes. This header works without exceptions being available. The parts that require
+exceptions being available are in their own header file [narrow](#user-content-H-narrow).
+
+See [GSL.util: Utilities](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-utilities)
+
+- [`gsl::narrow_cast`](#user-content-H-util-narrow_cast)
+- [`gsl::final_action`](#user-content-H-util-final_action)
+- [`gsl::at`](#user-content-H-util-at)
+
+### <a name="H-util-index" />`gsl::index`
+
+An alias to `std::ptrdiff_t`. It serves as the index type for all container indexes/subscripts/sizes.
+
+### <a name="H-util-narrow_cast" />`gsl::narrow_cast`
+
+`gsl::narrow_cast<T>(x)` is a named cast that is identical to a `static_cast<T>(x)`. It exists to make clear to static code analysis tools and to human readers that a lossy conversion is acceptable.
+
+Note: compare the throwing version [`gsl::narrow`](#user-content-H-narrow-narrow) in header [narrow](#user-content-H-narrow).
+
+See [ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-narrowing) and [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts-named)
+
+### <a name="H-util-final_action" />`gsl::final_action`
+
+```cpp
+template <class F>
+class final_action { ... };
+```
+
+`final_action` allows you to ensure something gets run at the end of a scope.
+
+See [E.19: Use a final_action object to express cleanup if no suitable resource handle is available](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Re-finally)
+
+#### Member functions
+
+```cpp
+explicit final_action(const F& ff) noexcept;
+explicit final_action(F&& ff) noexcept;
+```
+
+Construct an object with the action to invoke in the destructor.
+
+```cpp
+~final_action() noexcept;
+```
+
+The destructor will call the action that was passed in the constructor.
+
+```cpp
+final_action(final_action&& other) noexcept;
+final_action(const final_action&)   = delete;
+void operator=(const final_action&) = delete;
+void operator=(final_action&&)      = delete;
+```
+
+Move construction is allowed. Copy construction is deleted. Copy and move assignment are also explicitly deleted.
+
+#### <a name="H-util-finally" />Non-member functions
+```cpp
+template <class F>
+auto finally(F&& f) noexcept;
+```
+
+Creates a `gsl::final_action` object, deducing the template argument type from the type of the argument.
+
+### <a name="H-util-at" />`gsl::at`
+
+The function `gsl::at` offers a safe way to access data with index bounds checking.
+
+Note: `gsl::at` supports indexes up to `PTRDIFF_MAX`.
+
+See [ES.42: Keep use of pointers simple and straightforward](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-ptr)
+
+```cpp
+template <class T, std::size_t N>
+constexpr T& at(T (&arr)[N], const index i);
+```
+
+This overload returns a reference to the `i`s element of a C style array `arr`. It [`Expects`](#user-content-H-assert-expects) that the provided index is within the bounds of the array.
+
+```cpp
+template <class Cont>
+constexpr auto at(Cont& cont, const index i) -> decltype(cont[cont.size()]);
+```
+
+This overload returns a reference to the `i`s element of the container `cont`. It [`Expects`](#user-content-H-assert-expects) that the provided index is within the bounds of the array.
+
+```cpp
+template <class T>
+constexpr T at(const std::initializer_list<T> cont, const index i);
+```
+
+This overload returns a reference to the `i`s element of the initializer list `cont`. It [`Expects`](#user-content-H-assert-expects) that the provided index is within the bounds of the array.
+
+```cpp
+template <class T, std::size_t extent = std::dynamic_extent>
+constexpr auto at(std::span<T, extent> sp, const index i) -> decltype(sp[sp.size()]);
+```
+
+This overload returns a reference to the `i`s element of the `std::span` `sp`. It [`Expects`](#user-content-H-assert-expects) that the provided index is within the bounds of the array.
+
+For [`gsl::at`](#user-content-H-span_ext-at) for [`gsl::span`](#user-content-H-span-span) see header [`span_ext`](#user-content-H-span_ext).
+
+```cpp
+template <class T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value>>
+void swap(T& a, T& b);
+```
+
+Swaps the contents of two objects. Exists only to specialize `gsl::swap<T>(gsl::not_null<T>&, gsl::not_null<T>&)`.

include/CMakeLists.txt

@@ -1,4 +1,3 @@
-
 # Add include folders to the library and targets that consume it
 # the SYSTEM keyword suppresses warnings for users of the library
 #
@@ -7,14 +6,8 @@
 #
 # IE:
 #   #include <gsl/gsl>
-if(GSL_STANDALONE_PROJECT)
-    target_include_directories(GSL INTERFACE
-        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
-        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
-    )
+if(PROJECT_IS_TOP_LEVEL)
+    target_include_directories(GSL INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>)
 else()
-    target_include_directories(GSL SYSTEM INTERFACE
-        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
-        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
-    )
+    target_include_directories(GSL SYSTEM INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>)
 endif()

include/gsl/algorithm

@@ -17,8 +17,8 @@
 #ifndef GSL_ALGORITHM_H
 #define GSL_ALGORITHM_H
 
-#include <gsl/assert> // for Expects
-#include <gsl/span>   // for dynamic_extent, span
+#include "./assert" // for Expects
+#include "./span"   // for dynamic_extent, span
 
 #include <algorithm>   // for copy_n
 #include <cstddef>     // for ptrdiff_t

include/gsl/assert

@@ -14,8 +14,8 @@
 //
 ///////////////////////////////////////////////////////////////////////////////
 
-#ifndef GSL_CONTRACTS_H
-#define GSL_CONTRACTS_H
+#ifndef GSL_ASSERT_H
+#define GSL_ASSERT_H
 
 //
 // Temporary until MSVC STL supports no-exceptions mode.
@@ -46,18 +46,15 @@
 // Hopefully temporary until suppression standardization occurs
 //
 #if defined(__clang__)
-#define GSL_SUPPRESS(x) [[gsl::suppress("x")]]
+#define GSL_SUPPRESS(x) [[gsl::suppress(#x)]]
 #else
-#if defined(_MSC_VER) && !defined(__INTEL_COMPILER)
+#if defined(_MSC_VER) && !defined(__INTEL_COMPILER) && !defined(__NVCC__)
 #define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
 #else
 #define GSL_SUPPRESS(x)
 #endif // _MSC_VER
 #endif // __clang__
 
-#define GSL_STRINGIFY_DETAIL(x) #x
-#define GSL_STRINGIFY(x) GSL_STRINGIFY_DETAIL(x)
-
 #if defined(__clang__) || defined(__GNUC__)
 #define GSL_LIKELY(x) __builtin_expect(!!(x), 1)
 #define GSL_UNLIKELY(x) __builtin_expect(!!(x), 0)
@@ -133,4 +130,4 @@ namespace details
 #pragma clang diagnostic pop
 #endif
 
-#endif // GSL_CONTRACTS_H
+#endif // GSL_ASSERT_H

include/gsl/byte

@@ -17,24 +17,10 @@
 #ifndef GSL_BYTE_H
 #define GSL_BYTE_H
 
-//
-// make suppress attributes work for some compilers
-// Hopefully temporary until suppression standardization occurs
-//
-#if defined(__clang__)
-#define GSL_SUPPRESS(x) [[gsl::suppress("x")]]
-#else
-#if defined(_MSC_VER) && !defined(__INTEL_COMPILER)
-#define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
-#else
-#define GSL_SUPPRESS(x)
-#endif // _MSC_VER
-#endif // __clang__
+#include "./util" // for GSL_DEPRECATED
 
 #include <type_traits>
 
-// VS2017 15.8 added support for the __cpp_lib_byte definition
-// To do: drop _HAS_STD_BYTE when support for pre 15.8 expires
 #ifdef _MSC_VER
 
 #pragma warning(push)
@@ -45,18 +31,15 @@
 
 #ifndef GSL_USE_STD_BYTE
 // this tests if we are under MSVC and the standard lib has std::byte and it is enabled
-#if (defined(_HAS_STD_BYTE) && _HAS_STD_BYTE) ||                                                   \
-    (defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603)
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603
 
 #define GSL_USE_STD_BYTE 1
 
-#else // (defined(_HAS_STD_BYTE) && _HAS_STD_BYTE) || (defined(__cpp_lib_byte) && __cpp_lib_byte >=
-      // 201603)
+#else // defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603
 
 #define GSL_USE_STD_BYTE 0
 
-#endif // (defined(_HAS_STD_BYTE) && _HAS_STD_BYTE) || (defined(__cpp_lib_byte) && __cpp_lib_byte >=
-       // 201603)
+#endif // defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603
 #endif // GSL_USE_STD_BYTE
 
 #else // _MSC_VER
@@ -99,7 +82,14 @@ namespace gsl
 {
 #if GSL_USE_STD_BYTE
 
-using std::byte;
+namespace impl {
+// impl::byte is used by gsl::as_bytes so our own code does not trigger a deprecation warning as would be the case when we used gsl::byte.
+// Users of GSL should only use gsl::byte, not gsl::impl::byte.
+using byte = std::byte;
+}
+
+using byte GSL_DEPRECATED("Use std::byte instead.") = std::byte;
+
 using std::to_integer;
 
 #else // GSL_USE_STD_BYTE
@@ -110,25 +100,31 @@ enum class byte_may_alias byte : unsigned char
 {
 };
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+namespace impl {
+// impl::byte is used by gsl::as_bytes so our own code does not trigger a deprecation warning as would be the case when we used gsl::byte.
+// Users of GSL should only use gsl::byte, not gsl::impl::byte.
+using byte = gsl::byte;
+}
+
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator<<=(byte& b, IntegerType shift) noexcept
 {
     return b = byte(static_cast<unsigned char>(b) << shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator<<(byte b, IntegerType shift) noexcept
 {
     return byte(static_cast<unsigned char>(b) << shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator>>=(byte& b, IntegerType shift) noexcept
 {
     return b = byte(static_cast<unsigned char>(b) >> shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator>>(byte b, IntegerType shift) noexcept
 {
     return byte(static_cast<unsigned char>(b) >> shift);
@@ -166,7 +162,7 @@ constexpr byte operator^(byte l, byte r) noexcept
 
 constexpr byte operator~(byte b) noexcept { return byte(~static_cast<unsigned char>(b)); }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr IntegerType to_integer(byte b) noexcept
 {
     return static_cast<IntegerType>(b);
@@ -174,34 +170,24 @@ constexpr IntegerType to_integer(byte b) noexcept
 
 #endif // GSL_USE_STD_BYTE
 
-template <bool E, typename T>
-constexpr byte to_byte_impl(T t) noexcept
-{
-    static_assert(
-        E, "gsl::to_byte(t) must be provided an unsigned char, otherwise data loss may occur. "
-           "If you are calling to_byte with an integer contant use: gsl::to_byte<t>() version.");
-    return static_cast<byte>(t);
-}
-template <>
-// NOTE: need suppression since c++14 does not allow "return {t}"
-// GSL_SUPPRESS(type.4) // NO-FORMAT: attribute // TODO: suppression does not work
-constexpr byte to_byte_impl<true, unsigned char>(unsigned char t) noexcept
-{
-    return byte(t);
-}
 
 template <typename T>
-constexpr byte to_byte(T t) noexcept
+// NOTE: need suppression since c++14 does not allow "return {t}"
+// GSL_SUPPRESS(type.4) // NO-FORMAT: attribute // TODO: suppression does not work
+constexpr gsl::impl::byte to_byte(T t) noexcept
 {
-    return to_byte_impl<std::is_same<T, unsigned char>::value, T>(t);
+    static_assert(std::is_same<T, unsigned char>::value,
+                  "gsl::to_byte(t) must be provided an unsigned char, otherwise data loss may occur. "
+                  "If you are calling to_byte with an integer constant use: gsl::to_byte<t>() version.");
+    return gsl::impl::byte(t);
 }
 
 template <int I>
-constexpr byte to_byte() noexcept
+constexpr gsl::impl::byte to_byte() noexcept
 {
     static_assert(I >= 0 && I <= 255,
                   "gsl::byte only has 8 bits of storage, values must be in range 0-255");
-    return static_cast<byte>(I);
+    return static_cast<gsl::impl::byte>(I);
 }
 
 } // namespace gsl

include/gsl/gsl

@@ -17,16 +17,18 @@
 #ifndef GSL_GSL_H
 #define GSL_GSL_H
 
-#include <gsl/algorithm>   // copy
-#include <gsl/assert>      // Ensures/Expects
-#include <gsl/byte>        // byte
-#include <gsl/pointers>    // owner, not_null
-#include <gsl/span>        // span
-#include <gsl/string_span> // zstring, string_span, zstring_builder...
-#include <gsl/util>        // finally()/narrow_cast()...
+// IWYU pragma: begin_exports
+#include "./algorithm"   // copy
+#include "./assert"      // Ensures/Expects
+#include "./byte"        // byte
+#include "./pointers"    // owner, not_null
+#include "./span"        // span
+#include "./zstring"     // zstring
+#include "./util"        // finally()/narrow_cast()...
 
 #ifdef __cpp_exceptions
-#include <gsl/narrow> // narrow()
+#include "./narrow" // narrow()
 #endif
+// IWYU pragma: end_exports
 
 #endif // GSL_GSL_H

include/gsl/gsl_algorithm

@@ -1,4 +0,0 @@
-#pragma once
-#pragma message(                                                                                   \
-    "This header will soon be removed. Use <gsl/algorithm> instead of <gsl/gsl_algorithm>")
-#include <gsl/algorithm>

include/gsl/gsl_assert

@@ -1,3 +0,0 @@
-#pragma once
-#pragma message("This header will soon be removed. Use <gsl/assert> instead of <gsl/gsl_assert>")
-#include <gsl/assert>

include/gsl/gsl_byte

@@ -1,3 +0,0 @@
-#pragma once
-#pragma message("This header will soon be removed. Use <gsl/byte> instead of <gsl/gsl_byte>")
-#include <gsl/byte>

include/gsl/gsl_narrow

@@ -1,3 +0,0 @@
-#pragma once
-#pragma message("This header will soon be removed. Use <gsl/narrow> instead of <gsl/gsl_narrow>")
-#include <gsl/narrow>

include/gsl/gsl_util

@@ -1,3 +0,0 @@
-#pragma once
-#pragma message("This header will soon be removed. Use <gsl/util> instead of <gsl/gsl_util>")
-#include <gsl/util>

include/gsl/narrow

@@ -16,8 +16,9 @@
 
 #ifndef GSL_NARROW_H
 #define GSL_NARROW_H
-#include <gsl/assert> // for Expects
-#include <gsl/util>   // for narrow_cast
+#include "./assert"    // for GSL_SUPPRESS
+#include "./util"      // for narrow_cast
+#include <exception> // for std::exception
 namespace gsl
 {
 struct narrowing_error : public std::exception
@@ -29,9 +30,12 @@ struct narrowing_error : public std::exception
 template <class T, class U, typename std::enable_if<std::is_arithmetic<T>::value>::type* = nullptr>
 // clang-format off
 GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recognise noexcept(false)
+GSL_SUPPRESS(es.46) // NO-FORMAT: attribute // The warning suggests that a floating->unsigned conversion can occur
+                                            // in the static_cast below, and that gsl::narrow should be used instead.
+                                            // Suppress this warning, since gsl::narrow is defined in terms of
+                                            // static_cast
     // clang-format on
-    constexpr T narrow(U u) noexcept(false)
+    constexpr T narrow(U u)
 {
     constexpr const bool is_different_signedness =
         (std::is_signed<T>::value != std::is_signed<U>::value);
@@ -43,10 +47,18 @@ GSL_SUPPRESS(p.2) // NO-FORMAT: attribute // don't rely on undefined behavior
                                    // and cannot fit into the destination integral type), the resultant behavior is benign on the platforms
                                    // that we target (i.e., no hardware trap representations are hit).
 
+#if defined(__clang__) || defined(__GNUC__)
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wfloat-equal"
+#endif
+    // Note: NaN will always throw, since NaN != NaN
     if (static_cast<U>(t) != u || (is_different_signedness && ((t < T{}) != (u < U{}))))
     {
         throw narrowing_error{};
     }
+#if defined(__clang__) || defined(__GNUC__)
+    #pragma GCC diagnostic pop
+#endif
 
     return t;
 }
@@ -54,9 +66,8 @@ GSL_SUPPRESS(p.2) // NO-FORMAT: attribute // don't rely on undefined behavior
 template <class T, class U, typename std::enable_if<!std::is_arithmetic<T>::value>::type* = nullptr>
 // clang-format off
 GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
-GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recognise noexcept(false)
     // clang-format on
-    constexpr T narrow(U u) noexcept(false)
+    constexpr T narrow(U u)
 {
     const T t = narrow_cast<T>(u);
 

include/gsl/pointers

@@ -17,13 +17,14 @@
 #ifndef GSL_POINTERS_H
 #define GSL_POINTERS_H
 
-#include <gsl/assert> // for Ensures, Expects
+#include "./assert" // for Ensures, Expects
+#include "./util" // for GSL_DEPRECATED
 
-#include <algorithm>    // for forward
 #include <cstddef>      // for ptrdiff_t, nullptr_t, size_t
-#include <memory>       // for shared_ptr, unique_ptr
-#include <system_error> // for hash
+#include <functional>   // for less, greater
+#include <memory>       // for shared_ptr, unique_ptr, hash
 #include <type_traits>  // for enable_if_t, is_convertible, is_assignable
+#include <utility>      // for declval, forward
 
 #if !defined(GSL_NO_IOSTREAMS)
 #include <iosfwd> // for ostream
@@ -46,24 +47,39 @@ namespace details
         : std::true_type
     {
     };
+
+    // Resolves to the more efficient of `const T` or `const T&`, in the context of returning a const-qualified value
+    // of type T.
+    //
+    // Copied from cppfront's implementation of the CppCoreGuidelines F.16 (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-in)
+    template<typename T>
+    using value_or_reference_return_t = std::conditional_t<
+                                            sizeof(T) < 2*sizeof(void*) && std::is_trivially_copy_constructible<T>::value,
+                                            const T,
+                                            const T&>;
+
 } // namespace details
 
 //
 // GSL.owner: ownership pointers
 //
-using std::shared_ptr;
-using std::unique_ptr;
+template <typename... Ts>
+using shared_ptr GSL_DEPRECATED("Use std::shared_ptr instead") = std::shared_ptr<Ts...>;
+
+template <typename... Ts>
+using unique_ptr GSL_DEPRECATED("Use std::unique_ptr instead") = std::unique_ptr<Ts...>;
 
 //
 // owner
 //
-// owner<T> is designed as a bridge for code that must deal directly with owning pointers for some
-// reason
+// `gsl::owner<T>` is designed as a safety mechanism for code that must deal directly with raw pointers that own memory.
+// Ideally such code should be restricted to the implementation of low-level abstractions. `gsl::owner` can also be used
+// as a stepping point in converting legacy code to use more modern RAII constructs, such as smart pointers.
 //
 // T must be a pointer type
 // - disallow construction from any type other than pointer type
 //
-template <class T, class = std::enable_if_t<std::is_pointer<T>::value>>
+template <class T, std::enable_if_t<std::is_pointer<T>::value, bool> = true>
 using owner = T;
 
 //
@@ -86,27 +102,29 @@ class not_null
 public:
     static_assert(details::is_comparable_to_nullptr<T>::value, "T cannot be compared to nullptr.");
 
+    using element_type = T;
+
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr not_null(U&& u) : ptr_(std::forward<U>(u))
+    constexpr not_null(U&& u) noexcept(std::is_nothrow_move_constructible<T>::value) : ptr_(std::forward<U>(u))
     {
         Expects(ptr_ != nullptr);
     }
 
     template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
-    constexpr not_null(T u) : ptr_(std::move(u))
+    constexpr not_null(T u) noexcept(std::is_nothrow_move_constructible<T>::value) : ptr_(std::move(u))
     {
         Expects(ptr_ != nullptr);
     }
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr not_null(const not_null<U>& other) : not_null(other.get())
+    constexpr not_null(const not_null<U>& other) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null(other.get())
     {}
 
     not_null(const not_null& other) = default;
     not_null& operator=(const not_null& other) = default;
-    constexpr std::conditional_t<std::is_copy_constructible<T>::value, T, const T&> get() const
+    constexpr details::value_or_reference_return_t<T> get() const
+        noexcept(noexcept(details::value_or_reference_return_t<T>(std::declval<T&>())))
     {
-        Ensures(ptr_ != nullptr);
         return ptr_;
     }
 
@@ -127,10 +145,18 @@ public:
     not_null& operator-=(std::ptrdiff_t) = delete;
     void operator[](std::ptrdiff_t) const = delete;
 
+    void swap(not_null<T>& other) { std::swap(ptr_, other.ptr_); }
+
 private:
     T ptr_;
 };
 
+template <typename T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value, bool> = true>
+void swap(not_null<T>& a, not_null<T>& b)
+{
+    a.swap(b);
+}
+
 template <class T>
 auto make_not_null(T&& t) noexcept
 {
@@ -164,34 +190,34 @@ auto operator!=(const not_null<T>& lhs,
 
 template <class T, class U>
 auto operator<(const not_null<T>& lhs,
-               const not_null<U>& rhs) noexcept(noexcept(lhs.get() < rhs.get()))
-    -> decltype(lhs.get() < rhs.get())
+               const not_null<U>& rhs) noexcept(noexcept(std::less<>{}(lhs.get(), rhs.get())))
+    -> decltype(std::less<>{}(lhs.get(), rhs.get()))
 {
-    return lhs.get() < rhs.get();
+    return std::less<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
 auto operator<=(const not_null<T>& lhs,
-                const not_null<U>& rhs) noexcept(noexcept(lhs.get() <= rhs.get()))
-    -> decltype(lhs.get() <= rhs.get())
+                const not_null<U>& rhs) noexcept(noexcept(std::less_equal<>{}(lhs.get(), rhs.get())))
+    -> decltype(std::less_equal<>{}(lhs.get(), rhs.get()))
 {
-    return lhs.get() <= rhs.get();
+    return std::less_equal<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
 auto operator>(const not_null<T>& lhs,
-               const not_null<U>& rhs) noexcept(noexcept(lhs.get() > rhs.get()))
-    -> decltype(lhs.get() > rhs.get())
+               const not_null<U>& rhs) noexcept(noexcept(std::greater<>{}(lhs.get(), rhs.get())))
+    -> decltype(std::greater<>{}(lhs.get(), rhs.get()))
 {
-    return lhs.get() > rhs.get();
+    return std::greater<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
 auto operator>=(const not_null<T>& lhs,
-                const not_null<U>& rhs) noexcept(noexcept(lhs.get() >= rhs.get()))
-    -> decltype(lhs.get() >= rhs.get())
+                const not_null<U>& rhs) noexcept(noexcept(std::greater_equal<>{}(lhs.get(), rhs.get())))
+    -> decltype(std::greater_equal<>{}(lhs.get(), rhs.get()))
 {
-    return lhs.get() >= rhs.get();
+    return std::greater_equal<>{}(lhs.get(), rhs.get());
 }
 
 // more unwanted operators
@@ -204,14 +230,28 @@ not_null<T> operator+(const not_null<T>&, std::ptrdiff_t) = delete;
 template <class T>
 not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
 
+
+template <class T, class U = decltype(std::declval<const T&>().get()), bool = std::is_default_constructible<std::hash<U>>::value>
+struct not_null_hash
+{
+    std::size_t operator()(const T& value) const { return std::hash<U>{}(value.get()); }
+};
+
+template <class T, class U>
+struct not_null_hash<T, U, false>
+{
+    not_null_hash() = delete;
+    not_null_hash(const not_null_hash&) = delete;
+    not_null_hash& operator=(const not_null_hash&) = delete;
+};
+
 } // namespace gsl
 
 namespace std
 {
 template <class T>
-struct hash<gsl::not_null<T>>
+struct hash<gsl::not_null<T>> : gsl::not_null_hash<gsl::not_null<T>>
 {
-    std::size_t operator()(const gsl::not_null<T>& value) const { return hash<T>{}(value.get()); }
 };
 
 } // namespace std
@@ -241,22 +281,24 @@ class strict_not_null : public not_null<T>
 {
 public:
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr explicit strict_not_null(U&& u) : not_null<T>(std::forward<U>(u))
+    constexpr explicit strict_not_null(U&& u) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null<T>(std::forward<U>(u))
     {}
 
     template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
-    constexpr explicit strict_not_null(T u) : not_null<T>(u)
+    constexpr explicit strict_not_null(T u) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null<T>(std::move(u))
     {}
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr strict_not_null(const not_null<U>& other) : not_null<T>(other)
+    constexpr strict_not_null(const not_null<U>& other) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null<T>(other)
     {}
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr strict_not_null(const strict_not_null<U>& other) : not_null<T>(other)
+    constexpr strict_not_null(const strict_not_null<U>& other) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null<T>(other)
     {}
 
-    strict_not_null(strict_not_null&& other) = default;
+    // To avoid invalidating the "not null" invariant, the contained pointer is actually copied
+    // instead of moved. If it is a custom pointer, its constructor could in theory throw exceptions.
+    strict_not_null(strict_not_null&& other) noexcept(std::is_nothrow_copy_constructible<T>::value) = default;
     strict_not_null(const strict_not_null& other) = default;
     strict_not_null& operator=(const strict_not_null& other) = default;
     strict_not_null& operator=(const not_null<T>& other)
@@ -310,12 +352,8 @@ strict_not_null(T) -> strict_not_null<T>;
 namespace std
 {
 template <class T>
-struct hash<gsl::strict_not_null<T>>
+struct hash<gsl::strict_not_null<T>> : gsl::not_null_hash<gsl::strict_not_null<T>>
 {
-    std::size_t operator()(const gsl::strict_not_null<T>& value) const
-    {
-        return hash<T>{}(value.get());
-    }
 };
 
 } // namespace std

include/gsl/span

@@ -17,15 +17,20 @@
 #ifndef GSL_SPAN_H
 #define GSL_SPAN_H
 
-#include <gsl/assert> // for Expects
-#include <gsl/byte>   // for byte
-#include <gsl/util>   // for narrow_cast
+#include "./assert"   // for Expects
+#include "./byte"     // for gsl::impl::byte
+#include "./span_ext" // for span specialization of gsl::at and other span-related extensions
+#include "./util"     // for narrow_cast
 
-#include <array>        // for array
-#include <cstddef>      // for ptrdiff_t, size_t, nullptr_t
-#include <gsl/span_ext> // for span specialization of gsl::at and other span-related extensions
-#include <iterator>     // for reverse_iterator, distance, random_access_...
-#include <type_traits>  // for enable_if_t, declval, is_convertible, inte...
+#include <array>       // for array
+#include <cstddef>     // for ptrdiff_t, size_t, nullptr_t
+#include <iterator>    // for reverse_iterator, distance, random_access_...
+#include <memory>      // for pointer_traits
+#include <type_traits> // for enable_if_t, declval, is_convertible, inte...
+
+#if defined(__has_include) && __has_include(<version>)
+#include <version>
+#endif
 
 #if defined(_MSC_VER) && !defined(__clang__)
 #pragma warning(push)
@@ -37,7 +42,7 @@
 #pragma warning(disable : 4702) // unreachable code
 
 // Turn MSVC /analyze rules that generate too much noise. TODO: fix in the tool.
-#pragma warning(disable : 26495) // uninitalized member when constructor calls constructor
+#pragma warning(disable : 26495) // uninitialized member when constructor calls constructor
 #pragma warning(disable : 26446) // parser bug does not allow attributes on some templates
 
 #endif // _MSC_VER
@@ -48,7 +53,7 @@
 #define GSL_USE_STATIC_CONSTEXPR_WORKAROUND
 #endif // !(defined(__cplusplus) && (__cplusplus >= 201703L))
 
-// GCC 7 does not like the signed unsigned missmatch (size_t ptrdiff_t)
+// GCC 7 does not like the signed unsigned mismatch (size_t ptrdiff_t)
 // While there is a conversion from signed to unsigned, it happens at
 // compiletime, so the compiler wouldn't have to warn indiscriminately, but
 // could check if the source value actually doesn't fit into the target type
@@ -58,6 +63,14 @@
 #pragma GCC diagnostic ignored "-Wsign-conversion"
 #endif
 
+// Turn off clang unsafe buffer warnings as all accessed are guarded by runtime checks
+#if defined(__clang__)
+#if __has_warning("-Wunsafe-buffer-usage")
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunsafe-buffer-usage"
+#endif // __has_warning("-Wunsafe-buffer-usage")
+#endif // defined(__clang__)
+
 namespace gsl
 {
 
@@ -110,6 +123,9 @@ namespace details
     class span_iterator
     {
     public:
+#if defined(__cpp_lib_ranges) || (defined(_MSVC_STL_VERSION) && defined(__cpp_lib_concepts))
+        using iterator_concept = std::contiguous_iterator_tag;
+#endif // __cpp_lib_ranges
         using iterator_category = std::random_access_iterator_tag;
         using value_type = std::remove_cv_t<Type>;
         using difference_type = std::ptrdiff_t;
@@ -118,12 +134,15 @@ namespace details
 
 #ifdef _MSC_VER
         using _Unchecked_type = pointer;
+        using _Prevent_inheriting_unwrap = span_iterator;
 #endif // _MSC_VER
         constexpr span_iterator() = default;
 
         constexpr span_iterator(pointer begin, pointer end, pointer current)
             : begin_(begin), end_(end), current_(current)
-        {}
+        {
+            Expects(begin_ <= current_ && current <= end_);
+        }
 
         constexpr operator span_iterator<const Type>() const noexcept
         {
@@ -132,21 +151,18 @@ namespace details
 
         constexpr reference operator*() const noexcept
         {
-            Expects(begin_ && end_);
-            Expects(begin_ <= current_ && current_ < end_);
+            Expects(current_ != end_);
             return *current_;
         }
 
         constexpr pointer operator->() const noexcept
         {
-            Expects(begin_ && end_);
-            Expects(begin_ <= current_ && current_ < end_);
+            Expects(current_ != end_);
             return current_;
         }
         constexpr span_iterator& operator++() noexcept
         {
-            Expects(begin_ && current_ && end_);
-            Expects(current_ < end_);
+            Expects(current_ != end_);
             // clang-format off
             GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
             // clang-format on
@@ -163,8 +179,7 @@ namespace details
 
         constexpr span_iterator& operator--() noexcept
         {
-            Expects(begin_ && end_);
-            Expects(begin_ < current_);
+            Expects(begin_ != current_);
             --current_;
             return *this;
         }
@@ -206,6 +221,7 @@ namespace details
             if (n != 0) Expects(begin_ && current_ && end_);
             if (n > 0) Expects(current_ - begin_ >= n);
             if (n < 0) Expects(end_ - current_ >= -n);
+            GSL_SUPPRESS(bounds .1)
             current_ -= n;
             return *this;
         }
@@ -329,8 +345,26 @@ namespace details
         pointer begin_ = nullptr;
         pointer end_ = nullptr;
         pointer current_ = nullptr;
-    };
 
+        template <typename Ptr>
+        friend struct std::pointer_traits;
+    };
+}} // namespace gsl::details
+
+namespace std
+{
+template <class Type>
+struct pointer_traits<::gsl::details::span_iterator<Type>>
+{
+    using pointer = ::gsl::details::span_iterator<Type>;
+    using element_type = Type;
+    using difference_type = ptrdiff_t;
+
+    static constexpr element_type* to_address(const pointer i) noexcept { return i.current_; }
+};
+} // namespace std
+
+namespace gsl { namespace details {
     template <std::size_t Ext>
     class extent_type
     {
@@ -548,6 +582,8 @@ public:
     template <std::size_t Count>
     constexpr span<element_type, Count> first() const noexcept
     {
+        static_assert(Extent == dynamic_extent || Count <= Extent,
+                      "first() cannot extract more elements from a span than it contains.");
         Expects(Count <= size());
         return span<element_type, Count>{data(), Count};
     }
@@ -558,6 +594,8 @@ public:
         // clang-format on
         constexpr span<element_type, Count> last() const noexcept
     {
+        static_assert(Extent == dynamic_extent || Count <= Extent,
+                      "last() cannot extract more elements from a span than it contains.");
         Expects(Count <= size());
         return span<element_type, Count>{data() + (size() - Count), Count};
     }
@@ -569,6 +607,9 @@ public:
         constexpr auto subspan() const noexcept ->
         typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
     {
+        static_assert(Extent == dynamic_extent || (Extent >= Offset && (Count == dynamic_extent ||
+                                                                        Count <= Extent - Offset)),
+                      "subspan() cannot extract more elements from a span than it contains.");
         Expects((size() >= Offset) && (Count == dynamic_extent || (Count <= size() - Offset)));
         using type =
             typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type;
@@ -596,11 +637,7 @@ public:
     // [span.obs], span observers
     constexpr size_type size() const noexcept { return storage_.size(); }
 
-    constexpr size_type size_bytes() const noexcept
-    {
-        Expects(size() < dynamic_extent / sizeof(element_type));
-        return size() * sizeof(element_type);
-    }
+    constexpr size_type size_bytes() const noexcept { return size() * sizeof(element_type); }
 
     constexpr bool empty() const noexcept { return size() == 0; }
 
@@ -682,14 +719,11 @@ private:
         template <class OtherExtentType>
         constexpr storage_type(KnownNotNull data, OtherExtentType ext)
             : ExtentType(ext), data_(data.p)
-        {
-            Expects(ExtentType::size() != dynamic_extent);
-        }
+        {}
 
         template <class OtherExtentType>
         constexpr storage_type(pointer data, OtherExtentType ext) : ExtentType(ext), data_(data)
         {
-            Expects(ExtentType::size() != dynamic_extent);
             Expects(data || ExtentType::size() == 0);
         }
 
@@ -756,8 +790,15 @@ span(const Container&) -> span<Element>;
 #endif // ( defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L) )
 
 #if defined(GSL_USE_STATIC_CONSTEXPR_WORKAROUND)
+#if defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated" // Bug in clang-cl.exe which raises a C++17 -Wdeprecated warning about this static constexpr workaround in C++14 mode.
+#endif // defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
 template <class ElementType, std::size_t Extent>
 constexpr const typename span<ElementType, Extent>::size_type span<ElementType, Extent>::extent;
+#if defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
+#pragma clang diagnostic pop
+#endif // defined(__clang__) && defined(_MSC_VER) && defined(__cplusplus) && (__cplusplus < 201703L)
 #endif
 
 namespace details
@@ -783,28 +824,28 @@ namespace details
 
 // [span.objectrep], views of object representation
 template <class ElementType, std::size_t Extent>
-span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
+span<const gsl::impl::byte, details::calculate_byte_size<ElementType, Extent>::value>
 as_bytes(span<ElementType, Extent> s) noexcept
 {
-    using type = span<const byte, details::calculate_byte_size<ElementType, Extent>::value>;
+    using type = span<const gsl::impl::byte, details::calculate_byte_size<ElementType, Extent>::value>;
 
     // clang-format off
     GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
     // clang-format on
-    return type{reinterpret_cast<const byte*>(s.data()), s.size_bytes()};
+    return type{reinterpret_cast<const gsl::impl::byte*>(s.data()), s.size_bytes()};
 }
 
 template <class ElementType, std::size_t Extent,
           std::enable_if_t<!std::is_const<ElementType>::value, int> = 0>
-span<byte, details::calculate_byte_size<ElementType, Extent>::value>
+span<gsl::impl::byte, details::calculate_byte_size<ElementType, Extent>::value>
 as_writable_bytes(span<ElementType, Extent> s) noexcept
 {
-    using type = span<byte, details::calculate_byte_size<ElementType, Extent>::value>;
+    using type = span<gsl::impl::byte, details::calculate_byte_size<ElementType, Extent>::value>;
 
     // clang-format off
     GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
     // clang-format on
-    return type{reinterpret_cast<byte*>(s.data()), s.size_bytes()};
+    return type{reinterpret_cast<gsl::impl::byte*>(s.data()), s.size_bytes()};
 }
 
 } // namespace gsl
@@ -818,4 +859,10 @@ as_writable_bytes(span<ElementType, Extent> s) noexcept
 #pragma GCC diagnostic pop
 #endif // __GNUC__ > 6
 
+#if defined(__clang__)
+#if __has_warning("-Wunsafe-buffer-usage")
+#pragma clang diagnostic pop
+#endif // __has_warning("-Wunsafe-buffer-usage")
+#endif // defined(__clang__)
+
 #endif // GSL_SPAN_H

include/gsl/span_ext

@@ -27,8 +27,8 @@
 //
 ///////////////////////////////////////////////////////////////////////////////
 
-#include <gsl/assert> // GSL_KERNEL_MODE
-#include <gsl/util>   // for narrow_cast, narrow
+#include "./assert" // GSL_KERNEL_MODE
+#include "./util"   // for narrow_cast, narrow
 
 #include <cstddef> // for ptrdiff_t, size_t
 #include <utility>
@@ -41,7 +41,7 @@ namespace gsl
 {
 
 // [span.views.constants], constants
-constexpr const std::size_t dynamic_extent = narrow_cast<std::size_t>(-1);
+GSL_INLINE constexpr const std::size_t dynamic_extent = narrow_cast<std::size_t>(-1);
 
 template <class ElementType, std::size_t Extent = dynamic_extent>
 class span;
@@ -123,12 +123,14 @@ constexpr span<const typename Container::value_type> make_span(const Container&
 }
 
 template <class Ptr>
+GSL_DEPRECATED("This function is deprecated. See GSL issue #1092.")
 constexpr span<typename Ptr::element_type> make_span(Ptr& cont, std::size_t count)
 {
     return span<typename Ptr::element_type>(cont, count);
 }
 
 template <class Ptr>
+GSL_DEPRECATED("This function is deprecated. See GSL issue #1092.")
 constexpr span<typename Ptr::element_type> make_span(Ptr& cont)
 {
     return span<typename Ptr::element_type>(cont);

include/gsl/string_span

@@ -1,759 +0,0 @@
-///////////////////////////////////////////////////////////////////////////////
-//
-// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
-//
-// This code is licensed under the MIT License (MIT).
-//
-// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
-// THE SOFTWARE.
-//
-///////////////////////////////////////////////////////////////////////////////
-
-#ifndef GSL_STRING_SPAN_H
-#define GSL_STRING_SPAN_H
-
-#include <gsl/assert>   // for Ensures, Expects
-#include <gsl/span_ext> // for operator!=, operator==, dynamic_extent
-#include <gsl/util>     // for narrow_cast
-
-#include <algorithm> // for equal, lexicographical_compare
-#include <array>     // for array
-#include <cstddef>   // for size_t, nullptr_t
-#include <cstdint>   // for PTRDIFF_MAX
-#include <cstring>
-#include <string>      // for basic_string, allocator, char_traits
-#include <type_traits> // for declval, is_convertible, enable_if_t, add_...
-
-#if defined(_MSC_VER) && !defined(__clang__)
-#pragma warning(push)
-
-// Turn MSVC /analyze rules that generate too much noise. TODO: fix in the tool.
-#pragma warning(disable : 26446) // TODO: bug in parser - attributes and templates
-#pragma warning(disable : 26481) // TODO: suppress does not work inside templates sometimes
-#pragma warning(disable : 4996)  // use of functions & classes marked [[deprecated]]
-#endif                           // _MSC_VER
-
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
-#endif
-
-namespace gsl
-{
-//
-// czstring and wzstring
-//
-// These are "tag" typedefs for C-style strings (i.e. null-terminated character arrays)
-// that allow static analysis to help find bugs.
-//
-// There are no additional features/semantics that we can find a way to add inside the
-// type system for these types that will not either incur significant runtime costs or
-// (sometimes needlessly) break existing programs when introduced.
-//
-
-template <typename CharT, std::size_t Extent = dynamic_extent>
-using basic_zstring = CharT*;
-
-using czstring = basic_zstring<const char, dynamic_extent>;
-
-using cwzstring = basic_zstring<const wchar_t, dynamic_extent>;
-
-using cu16zstring = basic_zstring<const char16_t, dynamic_extent>;
-
-using cu32zstring = basic_zstring<const char32_t, dynamic_extent>;
-
-using zstring = basic_zstring<char, dynamic_extent>;
-
-using wzstring = basic_zstring<wchar_t, dynamic_extent>;
-
-using u16zstring = basic_zstring<char16_t, dynamic_extent>;
-
-using u32zstring = basic_zstring<char32_t, dynamic_extent>;
-
-namespace details
-{
-    template <class CharT>
-    [[deprecated("string_span was removed from the C++ Core Guidelines. For more information, see "
-                 "isocpp/CppCoreGuidelines PR#1680")]] constexpr std::size_t
-    string_length(const CharT* str, std::size_t n)
-    {
-        if (str == nullptr || n == dynamic_extent) return 0;
-
-        const span<const CharT> str_span{str, n};
-
-        std::size_t len = 0;
-        while (len < n && str_span[len]) len++;
-
-        return len;
-    }
-} // namespace details
-
-//
-// ensure_sentinel()
-//
-// Provides a way to obtain an span from a contiguous sequence
-// that ends with a (non-inclusive) sentinel value.
-//
-// Will fail-fast if sentinel cannot be found before max elements are examined.
-//
-template <typename T, const T Sentinel>
-[[deprecated("string_span was removed from the C++ Core Guidelines. For more information, see "
-             "isocpp/CppCoreGuidelines PR#1680")]] constexpr span<T, dynamic_extent>
-ensure_sentinel(T* seq, std::size_t max = static_cast<std::size_t>(-1))
-{
-    Ensures(seq != nullptr);
-
-    // clang-format off
-    GSL_SUPPRESS(f.23) // TODO: false positive // TODO: suppress does not work
-    // clang-format on
-    auto cur = seq;
-    Ensures(cur != nullptr); // workaround for removing the warning
-
-    // clang-format off
-    GSL_SUPPRESS(bounds.1) // TODO: suppress does not work
-    // clang-format on
-    while (static_cast<std::size_t>(cur - seq) < max && *cur != Sentinel) ++cur;
-    Ensures(*cur == Sentinel);
-    return {seq, static_cast<std::size_t>(cur - seq)};
-}
-
-//
-// ensure_z - creates a span for a zero terminated strings. The span will not contain the zero
-// termination. Will fail fast if a null-terminator cannot be found before the limit of size_type.
-//
-template <typename CharT>
-[[deprecated("string_span was removed from the C++ Core Guidelines. For more information, see "
-             "isocpp/CppCoreGuidelines PR#1680")]] constexpr span<CharT, dynamic_extent>
-ensure_z(CharT* const& sz, std::size_t max = static_cast<std::size_t>(-1))
-{
-    return ensure_sentinel<CharT, CharT(0)>(sz, max);
-}
-
-template <typename CharT, std::size_t N>
-constexpr span<CharT, dynamic_extent> ensure_z(CharT (&sz)[N])
-{
-    return ensure_z(&sz[0], N);
-}
-
-template <class Cont>
-[[deprecated(
-    "string_span was removed from the C++ Core Guidelines. For more information, see "
-    "isocpp/CppCoreGuidelines PR#1680")]] constexpr span<typename std::
-                                                             remove_pointer<
-                                                                 typename Cont::pointer>::type,
-                                                         dynamic_extent>
-ensure_z(Cont& cont)
-{
-    return ensure_z(cont.data(), cont.size());
-}
-
-template <typename CharT, std::size_t>
-class [[deprecated("string_span was removed from the C++ Core Guidelines. For more information, "
-                   "see isocpp/CppCoreGuidelines PR#1680")]] basic_string_span;
-
-namespace details
-{
-    template <typename T>
-    struct [[deprecated(
-        "string_span was removed from the C++ Core Guidelines. For more information, "
-        "see isocpp/CppCoreGuidelines PR#1680")]] is_basic_string_span_oracle : std::false_type{};
-
-    template <typename CharT, std::size_t Extent>
-    struct [[deprecated(
-        "string_span was removed from the C++ Core Guidelines. For more information, see "
-        "isocpp/CppCoreGuidelines PR#1680")]] is_basic_string_span_oracle<basic_string_span<CharT,
-                                                                                            Extent>>
-        : std::true_type{};
-
-    template <typename T>
-    struct [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                        "information, see isocpp/CppCoreGuidelines PR#1680")]] is_basic_string_span
-        : is_basic_string_span_oracle<std::remove_cv_t<T>>{};
-} // namespace details
-
-//
-// string_span and relatives
-//
-template <typename CharT, std::size_t Extent = dynamic_extent>
-class [[deprecated("string_span was removed from the C++ Core Guidelines. For more information, "
-                   "see isocpp/CppCoreGuidelines PR#1680")]] basic_string_span
-{
-public:
-    using element_type = CharT;
-    using value_type = std::remove_cv_t<element_type>;
-    using pointer = std::add_pointer_t<element_type>;
-    using reference = std::add_lvalue_reference_t<element_type>;
-    using const_reference = std::add_lvalue_reference_t<std::add_const_t<element_type>>;
-    using impl_type = span<element_type, Extent>;
-
-    using size_type = typename impl_type::size_type;
-    using iterator = typename impl_type::iterator;
-    using reverse_iterator = typename impl_type::reverse_iterator;
-
-    // default (empty)
-    constexpr basic_string_span() noexcept = default;
-
-    // copy
-    constexpr basic_string_span(const basic_string_span& other) noexcept = default;
-
-    // assign
-    constexpr basic_string_span& operator=(const basic_string_span& other) noexcept = default;
-
-    constexpr basic_string_span(pointer ptr, size_type length) : span_(ptr, length) {}
-    constexpr basic_string_span(pointer firstElem, pointer lastElem) : span_(firstElem, lastElem) {}
-
-    // From static arrays - if 0-terminated, remove 0 from the view
-    // All other containers allow 0s within the length, so we do not remove them
-    template <std::size_t N>
-    constexpr basic_string_span(element_type(&arr)[N]) : span_(remove_z(arr))
-    {}
-
-    template <std::size_t N, class ArrayElementType = std::remove_const_t<element_type>>
-    constexpr basic_string_span(std::array<ArrayElementType, N> & arr) noexcept : span_(arr)
-    {}
-
-    template <std::size_t N, class ArrayElementType = std::remove_const_t<element_type>>
-    constexpr basic_string_span(const std::array<ArrayElementType, N>& arr) noexcept : span_(arr)
-    {}
-
-    // Container signature should work for basic_string after C++17 version exists
-    template <class Traits, class Allocator>
-    // GSL_SUPPRESS(bounds.4) // TODO: parser bug
-    constexpr basic_string_span(std::basic_string<element_type, Traits, Allocator> & str)
-        : span_(&str[0], str.length())
-    {}
-
-    template <class Traits, class Allocator>
-    constexpr basic_string_span(const std::basic_string<element_type, Traits, Allocator>& str)
-        : span_(&str[0], str.length())
-    {}
-
-    // from containers. Containers must have a pointer type and data() function signatures
-    template <class Container,
-              class = std::enable_if_t<
-                  !details::is_basic_string_span<Container>::value &&
-                  std::is_convertible<typename Container::pointer, pointer>::value &&
-                  std::is_convertible<typename Container::pointer,
-                                      decltype(std::declval<Container>().data())>::value>>
-    constexpr basic_string_span(Container & cont) : span_(cont)
-    {}
-
-    template <class Container,
-              class = std::enable_if_t<
-                  !details::is_basic_string_span<Container>::value &&
-                  std::is_convertible<typename Container::pointer, pointer>::value &&
-                  std::is_convertible<typename Container::pointer,
-                                      decltype(std::declval<Container>().data())>::value>>
-    constexpr basic_string_span(const Container& cont) : span_(cont)
-    {}
-
-    // from string_span
-    template <
-        class OtherValueType, std::size_t OtherExtent,
-        class = std::enable_if_t<std::is_convertible<
-            typename basic_string_span<OtherValueType, OtherExtent>::impl_type, impl_type>::value>>
-    constexpr basic_string_span(basic_string_span<OtherValueType, OtherExtent> other)
-        : span_(other.data(), other.length())
-    {}
-
-    template <size_type Count>
-    constexpr basic_string_span<element_type, Count> first() const
-    {
-        return {span_.template first<Count>()};
-    }
-
-    constexpr basic_string_span<element_type, dynamic_extent> first(size_type count) const
-    {
-        return {span_.first(count)};
-    }
-
-    template <size_type Count>
-    constexpr basic_string_span<element_type, Count> last() const
-    {
-        return {span_.template last<Count>()};
-    }
-
-    constexpr basic_string_span<element_type, dynamic_extent> last(size_type count) const
-    {
-        return {span_.last(count)};
-    }
-
-    template <size_type Offset, size_type Count>
-    constexpr basic_string_span<element_type, Count> subspan() const
-    {
-        return {span_.template subspan<Offset, Count>()};
-    }
-
-    constexpr basic_string_span<element_type, dynamic_extent> subspan(
-        size_type offset, size_type count = dynamic_extent) const
-    {
-        return {span_.subspan(offset, count)};
-    }
-
-    constexpr reference operator[](size_type idx) const { return span_[idx]; }
-    constexpr reference operator()(size_type idx) const { return span_[idx]; }
-
-    constexpr pointer data() const { return span_.data(); }
-
-    constexpr size_type length() const noexcept { return span_.size(); }
-    constexpr size_type size() const noexcept { return span_.size(); }
-    constexpr size_type size_bytes() const noexcept { return span_.size_bytes(); }
-    constexpr size_type length_bytes() const noexcept { return span_.length_bytes(); }
-    constexpr bool empty() const noexcept { return size() == 0; }
-
-    constexpr iterator begin() const noexcept { return span_.begin(); }
-    constexpr iterator end() const noexcept { return span_.end(); }
-
-    constexpr reverse_iterator rbegin() const noexcept { return span_.rbegin(); }
-    constexpr reverse_iterator rend() const noexcept { return span_.rend(); }
-
-private:
-    static constexpr impl_type remove_z(pointer const& sz, std::size_t max)
-    {
-        return impl_type(sz, details::string_length(sz, max));
-    }
-
-    template <std::size_t N>
-    static constexpr impl_type remove_z(element_type(&sz)[N])
-    {
-        return remove_z(&sz[0], N);
-    }
-
-    impl_type span_;
-};
-
-template <std::size_t Extent = dynamic_extent>
-using string_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                               "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_string_span<char, Extent>;
-
-template <std::size_t Extent = dynamic_extent>
-using cstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_string_span<const char, Extent>;
-
-template <std::size_t Extent = dynamic_extent>
-using wstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_string_span<wchar_t, Extent>;
-
-template <std::size_t Extent = dynamic_extent>
-using cwstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                 "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_string_span<const wchar_t, Extent>;
-
-template <std::size_t Extent = dynamic_extent>
-using u16string_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                  "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_string_span<char16_t, Extent>;
-
-template <std::size_t Extent = dynamic_extent>
-using cu16string_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                   "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_string_span<const char16_t, Extent>;
-
-template <std::size_t Extent = dynamic_extent>
-using u32string_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                  "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_string_span<char32_t, Extent>;
-
-template <std::size_t Extent = dynamic_extent>
-using cu32string_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                   "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_string_span<const char32_t, Extent>;
-
-//
-// to_string() allow (explicit) conversions from string_span to string
-//
-
-template <typename CharT, std::size_t Extent>
-constexpr std::basic_string<typename std::remove_const<CharT>::type>
-to_string(basic_string_span<CharT, Extent> view)
-{
-    return {view.data(), narrow_cast<std::size_t>(view.length())};
-}
-
-template <typename CharT, typename Traits = typename std::char_traits<CharT>,
-          typename Allocator = std::allocator<CharT>, typename gCharT, std::size_t Extent>
-constexpr std::basic_string<CharT, Traits, Allocator>
-to_basic_string(basic_string_span<gCharT, Extent> view)
-{
-    return {view.data(), narrow_cast<std::size_t>(view.length())};
-}
-
-template <class ElementType, std::size_t Extent>
-constexpr basic_string_span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
-as_bytes(basic_string_span<ElementType, Extent> s) noexcept
-{
-    // clang-format off
-    GSL_SUPPRESS(type.1)
-    // clang-format on
-    return {reinterpret_cast<const byte*>(s.data()), s.size_bytes()};
-}
-
-template <class ElementType, std::size_t Extent,
-          class = std::enable_if_t<!std::is_const<ElementType>::value>>
-constexpr basic_string_span<byte, details::calculate_byte_size<ElementType, Extent>::value>
-as_writable_bytes(basic_string_span<ElementType, Extent> s) noexcept
-{
-    // clang-format off
-    GSL_SUPPRESS(type.1)
-    // clang-format on
-    return {reinterpret_cast<byte*>(s.data()), s.size_bytes()};
-}
-
-// zero-terminated string span, used to convert
-// zero-terminated spans to legacy strings
-template <typename CharT, std::size_t Extent = dynamic_extent>
-class [[deprecated("string_span was removed from the C++ Core Guidelines. For more information, "
-                   "see isocpp/CppCoreGuidelines PR#1680")]] basic_zstring_span
-{
-public:
-    using value_type = CharT;
-    using const_value_type = std::add_const_t<CharT>;
-
-    using pointer = std::add_pointer_t<value_type>;
-    using const_pointer = std::add_pointer_t<const_value_type>;
-
-    using zstring_type = basic_zstring<value_type, Extent>;
-    using const_zstring_type = basic_zstring<const_value_type, Extent>;
-
-    using impl_type = span<value_type, Extent>;
-    using string_span_type = basic_string_span<value_type, Extent>;
-
-    constexpr basic_zstring_span(impl_type s) : span_(s)
-    {
-        // expects a zero-terminated span
-        Expects(s.size() > 0);
-        Expects(s[s.size() - 1] == value_type{});
-    }
-
-    // copy
-    constexpr basic_zstring_span(const basic_zstring_span& other) = default;
-
-    // move
-    constexpr basic_zstring_span(basic_zstring_span && other) = default;
-
-    // assign
-    constexpr basic_zstring_span& operator=(const basic_zstring_span& other) = default;
-
-    // move assign
-    constexpr basic_zstring_span& operator=(basic_zstring_span&& other) = default;
-
-    constexpr bool empty() const noexcept { return false; }
-
-    constexpr string_span_type as_string_span() const noexcept
-    {
-        return {span_.data(), span_.size() - 1};
-    }
-    constexpr string_span_type ensure_z() const { return gsl::ensure_z(span_); }
-
-    constexpr const_zstring_type assume_z() const noexcept { return span_.data(); }
-
-private:
-    impl_type span_;
-};
-
-template <std::size_t Max = dynamic_extent>
-using zstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_zstring_span<char, Max>;
-
-template <std::size_t Max = dynamic_extent>
-using wzstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                 "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_zstring_span<wchar_t, Max>;
-
-template <std::size_t Max = dynamic_extent>
-using u16zstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                   "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_zstring_span<char16_t, Max>;
-
-template <std::size_t Max = dynamic_extent>
-using u32zstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                   "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_zstring_span<char32_t, Max>;
-
-template <std::size_t Max = dynamic_extent>
-using czstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                 "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_zstring_span<const char, Max>;
-
-template <std::size_t Max = dynamic_extent>
-using cwzstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For more "
-                                  "information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_zstring_span<const wchar_t, Max>;
-
-template <std::size_t Max = dynamic_extent>
-using cu16zstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For "
-                                    "more information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_zstring_span<const char16_t, Max>;
-
-template <std::size_t Max = dynamic_extent>
-using cu32zstring_span [[deprecated("string_span was removed from the C++ Core Guidelines. For "
-                                    "more information, see isocpp/CppCoreGuidelines PR#1680")]] =
-    basic_zstring_span<const char32_t, Max>;
-
-// operator ==
-template <class CharT, std::size_t Extent, class T,
-          class = std::enable_if_t<
-              details::is_basic_string_span<T>::value ||
-              std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>>>::value>>
-bool operator==(const gsl::basic_string_span<CharT, Extent>& one, const T& other)
-{
-    const gsl::basic_string_span<std::add_const_t<CharT>> tmp(other);
-    return std::equal(one.begin(), one.end(), tmp.begin(), tmp.end());
-}
-
-template <class CharT, std::size_t Extent, class T,
-          class = std::enable_if_t<
-              !details::is_basic_string_span<T>::value &&
-              std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>>>::value>>
-bool operator==(const T& one, const gsl::basic_string_span<CharT, Extent>& other)
-{
-    const gsl::basic_string_span<std::add_const_t<CharT>> tmp(one);
-    return std::equal(tmp.begin(), tmp.end(), other.begin(), other.end());
-}
-
-// operator !=
-template <typename CharT, std::size_t Extent = dynamic_extent, typename T,
-          typename = std::enable_if_t<std::is_convertible<
-              T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
-bool operator!=(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    return !(one == other);
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename = std::enable_if_t<
-        std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
-        !gsl::details::is_basic_string_span<T>::value>>
-bool operator!=(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    return !(one == other);
-}
-
-// operator<
-template <typename CharT, std::size_t Extent = dynamic_extent, typename T,
-          typename = std::enable_if_t<std::is_convertible<
-              T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
-bool operator<(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    const gsl::basic_string_span<std::add_const_t<CharT>, Extent> tmp(other);
-    return std::lexicographical_compare(one.begin(), one.end(), tmp.begin(), tmp.end());
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename = std::enable_if_t<
-        std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
-        !gsl::details::is_basic_string_span<T>::value>>
-bool operator<(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    gsl::basic_string_span<std::add_const_t<CharT>, Extent> tmp(one);
-    return std::lexicographical_compare(tmp.begin(), tmp.end(), other.begin(), other.end());
-}
-
-#ifndef _MSC_VER
-
-// VS treats temp and const containers as convertible to basic_string_span,
-// so the cases below are already covered by the previous operators
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator<(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    gsl::basic_string_span<std::add_const_t<CharT>, Extent> tmp(other);
-    return std::lexicographical_compare(one.begin(), one.end(), tmp.begin(), tmp.end());
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator<(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    gsl::basic_string_span<std::add_const_t<CharT>, Extent> tmp(one);
-    return std::lexicographical_compare(tmp.begin(), tmp.end(), other.begin(), other.end());
-}
-#endif
-
-// operator <=
-template <typename CharT, std::size_t Extent = dynamic_extent, typename T,
-          typename = std::enable_if_t<std::is_convertible<
-              T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
-bool operator<=(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    return !(other < one);
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename = std::enable_if_t<
-        std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
-        !gsl::details::is_basic_string_span<T>::value>>
-bool operator<=(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    return !(other < one);
-}
-
-#ifndef _MSC_VER
-
-// VS treats temp and const containers as convertible to basic_string_span,
-// so the cases below are already covered by the previous operators
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator<=(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    return !(other < one);
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator<=(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    return !(other < one);
-}
-#endif
-
-// operator>
-template <typename CharT, std::size_t Extent = dynamic_extent, typename T,
-          typename = std::enable_if_t<std::is_convertible<
-              T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
-bool operator>(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    return other < one;
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename = std::enable_if_t<
-        std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
-        !gsl::details::is_basic_string_span<T>::value>>
-bool operator>(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    return other < one;
-}
-
-#ifndef _MSC_VER
-
-// VS treats temp and const containers as convertible to basic_string_span,
-// so the cases below are already covered by the previous operators
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator>(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    return other < one;
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator>(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    return other < one;
-}
-#endif
-
-// operator >=
-template <typename CharT, std::size_t Extent = dynamic_extent, typename T,
-          typename = std::enable_if_t<std::is_convertible<
-              T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value>>
-bool operator>=(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    return !(one < other);
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename = std::enable_if_t<
-        std::is_convertible<T, gsl::basic_string_span<std::add_const_t<CharT>, Extent>>::value &&
-        !gsl::details::is_basic_string_span<T>::value>>
-bool operator>=(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    return !(one < other);
-}
-
-#ifndef _MSC_VER
-
-// VS treats temp and const containers as convertible to basic_string_span,
-// so the cases below are already covered by the previous operators
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator>=(gsl::basic_string_span<CharT, Extent> one, const T& other)
-{
-    return !(one < other);
-}
-
-template <
-    typename CharT, std::size_t Extent = dynamic_extent, typename T,
-    typename DataType = typename T::value_type,
-    typename = std::enable_if_t<
-        !gsl::details::is_span<T>::value && !gsl::details::is_basic_string_span<T>::value &&
-        std::is_convertible<DataType*, CharT*>::value &&
-        std::is_same<std::decay_t<decltype(std::declval<T>().size(), *std::declval<T>().data())>,
-                     DataType>::value>>
-bool operator>=(const T& one, gsl::basic_string_span<CharT, Extent> other)
-{
-    return !(one < other);
-}
-#endif
-} // namespace gsl
-
-#if defined(_MSC_VER) && !defined(__clang__)
-#pragma warning(pop)
-
-#endif // _MSC_VER
-
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic pop
-#endif
-#endif // GSL_STRING_SPAN_H

include/gsl/util

@@ -17,10 +17,11 @@
 #ifndef GSL_UTIL_H
 #define GSL_UTIL_H
 
-#include <gsl/assert> // for Expects
+#include "./assert" // for Expects
 
 #include <array>
 #include <cstddef>          // for ptrdiff_t, size_t
+#include <limits>           // for numeric_limits
 #include <initializer_list> // for initializer_list
 #include <type_traits>      // for is_signed, integral_constant
 #include <utility>          // for exchange, forward
@@ -39,12 +40,52 @@
 
 #endif // _MSC_VER
 
+// Turn off clang unsafe buffer warnings as all accessed are guarded by runtime checks
+#if defined(__clang__)
+#if __has_warning("-Wunsafe-buffer-usage")
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunsafe-buffer-usage"
+#endif // __has_warning("-Wunsafe-buffer-usage")
+#endif // defined(__clang__)
+
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
 #define GSL_NODISCARD [[nodiscard]]
 #else
 #define GSL_NODISCARD
 #endif // defined(__cplusplus) && (__cplusplus >= 201703L)
 
+#if defined(__cpp_inline_variables)
+#define GSL_INLINE inline
+#else
+#define GSL_INLINE
+#endif
+
+#if defined(__has_cpp_attribute)
+#if __has_cpp_attribute(deprecated)
+#define GSL_DEPRECATED(msg) [[deprecated(msg)]]
+#endif // __has_cpp_attribute(deprecated)
+#endif // defined(__has_cpp_attribute)
+
+#if !defined(GSL_DEPRECATED)
+#if defined(__cplusplus)
+#if __cplusplus >= 201309L
+#define GSL_DEPRECATED(msg) [[deprecated(msg)]]
+#endif // __cplusplus >= 201309L
+#endif // defined(__cplusplus)
+#endif // !defined(GSL_DEPRECATED)
+
+#if !defined(GSL_DEPRECATED)
+#if defined(_MSC_VER)
+#define GSL_DEPRECATED(msg) __declspec(deprecated(msg))
+#elif defined(__GNUC__)
+#define GSL_DEPRECATED(msg) __attribute__((deprecated(msg)))
+#endif // defined(_MSC_VER)
+#endif // !defined(GSL_DEPRECATED)
+
+#if !defined(GSL_DEPRECATED)
+#define GSL_DEPRECATED(msg)
+#endif // !defined(GSL_DEPRECATED)
+
 namespace gsl
 {
 //
@@ -59,40 +100,29 @@ template <class F>
 class final_action
 {
 public:
-    static_assert(!std::is_reference<F>::value && !std::is_const<F>::value &&
-                      !std::is_volatile<F>::value,
-                  "Final_action should store its callable by value");
+    explicit final_action(const F& ff) noexcept : f{ff} { }
+    explicit final_action(F&& ff) noexcept : f{std::move(ff)} { }
 
-    explicit final_action(F f) noexcept : f_(std::move(f)) {}
+    ~final_action() noexcept { if (invoke) f(); }
 
     final_action(final_action&& other) noexcept
-        : f_(std::move(other.f_)), invoke_(std::exchange(other.invoke_, false))
-    {}
+        : f(std::move(other.f)), invoke(std::exchange(other.invoke, false))
+    { }
 
-    final_action(const final_action&) = delete;
-    final_action& operator=(const final_action&) = delete;
-    final_action& operator=(final_action&&) = delete;
-
-    // clang-format off
-    GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // terminate if throws
-    // clang-format on
-    ~final_action() noexcept
-    {
-        if (invoke_) f_();
-    }
+    final_action(const final_action&)   = delete;
+    void operator=(const final_action&) = delete;
+    void operator=(final_action&&)      = delete;
 
 private:
-    F f_;
-    bool invoke_{true};
+    F f;
+    bool invoke = true;
 };
 
 // finally() - convenience function to generate a final_action
 template <class F>
-GSL_NODISCARD final_action<typename std::remove_cv<typename std::remove_reference<F>::type>::type>
-finally(F&& f) noexcept
+GSL_NODISCARD auto finally(F&& f) noexcept
 {
-    return final_action<typename std::remove_cv<typename std::remove_reference<F>::type>::type>(
-        std::forward<F>(f));
+    return final_action<std::decay_t<F>>{std::forward<F>(f)};
 }
 
 // narrow_cast(): a searchable way to do narrowing casts of values
@@ -115,6 +145,7 @@ GSL_SUPPRESS(bounds.2) // NO-FORMAT: attribute
     // clang-format on
     constexpr T& at(T (&arr)[N], const index i)
 {
+    static_assert(N <= static_cast<std::size_t>((std::numeric_limits<std::ptrdiff_t>::max)()), "We only support arrays up to PTRDIFF_MAX bytes.");
     Expects(i >= 0 && i < narrow_cast<index>(N));
     return arr[narrow_cast<std::size_t>(i)];
 }
@@ -141,12 +172,15 @@ GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
     return *(cont.begin() + i);
 }
 
+template <class T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value>>
+void swap(T& a, T& b) { std::swap(a, b); }
+
 #if defined(__cpp_lib_span) && __cpp_lib_span >= 202002L
-template <class T, size_t extent = std::dynamic_extent>
+template <class T, std::size_t extent = std::dynamic_extent>
 constexpr auto at(std::span<T, extent> sp, const index i) -> decltype(sp[sp.size()])
 {
     Expects(i >= 0 && i < narrow_cast<index>(sp.size()));
-    return sp[gsl::narrow_cast<size_t>(i)];
+    return sp[gsl::narrow_cast<std::size_t>(i)];
 }
 #endif // __cpp_lib_span >= 202002L
 } // namespace gsl
@@ -157,4 +191,10 @@ constexpr auto at(std::span<T, extent> sp, const index i) -> decltype(sp[sp.size
 
 #endif // _MSC_VER
 
+#if defined(__clang__)
+#if __has_warning("-Wunsafe-buffer-usage")
+#pragma clang diagnostic pop
+#endif // __has_warning("-Wunsafe-buffer-usage")
+#endif // defined(__clang__)
+
 #endif // GSL_UTIL_H

include/gsl/zstring

@@ -0,0 +1,58 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef GSL_ZSTRING_H
+#define GSL_ZSTRING_H
+
+#include "./span_ext" // for dynamic_extent
+
+#include <cstddef>   // for size_t, nullptr_t
+
+namespace gsl
+{
+//
+// czstring and wzstring
+//
+// These are "tag" typedefs for C-style strings (i.e. null-terminated character arrays)
+// that allow static analysis to help find bugs.
+//
+// There are no additional features/semantics that we can find a way to add inside the
+// type system for these types that will not either incur significant runtime costs or
+// (sometimes needlessly) break existing programs when introduced.
+//
+
+template <typename CharT, std::size_t Extent = dynamic_extent>
+using basic_zstring = CharT*;
+
+using czstring = basic_zstring<const char, dynamic_extent>;
+
+using cwzstring = basic_zstring<const wchar_t, dynamic_extent>;
+
+using cu16zstring = basic_zstring<const char16_t, dynamic_extent>;
+
+using cu32zstring = basic_zstring<const char32_t, dynamic_extent>;
+
+using zstring = basic_zstring<char, dynamic_extent>;
+
+using wzstring = basic_zstring<wchar_t, dynamic_extent>;
+
+using u16zstring = basic_zstring<char16_t, dynamic_extent>;
+
+using u32zstring = basic_zstring<char32_t, dynamic_extent>;
+
+} // namespace gsl
+
+#endif // GSL_ZSTRING_H

pipelines/jobs.yml

@@ -1,43 +0,0 @@
-parameters:
-  CXXVersions: [ 14, 17, 20 ]
-  buildTypes: [ 'Debug', 'Release' ]
-  image: ''
-
-  compiler: ''
-  compilerVersions: ["default"] # if default value, simply uses whatever version is on the machine.
-                                # the text of this default value doesn't actually matter.
-  setupfile: ''
-  extraCmakeArgs: ''
-
-jobs:
-- ${{ each compilerVersion in parameters.compilerVersions }}:
-  - ${{ each CXXVersion in parameters.CXXVersions }}:
-    - ${{ each buildType in parameters.buildTypes }}:
-      - job:
-        displayName: ${{ format('{0} {1} C++{2} {3}', parameters.compiler, compilerVersion, CXXVersion, buildType) }}
-        pool:
-            vmImage: ${{ parameters.image }}
-        continueOnError: false
-
-        steps:
-          - ${{ if not(eq(parameters.setupfile, '')) }}:
-            - template: ${{ parameters.setupfile }}
-              parameters:
-                version: ${{ compilerVersion }}
-
-          - task: CMake@1
-            name: Configure
-            inputs:
-              workingDirectory: build
-              cmakeArgs: '-DGSL_CXX_STANDARD=${{ CXXVersion }} -DCMAKE_BUILD_TYPE=${{ buildType }} -DCI_TESTING:BOOL=ON -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -Werror=dev ${{ parameters.extraCmakeArgs }} .. '
-
-          - task: CMake@1
-            name: Build
-            inputs:
-              workingDirectory: build
-              cmakeArgs: '--build . '
-
-          - script: ctest . --output-on-failure --no-compress-output
-            name: CTest
-            workingDirectory: build
-            failOnStderr: true

pipelines/setup_apple.yml

@@ -1,9 +0,0 @@
-parameters:
-    version: 0
-
-steps:
-  - script: |
-      if [ "${{ parameters.version }}" != "default" ]; then sudo xcode-select -switch /Applications/Xcode_${{ parameters.version }}.app; fi
-
-    displayName: "Setup Xcode Version"
-    failOnStderr: true

pipelines/setup_clang.yml

@@ -1,13 +0,0 @@
-parameters:
-    version: 0
-
-steps:
-  - script: |
-      echo "##vso[task.setvariable variable=CXX;]${CXX}"
-      echo "##vso[task.setvariable variable=CC;]${CC}"
-
-    displayName: "Setup Clang Version"
-    failOnStderr: true
-    env:
-      CC: clang-${{ parameters.version }}
-      CXX: clang++-${{ parameters.version }}

pipelines/setup_gcc.yml

@@ -1,14 +0,0 @@
-parameters:
-    version: 0
-
-steps:
-  - script: |
-      echo "##vso[task.setvariable variable=CXX;]${CXX}"
-      echo "##vso[task.setvariable variable=CC;]${CC}"
-      if [ "${{ parameters.version }}" = "11" ]; then sudo apt-get install $CXX; fi
-
-    displayName: "Setup GCC Version"
-    failOnStderr: true
-    env:
-      CC: gcc-${{ parameters.version }}
-      CXX: g++-${{ parameters.version }}

tests/CMakeLists.txt

@@ -1,7 +1,12 @@
-cmake_minimum_required(VERSION 3.0.2)
+cmake_minimum_required(VERSION 3.14...3.16)
 
-project(GSLTests CXX)
-enable_testing()  # again, for support standalone testing
+project(GSLTests LANGUAGES CXX)
+
+set(GSL_CXX_STANDARD "14" CACHE STRING "Use c++ standard")
+
+set(CMAKE_CXX_STANDARD ${GSL_CXX_STANDARD})
+set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 include(FindPkgConfig)
 include(ExternalProject)
@@ -52,16 +57,21 @@ if (NOT GTestMain_FOUND)
 endif()
 
 if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
-    # CMake has been started independently in this directory with tests. Do
-    # import the globally installed Guidelines Support Library and test it
-    # instead of the current version from the include/ folder.
-    find_package(Microsoft.GSL REQUIRED)
+    find_package(Microsoft.GSL CONFIG REQUIRED)
+    enable_testing()
+
+    if (NOT DEFINED Microsoft.GSL_VERSION)
+        message(FATAL_ERROR "Microsoft.GSL_VERSION not defined!")
+    endif()
+
+    message(STATUS "Microsoft.GSL_VERSION = ${Microsoft.GSL_VERSION}")
 endif()
 
 if (MSVC AND (GSL_CXX_STANDARD GREATER_EQUAL 17))
     set(GSL_CPLUSPLUS_OPT -Zc:__cplusplus -permissive-)
 endif()
 
+include(CheckCXXCompilerFlag)
 # this interface adds compile options to how the tests are run
 # please try to keep entries ordered =)
 add_library(gsl_tests_config INTERFACE)
@@ -88,6 +98,7 @@ if(MSVC) # MSVC or simulating MSVC
         >
         $<$<CXX_COMPILER_ID:Clang>:
           -Weverything
+          -Wfloat-equal
           -Wno-c++98-compat
           -Wno-c++98-compat-pedantic
           -Wno-covered-switch-default # GTest
@@ -99,12 +110,20 @@ if(MSVC) # MSVC or simulating MSVC
           -Wno-shift-sign-overflow # GTest gtest-port.h
           -Wno-undef # GTest
           -Wno-used-but-marked-unused # GTest EXPECT_DEATH
+          -Wno-switch-default # GTest EXPECT_DEATH
           $<$<EQUAL:${GSL_CXX_STANDARD},14>: # no support for [[maybe_unused]]
             -Wno-unused-member-function
             -Wno-unused-variable
+            $<$<VERSION_EQUAL:$<CXX_COMPILER_VERSION>,15.0.1>:
+              -Wno-deprecated # False positive in MSVC Clang 15.0.1 raises a C++17 warning
+            >
           >
         >
     )
+    check_cxx_compiler_flag("-Wno-reserved-identifier" WARN_RESERVED_ID)
+    if (WARN_RESERVED_ID)
+      target_compile_options(gsl_tests_config INTERFACE "-Wno-reserved-identifier")
+    endif()
 else()
     target_compile_options(gsl_tests_config INTERFACE
         -fno-strict-aliasing
@@ -117,6 +136,7 @@ else()
         -Wpedantic
         -Wshadow
         -Wsign-conversion
+        -Wfloat-equal
         -Wno-deprecated-declarations # Allow tests for [[deprecated]] elements
         $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
           -Weverything
@@ -127,6 +147,7 @@ else()
           -Wno-global-constructors # GTest
           -Wno-missing-prototypes
           -Wno-padded
+          -Wno-switch-default
           -Wno-unknown-attributes
           -Wno-used-but-marked-unused # GTest EXPECT_DEATH
           -Wno-weak-vtables
@@ -167,6 +188,11 @@ else()
         >
     )
 endif(MSVC)
+check_cxx_compiler_flag("-Wno-unsafe-buffer-usage" WARN_UNSAFE_BUFFER)
+if (WARN_UNSAFE_BUFFER)
+  # This test uses very greedy heuristics such as "no pointer arithmetic on raw buffer"
+  target_compile_options(gsl_tests_config INTERFACE "-Wno-unsafe-buffer-usage")
+endif()
 
 # for tests to find the gtest header
 target_include_directories(gsl_tests_config SYSTEM INTERFACE
@@ -180,11 +206,12 @@ add_executable(gsl_tests
     byte_tests.cpp
     notnull_tests.cpp
     owner_tests.cpp
+    pointers_tests.cpp
     span_compatibility_tests.cpp
     span_ext_tests.cpp
     span_tests.cpp
     strict_notnull_tests.cpp
-    string_span_tests.cpp
+    
     utils_tests.cpp
 )
 
@@ -222,12 +249,22 @@ if(MSVC) # MSVC or simulating MSVC
         >
         $<$<CXX_COMPILER_ID:Clang>:
           -Weverything
+          -Wfloat-equal
           -Wno-c++98-compat
           -Wno-c++98-compat-pedantic
           -Wno-missing-prototypes
           -Wno-unknown-attributes
+          $<$<EQUAL:${GSL_CXX_STANDARD},14>:
+            $<$<VERSION_EQUAL:$<CXX_COMPILER_VERSION>,15.0.1>: 
+              -Wno-deprecated # False positive in MSVC Clang 15.0.1 raises a C++17 warning
+            >
+          >
         >
     )
+    check_cxx_compiler_flag("-Wno-reserved-identifier" WARN_RESERVED_ID)
+    if (WARN_RESERVED_ID)
+      target_compile_options(gsl_tests_config_noexcept INTERFACE "-Wno-reserved-identifier")
+    endif()
 else()
     target_compile_options(gsl_tests_config_noexcept INTERFACE
         -fno-exceptions
@@ -241,6 +278,7 @@ else()
         -Wpedantic
         -Wshadow
         -Wsign-conversion
+        -Wfloat-equal
         $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
           -Weverything
           -Wno-c++98-compat
@@ -267,6 +305,11 @@ else()
         >
     )
 endif(MSVC)
+check_cxx_compiler_flag("-Wno-unsafe-buffer-usage" WARN_UNSAFE_BUFFER)
+if (WARN_UNSAFE_BUFFER)
+  # This test uses very greedy heuristics such as "no pointer arithmetic on raw buffer"
+  target_compile_options(gsl_tests_config_noexcept INTERFACE "-Wno-unsafe-buffer-usage")
+endif()
 
 add_executable(gsl_noexcept_tests no_exception_ensure_tests.cpp)
 target_link_libraries(gsl_noexcept_tests

tests/CMakeLists.txt.in

@@ -1,10 +1,10 @@
-cmake_minimum_required(VERSION 3.0.2)
+cmake_minimum_required(VERSION 3.13)
 project(googletest-download NONE)
 
 include(ExternalProject)
 ExternalProject_Add(googletest
   GIT_REPOSITORY    https://github.com/google/googletest.git
-  GIT_TAG           1b18723e874b256c1e39378c6774a90701d70f7a
+  GIT_TAG           v1.14.0
   SOURCE_DIR        "${CMAKE_CURRENT_BINARY_DIR}/googletest-src"
   BINARY_DIR        "${CMAKE_CURRENT_BINARY_DIR}/googletest-build"
   CONFIGURE_COMMAND ""

tests/algorithm_tests.cpp

@@ -188,7 +188,7 @@ TEST(algorithm_tests, incompatible_type)
     span<int> src_span_dyn(src);
     span<int, 4> src_span_static(src);
     span<int*> dst_span_dyn(dst);
-    span<int*, 4> dst_span_static(dst);
+    span<int*, 4> dst_span_static(gsl::make_span(dst));
 
     // every line should produce a compilation error
     copy(src_span_dyn, dst_span_dyn);

tests/assertion_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include "deathTestCommon.h"
-#include <gsl/assert> // for fail_fast (ptr only), Ensures, Expects
+#include <gsl/assert> // for Ensures, Expects
 #include <gtest/gtest.h>
 
 using namespace gsl;

tests/at_tests.cpp

@@ -20,6 +20,7 @@
 
 #include <array>            // for array
 #include <cstddef>          // for size_t
+#include <exception>        // for terminate
 #include <initializer_list> // for initializer_list
 #include <vector>           // for vector
 #if defined(__cplusplus) && __cplusplus >= 202002L

tests/byte_tests.cpp

@@ -16,8 +16,12 @@
 
 #include <gtest/gtest.h>
 
+#define GSL_USE_STD_BYTE 0
 #include <gsl/byte> // for to_byte, to_integer, byte, operator&, ope...
 
+#include <type_traits>
+#include <utility>
+
 using namespace std;
 using namespace gsl;
 
@@ -33,41 +37,46 @@ int modify_both(gsl::byte& b, int& i)
 TEST(byte_tests, construction)
 {
     {
-        const byte b = static_cast<byte>(4);
+        const gsl::byte b = static_cast<gsl::byte>(4);
         EXPECT_TRUE(static_cast<unsigned char>(b) == 4);
     }
 
-    // clang-format off
-    GSL_SUPPRESS(es.49)
-    // clang-format on
     {
-        const byte b = byte(12);
+        const gsl::byte b = gsl::byte(12);
         EXPECT_TRUE(static_cast<unsigned char>(b) == 12);
     }
 
     {
-        const byte b = to_byte<12>();
+        const gsl::byte b = to_byte<12>();
         EXPECT_TRUE(static_cast<unsigned char>(b) == 12);
     }
     {
         const unsigned char uc = 12;
-        const byte b = to_byte(uc);
+        const gsl::byte b = to_byte(uc);
         EXPECT_TRUE(static_cast<unsigned char>(b) == 12);
     }
 
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
     {
-        const byte b{14};
+        const gsl::byte b{14};
         EXPECT_TRUE(static_cast<unsigned char>(b) == 14);
     }
 #endif
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+    to_byte(char{});
+    to_byte(3);
+    to_byte(3u);
+    to_byte<-1>();
+    to_byte<256u>();
+#endif
 }
 
 TEST(byte_tests, bitwise_operations)
 {
-    const byte b = to_byte<0xFF>();
+    const gsl::byte b = to_byte<0xFF>();
 
-    byte a = to_byte<0x00>();
+    gsl::byte a = to_byte<0x00>();
     EXPECT_TRUE((b | a) == to_byte<0xFF>());
     EXPECT_TRUE(a == to_byte<0x00>());
 
@@ -101,7 +110,7 @@ TEST(byte_tests, bitwise_operations)
 
 TEST(byte_tests, to_integer)
 {
-    const byte b = to_byte<0x12>();
+    const gsl::byte b = to_byte<0x12>();
 
     EXPECT_TRUE(0x12 == gsl::to_integer<char>(b));
     EXPECT_TRUE(0x12 == gsl::to_integer<short>(b));
@@ -120,12 +129,50 @@ TEST(byte_tests, to_integer)
 TEST(byte_tests, aliasing)
 {
     int i{0};
-    const int res = modify_both(reinterpret_cast<byte&>(i), i);
+    const int res = modify_both(reinterpret_cast<gsl::byte&>(i), i);
     EXPECT_TRUE(res == i);
 }
 
-} // namespace
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-copy(src_span_static, dst_span_static);
-#endif
+template <typename U, typename = void>
+static constexpr bool LShiftCompilesFor = false;
+template <typename U>
+static constexpr bool LShiftCompilesFor<
+    U, void_t<decltype(gsl::operator<< <float>(declval<gsl::byte>(), declval<U>()))>> = true;
+static_assert(!LShiftCompilesFor<float>, "!LShiftCompilesFor<float>");
+
+template <typename U, typename = void>
+static constexpr bool RShiftCompilesFor = false;
+template <typename U>
+static constexpr bool RShiftCompilesFor<
+    U, void_t<decltype(gsl::operator>> <U>(declval<gsl::byte>(), declval<U>()))>> = true;
+static_assert(!RShiftCompilesFor<float>, "!RShiftCompilesFor<float>");
+
+template <typename U, typename = void>
+static constexpr bool LShiftAssignCompilesFor = false;
+template <typename U>
+static constexpr bool LShiftAssignCompilesFor<
+    U, void_t<decltype(gsl::operator<<= <U>(declval<gsl::byte&>(), declval<U>()))>> = true;
+static_assert(!LShiftAssignCompilesFor<float>, "!LShiftAssignCompilesFor<float>");
+
+template <typename U, typename = void>
+static constexpr bool RShiftAssignCompilesFor = false;
+template <typename U>
+static constexpr bool RShiftAssignCompilesFor<
+    U, void_t<decltype(gsl::operator>>= <U>(declval<gsl::byte&>(), declval<U>()))>> = true;
+static_assert(!RShiftAssignCompilesFor<float>, "!RShiftAssignCompilesFor<float>");
+
+template <typename U, typename = void>
+static constexpr bool ToIntegerCompilesFor = false;
+template <typename U>
+static constexpr bool
+    ToIntegerCompilesFor<U, void_t<decltype(gsl::to_integer<U>(gsl::byte{}))>> = true;
+static_assert(!ToIntegerCompilesFor<float>, "!ToIntegerCompilesFor<float>");
+
+} // namespace

tests/notnull_tests.cpp

@@ -18,16 +18,25 @@
 
 #include <gsl/pointers> // for not_null, operator<, operator<=, operator>
 
-#include <algorithm> // for addressof
-#include <memory>    // for shared_ptr, make_shared, operator<, opera...
-#include <sstream>   // for operator<<, ostringstream, basic_ostream:...
-#include <stdint.h>  // for uint16_t
-#include <string>    // for basic_string, operator==, string, operator<<
-#include <typeinfo>  // for type_info
+#include <algorithm>   // for addressof
+#include <cstdint>     // for uint16_t
+#include <memory>      // for shared_ptr, make_shared, operator<, opera...
+#include <sstream>     // for operator<<, ostringstream, basic_ostream:...
+#include <string>      // for basic_string, operator==, string, operator<<
+#include <type_traits> // for declval
+#include <typeinfo>    // for type_info
+#include <variant>     // for variant, monostate, get
 
 #include "deathTestCommon.h"
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 struct MyBase
 {
 };
@@ -52,7 +61,7 @@ template <typename T>
 struct CustomPtr
 {
     CustomPtr(T* p) : p_(p) {}
-    operator T*() { return p_; }
+    operator T*() const { return p_; }
     bool operator!=(std::nullptr_t) const { return p_ != nullptr; }
     T* p_ = nullptr;
 };
@@ -140,16 +149,39 @@ bool helper_const(not_null<const int*> p) { return *p == 12; }
 int* return_pointer() { return nullptr; }
 } // namespace
 
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_A = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_A<U, void_t<decltype(gsl::not_null<void*>{std::declval<U>()})>> = true;
+
+template <typename U, int N, typename = void>
+static constexpr bool CtorCompilesFor_B = false;
+template <typename U, int N>
+static constexpr bool CtorCompilesFor_B<U, N, void_t<decltype(gsl::not_null<U>{N})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool DefaultCtorCompilesFor = false;
+template <typename U>
+static constexpr bool DefaultCtorCompilesFor<U, void_t<decltype(gsl::not_null<U>{})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_C = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_C<U, void_t<decltype(gsl::not_null<U*>{std::declval<std::unique_ptr<U>>()})>> =
+        true;
+
 TEST(notnull_tests, TestNotNullConstructors)
 {
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        not_null<int*> p = nullptr;          // yay...does not compile!
-        not_null<std::vector<char>*> p1 = 0; // yay...does not compile!
-        not_null<int*> p2;                   // yay...does not compile!
-        std::unique_ptr<int> up = std::make_unique<int>(120);
-        not_null<int*> p3 = up;
+        static_assert(CtorCompilesFor_A<void*>, "CtorCompilesFor_A<void*>");
+        static_assert(!CtorCompilesFor_A<std::nullptr_t>, "!CtorCompilesFor_A<std::nullptr_t>");
+        static_assert(!CtorCompilesFor_B<void*, 0>, "!CtorCompilesFor_B<void*, 0>");
+        static_assert(!DefaultCtorCompilesFor<void*>, "!DefaultCtorCompilesFor<void*>");
+        static_assert(!CtorCompilesFor_C<int>, "CtorCompilesFor_C<int>");
 
+#ifdef CONFIRM_COMPILATION_ERRORS
         // Forbid non-nullptr assignable types
         not_null<std::vector<int>> f(std::vector<int>{1});
         not_null<int> z(10);
@@ -177,6 +209,14 @@ TEST(notnull_tests, TestNotNullConstructors)
         EXPECT_DEATH((not_null<decltype(pi)>(pi)), expected);
     }
 
+    {
+        // from unique pointer
+        not_null<std::unique_ptr<int>> x(
+            std::make_unique<int>(10)); // unique_ptr<int> is nullptr assignable
+
+        EXPECT_DEATH((not_null<std::unique_ptr<int>>(std::unique_ptr<int>{})), expected);
+    }
+
     {
         // from pointer to local
         int t = 42;
@@ -267,6 +307,27 @@ TEST(notnull_tests, TestNotNullostream)
     ostream_helper<std::string>("string");
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool AssignmentCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    AssignmentCompilesFor<U, V,
+                          void_t<decltype(std::declval<gsl::not_null<U*>&>().operator=(
+                              std::declval<gsl::not_null<V*>&>()))>> = true;
+
+template <typename U, typename V, typename = void>
+static constexpr bool SCastCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    SCastCompilesFor<U, V, void_t<decltype(static_cast<U*>(std::declval<gsl::not_null<V*>&>()))>> =
+        true;
+
+template <typename U, typename V, typename = void>
+static constexpr bool RCastCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool RCastCompilesFor<
+    U, V, void_t<decltype(reinterpret_cast<U*>(std::declval<gsl::not_null<V*>&>()))>> = true;
+
 TEST(notnull_tests, TestNotNullCasting)
 {
     MyBase base;
@@ -279,15 +340,30 @@ TEST(notnull_tests, TestNotNullCasting)
     q = p; // allowed with heterogeneous copy ctor
     EXPECT_TRUE(q == p);
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    q = u; // no viable conversion possible between MyBase* and Unrelated*
-    p = q; // not possible to implicitly convert MyBase* to MyDerived*
+    static_assert(AssignmentCompilesFor<MyBase, MyDerived>,
+                  "AssignmentCompilesFor<MyBase, MyDerived>");
+    static_assert(!AssignmentCompilesFor<MyBase, Unrelated>,
+                  "!AssignmentCompilesFor<MyBase, Unrelated>");
+    static_assert(!AssignmentCompilesFor<Unrelated, MyDerived>,
+                  "!AssignmentCompilesFor<Unrelated, MyDerived>");
+    static_assert(!AssignmentCompilesFor<MyDerived, MyBase>,
+                  "!AssignmentCompilesFor<MyDerived, MyBase>");
+
+    static_assert(SCastCompilesFor<MyDerived, MyDerived>, "SCastCompilesFor<MyDerived, MyDerived>");
+    static_assert(SCastCompilesFor<MyBase, MyDerived>, "SCastCompilesFor<MyBase, MyDerived>");
+    static_assert(!SCastCompilesFor<MyDerived, MyBase>, "!SCastCompilesFor<MyDerived, MyBase>");
+    static_assert(!SCastCompilesFor<Unrelated, MyDerived>,
+                  "!SCastCompilesFor<Unrelated, MyDerived>");
+    static_assert(!RCastCompilesFor<MyDerived, MyDerived>,
+                  "!SCastCompilesFor<MyDerived, MyDerived>");
+    static_assert(!RCastCompilesFor<Unrelated, MyDerived>,
+                  "!SCastCompilesFor<Unrelated, MyDerived>");
 
-    not_null<Unrelated*> r = p;
-    not_null<Unrelated*> s = reinterpret_cast<Unrelated*>(p);
-#endif
     not_null<Unrelated*> t(reinterpret_cast<Unrelated*>(p.get()));
     EXPECT_TRUE(reinterpret_cast<void*>(p.get()) == reinterpret_cast<void*>(t.get()));
+
+    (void) static_cast<MyDerived*>(p);
+    (void) static_cast<MyBase*>(p);
 }
 
 TEST(notnull_tests, TestNotNullAssignment)
@@ -429,6 +505,18 @@ TEST(notnull_tests, TestNotNullCustomPtrComparison)
 
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
 
+template <typename U, typename = void>
+static constexpr bool TypeDeductionCtorCompilesFor = false;
+template <typename U>
+static constexpr bool
+    TypeDeductionCtorCompilesFor<U, void_t<decltype(not_null{std::declval<U>()})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool TypeDeductionHelperCompilesFor = false;
+template <typename U>
+static constexpr bool
+    TypeDeductionHelperCompilesFor<U, void_t<decltype(helper(not_null{std::declval<U>()}))>> = true;
+
 TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
 {
     {
@@ -441,6 +529,18 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         EXPECT_TRUE(*x == 42);
     }
 
+    {
+        const int i = 42;
+
+        not_null x{&i};
+        static_assert(TypeDeductionHelperCompilesFor<int*>, "TypeDeductionHelperCompilesFor<int*>");
+        static_assert(!TypeDeductionHelperCompilesFor<const int*>,
+                      "!TypeDeductionHelperCompilesFor<const int*>");
+        helper_const(not_null{&i});
+
+        EXPECT_TRUE(*x == 42);
+    }
+
     {
         int i = 42;
         int* p = &i;
@@ -452,6 +552,16 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         EXPECT_TRUE(*x == 42);
     }
 
+    {
+        const int i = 42;
+        const int* p = &i;
+
+        not_null x{p};
+        helper_const(not_null{p});
+
+        EXPECT_TRUE(*x == 42);
+    }
+
     const auto terminateHandler = std::set_terminate([] {
         std::cerr << "Expected Death. TestNotNullConstructorTypeDeduction";
         std::abort();
@@ -481,16 +591,35 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         EXPECT_DEATH(helper_const(not_null{p}), expected);
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        not_null x{nullptr};
-        helper(not_null{nullptr});
-        helper_const(not_null{nullptr});
-    }
+    static_assert(TypeDeductionCtorCompilesFor<void*>, "TypeDeductionCtorCompilesFor<void*>");
+#if defined(_MSC_VER) && !defined(__clang__)
+    // Fails on gcc, clang, xcode, VS clang with
+    // "error : no type named 'type' in 'std::enable_if<false>'; 'enable_if' cannot be used to
+    // disable this declaration"
+    static_assert(!TypeDeductionCtorCompilesFor<std::nullptr_t>,
+                  "!TypeDeductionCtorCompilesFor<std::nullptr_t>");
+    static_assert(!TypeDeductionHelperCompilesFor<std::nullptr_t>,
+                  "!TypeDeductionHelperCompilesFor<std::nullptr_t>");
 #endif
 }
+
+TEST(notnull_tests, TestVariantEmplace)
+{
+    int i = 0;
+    std::variant<std::monostate, not_null<int*>> v;
+    v.emplace<not_null<int*>>(&i);
+
+    EXPECT_FALSE(v.valueless_by_exception());
+    EXPECT_TRUE(v.index() == 1);
+    EXPECT_TRUE(std::get<not_null<int*>>(v) == &i);
+}
 #endif // #if defined(__cplusplus) && (__cplusplus >= 201703L)
 
+template <typename U, typename = void>
+static constexpr bool HelperCompilesFor = false;
+template <typename U>
+static constexpr bool HelperCompilesFor<U, void_t<decltype(helper(std::declval<U>()))>> = true;
+
 TEST(notnull_tests, TestMakeNotNull)
 {
     {
@@ -503,6 +632,17 @@ TEST(notnull_tests, TestMakeNotNull)
         EXPECT_TRUE(*x == 42);
     }
 
+    {
+        const int i = 42;
+
+        const auto x = make_not_null(&i);
+        static_assert(HelperCompilesFor<gsl::not_null<int*>>,
+                      "HelperCompilesFor<gsl::not_null<int*>>");
+        helper_const(make_not_null(&i));
+
+        EXPECT_TRUE(*x == 42);
+    }
+
     {
         int i = 42;
         int* p = &i;
@@ -514,6 +654,18 @@ TEST(notnull_tests, TestMakeNotNull)
         EXPECT_TRUE(*x == 42);
     }
 
+    {
+        const int i = 42;
+        const int* p = &i;
+
+        const auto x = make_not_null(p);
+        static_assert(!HelperCompilesFor<gsl::not_null<const int*>>,
+                      "!HelperCompilesFor<gsl::not_null<const int*>>");
+        helper_const(make_not_null(p));
+
+        EXPECT_TRUE(*x == 42);
+    }
+
     const auto terminateHandler = std::set_terminate([] {
         std::cerr << "Expected Death. TestMakeNotNull";
         std::abort();
@@ -556,15 +708,31 @@ TEST(notnull_tests, TestMakeNotNull)
 
 TEST(notnull_tests, TestStdHash)
 {
-    int x = 42;
-    int y = 99;
-    not_null<int*> nn{&x};
-    const not_null<int*> cnn{&x};
+    {
+        int x = 42;
+        int y = 99;
+        not_null<int*> nn{&x};
+        const not_null<int*> cnn{&x};
 
-    std::hash<not_null<int*>> hash_nn;
-    std::hash<int*> hash_intptr;
+        std::hash<not_null<int*>> hash_nn;
+        std::hash<int*> hash_intptr;
 
-    EXPECT_TRUE(hash_nn(nn) == hash_intptr(&x));
-    EXPECT_FALSE(hash_nn(nn) == hash_intptr(&y));
-    EXPECT_FALSE(hash_nn(nn) == hash_intptr(nullptr));
+        EXPECT_TRUE(hash_nn(nn) == hash_intptr(&x));
+        EXPECT_FALSE(hash_nn(nn) == hash_intptr(&y));
+        EXPECT_FALSE(hash_nn(nn) == hash_intptr(nullptr));
+    }
+
+    {
+        const int x = 42;
+        const int y = 99;
+        not_null<const int*> nn{&x};
+        const not_null<const int*> cnn{&x};
+
+        std::hash<not_null<const int*>> hash_nn;
+        std::hash<const int*> hash_intptr;
+
+        EXPECT_TRUE(hash_nn(nn) == hash_intptr(&x));
+        EXPECT_FALSE(hash_nn(nn) == hash_intptr(&y));
+        EXPECT_FALSE(hash_nn(nn) == hash_intptr(nullptr));
+    }
 }

tests/owner_tests.cpp

@@ -17,6 +17,7 @@
 #include <gtest/gtest.h>
 
 #include <gsl/pointers> // for owner
+#include <type_traits>  // for declval
 
 using namespace gsl;
 
@@ -32,12 +33,18 @@ TEST(owner_tests, basic_test)
     delete p;
 }
 
-TEST(owner_tests, check_pointer_constraint)
-{
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        owner<int> integerTest = 10;
-        owner<std::shared_ptr<int>> sharedPtrTest(new int(10));
-    }
-#endif
-}
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
+template <typename U, typename = void>
+static constexpr bool OwnerCompilesFor = false;
+template <typename U>
+static constexpr bool OwnerCompilesFor<U, void_t<decltype(gsl::owner<U>{})>> =
+        true;
+static_assert(OwnerCompilesFor<int*>, "OwnerCompilesFor<int*>");
+static_assert(!OwnerCompilesFor<int>, "!OwnerCompilesFor<int>");
+static_assert(!OwnerCompilesFor<std::shared_ptr<int>>, "!OwnerCompilesFor<std::shared_ptr<int>>");

tests/pointers_tests.cpp

@@ -0,0 +1,97 @@
+#include <gtest/gtest.h>
+
+#include <gsl/pointers>
+
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
+namespace
+{
+// Custom pointer type that can be used for gsl::not_null, but for which these cannot be swapped.
+struct NotMoveAssignableCustomPtr
+{
+    NotMoveAssignableCustomPtr() = default;
+    NotMoveAssignableCustomPtr(const NotMoveAssignableCustomPtr&) = default;
+    NotMoveAssignableCustomPtr& operator=(const NotMoveAssignableCustomPtr&) = default;
+    NotMoveAssignableCustomPtr(NotMoveAssignableCustomPtr&&) = default;
+    NotMoveAssignableCustomPtr& operator=(NotMoveAssignableCustomPtr&&) = delete;
+
+    bool operator!=(std::nullptr_t) const { return true; }
+
+    int dummy{}; // Without this clang warns, that NotMoveAssignableCustomPtr() is unneeded
+};
+
+template <typename U, typename = void>
+static constexpr bool SwapCompilesFor = false;
+template <typename U>
+static constexpr bool
+    SwapCompilesFor<U, void_t<decltype(gsl::swap<U>(std::declval<gsl::not_null<U>&>(),
+                                                    std::declval<gsl::not_null<U>&>()))>> = true;
+
+TEST(pointers_test, swap)
+{
+    // taken from gh-1129:
+    {
+        gsl::not_null<std::unique_ptr<int>> a(std::make_unique<int>(0));
+        gsl::not_null<std::unique_ptr<int>> b(std::make_unique<int>(1));
+
+        EXPECT_TRUE(*a == 0);
+        EXPECT_TRUE(*b == 1);
+
+        gsl::swap(a, b);
+
+        EXPECT_TRUE(*a == 1);
+        EXPECT_TRUE(*b == 0);
+
+        // Make sure our custom ptr can be used with not_null. The shared_pr is to prevent "unused"
+        // compiler warnings.
+        const auto shared_custom_ptr{std::make_shared<NotMoveAssignableCustomPtr>()};
+        gsl::not_null<NotMoveAssignableCustomPtr> c{*shared_custom_ptr};
+        EXPECT_TRUE(c.get() != nullptr);
+    }
+
+    {
+        gsl::strict_not_null<std::unique_ptr<int>> a{std::make_unique<int>(0)};
+        gsl::strict_not_null<std::unique_ptr<int>> b{std::make_unique<int>(1)};
+
+        EXPECT_TRUE(*a == 0);
+        EXPECT_TRUE(*b == 1);
+
+        gsl::swap(a, b);
+
+        EXPECT_TRUE(*a == 1);
+        EXPECT_TRUE(*b == 0);
+    }
+
+    {
+        gsl::not_null<std::unique_ptr<int>> a{std::make_unique<int>(0)};
+        gsl::strict_not_null<std::unique_ptr<int>> b{std::make_unique<int>(1)};
+
+        EXPECT_TRUE(*a == 0);
+        EXPECT_TRUE(*b == 1);
+
+        gsl::swap(a, b);
+
+        EXPECT_TRUE(*a == 1);
+        EXPECT_TRUE(*b == 0);
+    }
+
+    static_assert(!SwapCompilesFor<NotMoveAssignableCustomPtr>,
+                  "!SwapCompilesFor<NotMoveAssignableCustomPtr>");
+}
+
+TEST(pointers_test, member_types)
+{
+    static_assert(std::is_same<gsl::not_null<int*>::element_type, int*>::value,
+                  "check member type: element_type");
+}
+
+} // namespace

tests/span_compatibility_tests.cpp

@@ -519,7 +519,7 @@ TEST(span_compatibility_tests, assertion_tests)
 
 // assertions for span's definition
 static_assert(std::is_same<decltype(gsl::dynamic_extent), const std::size_t>::value,
-              "gsl::dynamic_extent must be respresented as std::size_t");
+              "gsl::dynamic_extent must be represented as std::size_t");
 static_assert(gsl::dynamic_extent == static_cast<std::size_t>(-1),
               "gsl::dynamic_extent must be defined as the max value of std::size_t");
 
@@ -1005,12 +1005,18 @@ static_assert(std::is_convertible<const std::array<int, 3>&, gsl::span<const int
               "std::is_convertible<const std::array<int, 3>&, gsl::span<const int>>");
 
 #if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 template <typename U, typename = void>
 static constexpr bool AsWritableBytesCompilesFor = false;
 
 template <typename U>
 static constexpr bool
-    AsWritableBytesCompilesFor<U, void_t<decltype(as_writable_bytes(declval<U>()))>> = true;
+    AsWritableBytesCompilesFor<U, ::void_t<decltype(as_writable_bytes(declval<U>()))>> = true;
 
 static_assert(AsWritableBytesCompilesFor<gsl::span<int>>,
               "AsWritableBytesCompilesFor<gsl::span<int>>");
@@ -1020,4 +1026,3 @@ static_assert(!AsWritableBytesCompilesFor<gsl::span<const int>>,
               "!AsWritableBytesCompilesFor<gsl::span<const int>>");
 static_assert(!AsWritableBytesCompilesFor<gsl::span<const int, 9>>,
               "!AsWritableBytesCompilesFor<gsl::span<const int, 9>>");
-#endif // __cplusplus >= 201703l

tests/span_ext_tests.cpp

@@ -19,9 +19,10 @@
 #include <gsl/span> // for span and span_ext
 #include <gsl/util> // for narrow_cast, at
 
-#include <array>    // for array
-#include <iostream> // for cerr
-#include <vector>   // for vector
+#include <array>     // for array
+#include <exception> // for terminate
+#include <iostream>  // for cerr
+#include <vector>    // for vector
 
 using namespace std;
 using namespace gsl;
@@ -193,10 +194,28 @@ TEST(span_ext_test, make_span_from_container_constructor)
 
 TEST(span_test, interop_with_gsl_at)
 {
-    int arr[5] = {1, 2, 3, 4, 5};
-    gsl::span<int> s{arr};
-    EXPECT_TRUE(at(s, 0) == 1);
-    EXPECT_TRUE(at(s, 1) == 2);
+    std::vector<int> vec{1, 2, 3, 4, 5};
+    gsl::span<int> sp{vec};
+
+    std::vector<int> cvec{1, 2, 3, 4, 5};
+    gsl::span<int> csp{cvec};
+
+    for (gsl::index i = 0; i < gsl::narrow_cast<gsl::index>(vec.size()); ++i)
+    {
+        EXPECT_TRUE(&gsl::at(sp, i) == &vec[gsl::narrow_cast<size_t>(i)]);
+        EXPECT_TRUE(&gsl::at(csp, i) == &cvec[gsl::narrow_cast<size_t>(i)]);
+    }
+
+    const auto terminateHandler = std::set_terminate([] {
+        std::cerr << "Expected Death. interop_with_gsl_at";
+        std::abort();
+    });
+    const auto expected = GetExpectedDeathString(terminateHandler);
+
+    EXPECT_DEATH(gsl::at(sp, -1), expected);
+    EXPECT_DEATH(gsl::at(sp, gsl::narrow_cast<gsl::index>(sp.size())), expected);
+    EXPECT_DEATH(gsl::at(csp, -1), expected);
+    EXPECT_DEATH(gsl::at(csp, gsl::narrow_cast<gsl::index>(sp.size())), expected);
 }
 
 TEST(span_ext_test, iterator_free_functions)

tests/span_tests.cpp

@@ -40,11 +40,21 @@
 #endif // __has_include(<string_view>)
 #endif // __has_include
 #endif // (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L))
+#if defined(__cplusplus) && __cplusplus >= 202002L
+#include <span>
+#endif // __cplusplus >= 202002L
 
 #include "deathTestCommon.h"
 
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 namespace
 {
 
@@ -59,8 +69,7 @@ struct AddressOverloaded
 #if (__cplusplus > 201402L)
     [[maybe_unused]]
 #endif
-    AddressOverloaded
-    operator&() const
+    AddressOverloaded operator&() const
     {
         return {};
     }
@@ -213,6 +222,12 @@ TEST(span_test, from_pointer_length_constructor)
 
 TEST(span_test, from_pointer_pointer_construction)
 {
+    // const auto terminateHandler = std::set_terminate([] {
+    //     std::cerr << "Expected Death. from_pointer_pointer_construction";
+    //     std::abort();
+    // });
+    // const auto expected = GetExpectedDeathString(terminateHandler);
+
     int arr[4] = {1, 2, 3, 4};
 
     {
@@ -242,19 +257,11 @@ TEST(span_test, from_pointer_pointer_construction)
         EXPECT_TRUE(s.data() == &arr[0]);
     }
 
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
-    //{
+    //{ // this test succeeds on all platforms, gsl::span is more relaxed than std::span where this would be UB
     //    auto workaround_macro = [&]() { span<int> s{&arr[1], &arr[0]}; };
     //    EXPECT_DEATH(workaround_macro(), expected);
     //}
 
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
-    //{
-    //    int* p = nullptr;
-    //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
-    //    EXPECT_DEATH(workaround_macro(), expected);
-    //}
-
     {
         int* p = nullptr;
         span<int> s{p, p};
@@ -268,19 +275,21 @@ TEST(span_test, from_pointer_pointer_construction)
         EXPECT_TRUE(s.size() == 0);
         EXPECT_TRUE(s.data() == nullptr);
     }
-
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
-    //{
-    //    int* p = nullptr;
-    //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
-    //    EXPECT_DEATH(workaround_macro(), expected);
-    //}
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool CtorCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool CtorCompilesFor<U, V, void_t<decltype(U{std::declval<V>()})>> = true;
+
 TEST(span_test, from_array_constructor)
 {
     int arr[5] = {1, 2, 3, 4, 5};
 
+    static_assert(!CtorCompilesFor<span<int, 6>, int[5]>, "!CtorCompilesFor<span<int, 6>, int[5]>");
+    static_assert(!CtorCompilesFor<span<int, 0>, int[5]>, "!CtorCompilesFor<span<int, 0>, int[5]>");
+    static_assert(!CtorCompilesFor<span<int>, int[2][3]>, "!CtorCompilesFor<span<int>, int[2][3]>");
+
     {
         const span<int> s{arr};
         EXPECT_TRUE(s.size() == 5);
@@ -295,70 +304,28 @@ TEST(span_test, from_array_constructor)
 
     int arr2d[2][3] = {1, 2, 3, 4, 5, 6};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 6> s{arr};
-    }
+    static_assert(!CtorCompilesFor<span<int, 0>, int[2][3]>,
+                  "!CtorCompilesFor<span<int, 0>, int[2][3]>");
+    static_assert(!CtorCompilesFor<span<int, 6>, int[2][3]>,
+                  "!CtorCompilesFor<span<int, 6>, int[2][3]>");
 
-    {
-        span<int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr[0]);
-    }
-
-    {
-        span<int> s{arr2d};
-        EXPECT_TRUE(s.size() == 6);
-        EXPECT_TRUE(s.data() == &arr2d[0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[5] == 6);
-    }
-
-    {
-        span<int, 0> s{arr2d};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr2d[0][0]);
-    }
-
-    {
-        span<int, 6> s{arr2d};
-    }
-#endif
     {
         const span<int[3]> s{std::addressof(arr2d[0]), 1};
         EXPECT_TRUE(s.size() == 1);
         EXPECT_TRUE(s.data() == std::addressof(arr2d[0]));
     }
 
-    int arr3d[2][3][2] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
+    int arr3d[2][3][2] = {{{1, 2}, {3, 4}, {5, 6}}, {{7, 8}, {9, 10}, {11, 12}}};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int> s{arr3d};
-        EXPECT_TRUE(s.size() == 12);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[11] == 12);
-    }
+    static_assert(!CtorCompilesFor<span<int>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 0>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 0>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 11>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 11>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 12>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 12>, int[2][3][2]>");
 
-    {
-        span<int, 0> s{arr3d};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-    }
-
-    {
-        span<int, 11> s{arr3d};
-    }
-
-    {
-        span<int, 12> s{arr3d};
-        EXPECT_TRUE(s.size() == 12);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[5] == 6);
-    }
-#endif
     {
         const span<int[3][2]> s{std::addressof(arr3d[0]), 1};
         EXPECT_TRUE(s.size() == 1);
@@ -386,6 +353,13 @@ TEST(span_test, from_dynamic_array_constructor)
     delete[] arr;
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool ConversionCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    ConversionCompilesFor<U, V, void_t<decltype(std::declval<void (*)(U)>()(std::declval<V>()))>> =
+        true;
+
 TEST(span_test, from_std_array_constructor)
 {
     std::array<int, 4> arr = {1, 2, 3, 4};
@@ -425,43 +399,31 @@ TEST(span_test, from_std_array_constructor)
         EXPECT_TRUE(ao_arr.data() == fs.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
+    static_assert(!CtorCompilesFor<span<int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 2>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>");
 
-        span<const int, 2> cs{arr};
-        EXPECT_TRUE(cs.size() == 2);
-        EXPECT_TRUE(cs.data() == arr.data());
-    }
+    static_assert(!CtorCompilesFor<span<int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 0>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>");
 
-    {
-        span<int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
+    static_assert(!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>");
 
-        span<const int, 0> cs{arr};
-        EXPECT_TRUE(cs.size() == 0);
-        EXPECT_TRUE(cs.data() == arr.data());
-    }
-
-    {
-        span<int, 5> s{arr};
-    }
-
-    {
-        auto get_an_array = []() -> std::array<int, 4> { return {1, 2, 3, 4}; };
-        auto take_a_span = [](span<int> s) { static_cast<void>(s); };
-        // try to take a temporary std::array
-        take_a_span(get_an_array());
-    }
+#if !defined(_MSC_VER) || (_MSC_VER > 1943) || (__cplusplus >= 201703L)
+    // Fails on "Visual Studio 16 2019/Visual Studio 17 2022, windows-2019/2022, Debug/Release, 14".
+    static_assert(!ConversionCompilesFor<span<int>, std::array<int, 4>>,
+                  "!ConversionCompilesFor<span<int>, std::array<int, 4>>");
 #endif
 
     {
         auto get_an_array = []() -> std::array<int, 4> { return {1, 2, 3, 4}; };
-        auto take_a_span = [](span<const int> s) { static_cast<void>(s); };
+        auto take_a_span = [](span<const int>) {};
         // try to take a temporary std::array
+        static_assert(ConversionCompilesFor<span<const int>, std::array<int, 4>>,
+                      "ConversionCompilesFor<span<const int>, std::array<int, 4>>");
         take_a_span(get_an_array());
     }
 }
@@ -490,23 +452,12 @@ TEST(span_test, from_const_std_array_constructor)
         EXPECT_TRUE(s.data() == ao_arr.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<const int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 5> s{arr};
-    }
-#endif
+    static_assert(!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>");
 
     {
         auto get_an_array = []() -> const std::array<int, 4> { return {1, 2, 3, 4}; };
@@ -532,27 +483,14 @@ TEST(span_test, from_std_array_const_constructor)
         EXPECT_TRUE(s.data() == arr.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<const int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 5> s{arr};
-    }
-
-    {
-        span<int, 4> s{arr};
-    }
-#endif
+    static_assert(!CtorCompilesFor<span<const int, 2>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 5>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 5>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<int, 5>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, const std::array<int, 4>&>");
 }
 
 TEST(span_test, from_container_constructor)
@@ -574,32 +512,28 @@ TEST(span_test, from_container_constructor)
     const std::string cstr = "hello";
 
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        span<char> s{str};
-        EXPECT_TRUE(s.size() == str.size());
-         EXPECT_TRUE(s.data() == str.data()));
-#endif
-         span<const char> cs{str};
-         EXPECT_TRUE(cs.size() == str.size());
-         EXPECT_TRUE(cs.data() == str.data());
+        static_assert(CtorCompilesFor<span<char>, std::string&> == (__cplusplus >= 201703L),
+                      "CtorCompilesFor<span<char>, std::string&> == (__cplusplus >= 201703L)");
+
+        span<const char> cs{str};
+        EXPECT_TRUE(cs.size() == str.size());
+        EXPECT_TRUE(cs.data() == str.data());
     }
 
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        span<char> s{cstr};
-#endif
+        static_assert(!CtorCompilesFor<span<char>, const std::string&>,
+                      "!CtorCompilesFor<span<char>, const std::string&>");
+
         span<const char> cs{cstr};
         EXPECT_TRUE(cs.size() == cstr.size());
         EXPECT_TRUE(cs.data() == cstr.data());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_vector = []() -> std::vector<int> { return {}; };
-        auto use_span = [](span<int> s) { static_cast<void>(s); };
-        use_span(get_temp_vector());
-#endif
-    }
+#if !defined(_MSC_VER) || (_MSC_VER > 1943) || (__cplusplus >= 201703L)
+    // Fails on "Visual Studio 16 2019/Visual Studio 17 2022, windows-2019/2022, Debug/Release, 14".
+    static_assert(!ConversionCompilesFor<span<int>, std::vector<int>>,
+                  "!ConversionCompilesFor<span<int>, std::vector<int>>");
+#endif // !defined(_MSC_VER) || (_MSC_VER > 1942) || (__cplusplus >= 201703L)
 
     {
         auto get_temp_vector = []() -> std::vector<int> { return {}; };
@@ -607,13 +541,8 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_vector());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_string = []() -> std::string { return {}; };
-        auto use_span = [](span<char> s) { static_cast<void>(s); };
-        use_span(get_temp_string());
-#endif
-    }
+    static_assert(!ConversionCompilesFor<span<char>, std::string>,
+                  "!ConversionCompilesFor<span<char>, std::string>");
 
     {
         auto get_temp_string = []() -> std::string { return {}; };
@@ -621,13 +550,10 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_string());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_vector = []() -> const std::vector<int> { return {}; };
-        auto use_span = [](span<const char> s) { static_cast<void>(s); };
-        use_span(get_temp_vector());
-#endif
-    }
+    static_assert(!ConversionCompilesFor<span<const char>, const std::vector<int>>,
+                  "!ConversionCompilesFor<span<const char>, const std::vector<int>>");
+    static_assert(!ConversionCompilesFor<span<char>, const std::string>,
+                  "!ConversionCompilesFor<span<char>, const std::string>");
 
     {
         auto get_temp_string = []() -> const std::string { return {}; };
@@ -635,46 +561,73 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_string());
     }
 
+    static_assert(!CtorCompilesFor<span<int>, std::map<int, int>&>,
+                  "!CtorCompilesFor<span<int>, std::map<int, int>&>");
+}
+
+TEST(span_test, from_convertible_span_constructor)
+{
+    const auto terminateHandler = std::set_terminate([] {
+        std::cerr << "Expected Death. from_convertible_span_constructor";
+        std::abort();
+    });
+    const auto expected = GetExpectedDeathString(terminateHandler);
+
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        std::map<int, int> m;
-        span<int> s{m};
-#endif
+        span<DerivedClass> avd;
+        span<const DerivedClass> avcd = avd;
+        static_cast<void>(avcd);
     }
-}
 
-TEST(span_test, from_convertible_span_constructor){{span<DerivedClass> avd;
-span<const DerivedClass> avcd = avd;
-static_cast<void>(avcd);
-}
+    {
+        std::array<DerivedClass, 2> arr{};
+        span<DerivedClass, 2> avd{arr};
+        span<const DerivedClass, 2> avcd = avd;
+        static_cast<void>(avcd);
+    }
 
-{
-#ifdef CONFIRM_COMPILATION_ERRORS
-    span<DerivedClass> avd;
-    span<BaseClass> avb = avd;
-    static_cast<void>(avb);
-#endif
-}
+    {
+        std::array<DerivedClass, 2> arr{};
+        span<DerivedClass, 2> avd{arr};
+        span<const DerivedClass> avcd = avd;
+        static_cast<void>(avcd);
+    }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-{
-    span<int> s;
-    span<unsigned int> s2 = s;
-    static_cast<void>(s2);
-}
+    {
+        std::array<DerivedClass, 2> arr{};
+        span<DerivedClass> avd{arr};
+        span<const DerivedClass, 2> avcd{avd};
+        static_cast<void>(avcd);
+    }
 
-{
-    span<int> s;
-    span<const unsigned int> s2 = s;
-    static_cast<void>(s2);
-}
+    {
+        std::array<DerivedClass, 2> arr{};
+        span<DerivedClass> avd{arr};
+        using T = span<const DerivedClass, 1>;
+        EXPECT_DEATH(T{avd}, expected);
+    }
 
-{
-    span<int> s;
-    span<short> s2 = s;
-    static_cast<void>(s2);
-}
-#endif
+    {
+        std::array<DerivedClass, 1> arr{};
+        span<DerivedClass> avd{arr};
+        using T = span<const DerivedClass, 2>;
+        EXPECT_DEATH(T{avd}, expected);
+    }
+
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 2>, span<DerivedClass>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 2>, span<DerivedClass>&>");
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 1>, span<DerivedClass, 2>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 1>, span<DerivedClass, 2>&>");
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 3>, span<DerivedClass, 2>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 3>, span<DerivedClass, 2>&>");
+    static_assert(!ConversionCompilesFor<span<BaseClass, 3>, span<DerivedClass>&>,
+                  "!ConversionCompilesFor<span<BaseClass, 3>, span<DerivedClass>&>");
+    static_assert(!ConversionCompilesFor<span<unsigned int>, span<int>&>,
+                  "!ConversionCompilesFor<span<unsigned int>, span<int>&>");
+    static_assert(!ConversionCompilesFor<span<const unsigned int>, span<int>&>,
+                  "!ConversionCompilesFor<span<const unsigned int>, span<int>&>");
+    static_assert(!ConversionCompilesFor<span<short>, span<int>&>,
+                  "!ConversionCompilesFor<span<short>, span<int>&>");
 }
 
 TEST(span_test, copy_move_and_assignment)
@@ -734,10 +687,9 @@ TEST(span_test, first)
     {
         span<int, 5> av = arr;
 #ifdef CONFIRM_COMPILATION_ERRORS
-        EXPECT_TRUE(av.first<6>().size() == 6);
-        EXPECT_TRUE(av.first<-1>().size() == -1);
+        (void) av.first<6>();
 #endif
-        EXPECT_DEATH(av.first(6).size(), expected);
+        EXPECT_DEATH(av.first(6), expected);
     }
 
     {
@@ -778,9 +730,9 @@ TEST(span_test, last)
     {
         span<int, 5> av = arr;
 #ifdef CONFIRM_COMPILATION_ERRORS
-        EXPECT_TRUE(av.last<6>().size() == 6);
+        (void) av.last<6>();
 #endif
-        EXPECT_DEATH(av.last(6).size(), expected);
+        EXPECT_DEATH(av.last(6), expected);
     }
 
     {
@@ -805,6 +757,9 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<2, 2>().size()) == 2);
         EXPECT_TRUE(decltype(av.subspan<2, 2>())::extent == 2);
         EXPECT_TRUE(av.subspan(2, 2).size() == 2);
+
+        EXPECT_TRUE((av.subspan<2, 3>().size()) == 3);
+        EXPECT_TRUE(decltype(av.subspan<2, 3>())::extent == 3);
         EXPECT_TRUE(av.subspan(2, 3).size() == 3);
     }
 
@@ -821,8 +776,12 @@ TEST(span_test, subspan)
         EXPECT_TRUE(decltype(av.subspan<0, 5>())::extent == 5);
         EXPECT_TRUE(av.subspan(0, 5).size() == 5);
 
-        EXPECT_DEATH(av.subspan(0, 6).size(), expected);
-        EXPECT_DEATH(av.subspan(1, 5).size(), expected);
+#ifdef CONFIRM_COMPILATION_ERRORS
+        (void) av.subspan<0, 6>();
+        (void) av.subspan<1, 5>();
+#endif
+        EXPECT_DEATH(av.subspan(0, 6), expected);
+        EXPECT_DEATH(av.subspan(1, 5), expected);
     }
 
     {
@@ -830,14 +789,22 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<4, 0>().size()) == 0);
         EXPECT_TRUE(decltype(av.subspan<4, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(4, 0).size() == 0);
+
+        EXPECT_TRUE((av.subspan<5, 0>().size()) == 0);
+        EXPECT_TRUE(decltype(av.subspan<5, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(5, 0).size() == 0);
-        EXPECT_DEATH(av.subspan(6, 0).size(), expected);
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+        (void) av.subspan<6, 0>();
+#endif
+        EXPECT_DEATH(av.subspan(6, 0), expected);
     }
 
     {
         span<int, 5> av = arr;
         EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(decltype(av.subspan<1>())::extent == 4);
+        EXPECT_TRUE(av.subspan(1).size() == 4);
     }
 
     {
@@ -845,35 +812,58 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<0, 0>().size()) == 0);
         EXPECT_TRUE(decltype(av.subspan<0, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(0, 0).size() == 0);
-        EXPECT_DEATH((av.subspan<1, 0>().size()), expected);
+
+        EXPECT_DEATH((av.subspan<1, 0>()), expected);
+        EXPECT_DEATH((av.subspan(1, 0)), expected);
     }
 
     {
         span<int> av;
+        EXPECT_TRUE((av.subspan<0>().size()) == 0);
+        EXPECT_TRUE(decltype(av.subspan<0>())::extent == dynamic_extent);
         EXPECT_TRUE(av.subspan(0).size() == 0);
-        EXPECT_DEATH(av.subspan(1).size(), expected);
+
+        EXPECT_DEATH(av.subspan<1>(), expected);
+        EXPECT_TRUE(decltype(av.subspan<1>())::extent == dynamic_extent);
+        EXPECT_DEATH(av.subspan(1), expected);
     }
 
     {
         span<int> av = arr;
         EXPECT_TRUE(av.subspan(0).size() == 5);
+        EXPECT_TRUE(av.subspan<0>().size() == 5);
         EXPECT_TRUE(av.subspan(1).size() == 4);
+        EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(av.subspan(4).size() == 1);
+        EXPECT_TRUE(av.subspan<4>().size() == 1);
         EXPECT_TRUE(av.subspan(5).size() == 0);
-        EXPECT_DEATH(av.subspan(6).size(), expected);
+        EXPECT_TRUE(av.subspan<5>().size() == 0);
+        EXPECT_DEATH(av.subspan(6), expected);
+        EXPECT_DEATH(av.subspan<6>(), expected);
         const auto av2 = av.subspan(1);
         for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == static_cast<int>(i) + 2);
+        const auto av3 = av.subspan<1>();
+        for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av3[i] == static_cast<int>(i) + 2);
     }
 
     {
         span<int, 5> av = arr;
         EXPECT_TRUE(av.subspan(0).size() == 5);
+        EXPECT_TRUE(av.subspan<0>().size() == 5);
         EXPECT_TRUE(av.subspan(1).size() == 4);
+        EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(av.subspan(4).size() == 1);
+        EXPECT_TRUE(av.subspan<4>().size() == 1);
         EXPECT_TRUE(av.subspan(5).size() == 0);
-        EXPECT_DEATH(av.subspan(6).size(), expected);
+        EXPECT_TRUE(av.subspan<5>().size() == 0);
+        EXPECT_DEATH(av.subspan(6), expected);
+#ifdef CONFIRM_COMPILATION_ERRORS
+        EXPECT_DEATH(av.subspan<6>(), expected);
+#endif
         const auto av2 = av.subspan(1);
         for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == static_cast<int>(i) + 2);
+        const auto av3 = av.subspan<1>();
+        for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av3[i] == static_cast<int>(i) + 2);
     }
 }
 
@@ -1048,15 +1038,21 @@ TEST(span_test, rbegin_rend)
     }
 }
 
+template <typename U, typename = void>
+static constexpr bool AsWritableBytesCompilesFor = false;
+template <typename U>
+static constexpr bool
+    AsWritableBytesCompilesFor<U, void_t<decltype(as_writable_bytes(std::declval<U>()))>> = true;
+
 TEST(span_test, as_bytes)
 {
-    const auto terminateHandler = std::set_terminate([] {
-        std::cerr << "Expected Death. as_bytes";
-        std::abort();
-    });
-    const auto expected = GetExpectedDeathString(terminateHandler);
-
     int a[] = {1, 2, 3, 4};
+
+    static_assert(AsWritableBytesCompilesFor<span<int>>, "AsWriteableBytesCompilesFor<span<int>>");
+    // you should not be able to get writeable bytes for const objects
+    static_assert(!AsWritableBytesCompilesFor<span<const int>>,
+                  "!AsWriteableBytesCompilesFor<span<const int>>");
+
     {
         const span<const int> s = a;
         EXPECT_TRUE(s.size() == 4);
@@ -1081,29 +1077,12 @@ TEST(span_test, as_bytes)
         EXPECT_TRUE(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
         EXPECT_TRUE(bs.size() == s.size_bytes());
     }
-
-    int b[5] = {1, 2, 3, 4, 5};
-    {
-        span<int> sp(std::begin(b), static_cast<size_t>(-2));
-        EXPECT_DEATH((void) sp.size_bytes(), expected);
-    }
 }
 
 TEST(span_test, as_writable_bytes)
 {
     int a[] = {1, 2, 3, 4};
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        // you should not be able to get writeable bytes for const objects
-        span<const int> s = a;
-        EXPECT_TRUE(s.size() == 4);
-        span<const byte> bs = as_writable_bytes(s);
-        EXPECT_TRUE(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
-        EXPECT_TRUE(bs.size() == s.size_bytes());
-#endif
-    }
-
     {
         span<int> s;
         const auto bs = as_writable_bytes(s);
@@ -1143,30 +1122,15 @@ TEST(span_test, fixed_size_conversions)
         static_cast<void>(s);
     }
 
-// initialization or assignment to static span that REDUCES size is NOT ok
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 2> s = arr;
-    }
-    {
-        span<int, 2> s2 = s4;
-        static_cast<void>(s2);
-    }
-#endif
+    // initialization or assignment to static span that REDUCES size is NOT ok
+    static_assert(!ConversionCompilesFor<span<int, 2>, int[4]>,
+                  "!ConversionCompilesFor<span<int, 2>, int[4]>");
+    static_assert(!ConversionCompilesFor<span<int, 2>, span<int, 4>>,
+                  "!ConversionCompilesFor<span<int, 2>, span<int, 4>>");
 
     // even when done dynamically
-    {
-        /*
-        // this now results in a compile-time error, rather than runtime.
-        // There is no suitable conversion from dynamic span to fixed span.
-        span<int> s = arr;
-        auto f = [&]() {
-            const span<int, 2> s2 = s;
-            static_cast<void>(s2);
-        };
-        EXPECT_DEATH(f(), expected);
-        */
-    }
+    static_assert(!ConversionCompilesFor<span<int, 2>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 2>, span<int>>");
 
     // but doing so explicitly is ok
 
@@ -1180,32 +1144,24 @@ TEST(span_test, fixed_size_conversions)
         static_cast<void>(s1);
     }
 
-    /*
-     // this is not a legal operation in std::span, so we are no longer supporting it
-     // conversion from span<int, 4> to span<int, dynamic_extent> via call to `first`
-     // then convert from span<int, dynamic_extent> to span<int, 1>
-     // The dynamic to fixed extents are not supported in the standard
-     // to make this work, span<int, 1> would need to be span<int>.
-     {
-
-         // NB: implicit conversion to span<int,1> from span<int>
-         span<int, 1> s1 = s4.first(1);
-         static_cast<void>(s1);
-     }
-     */
+    // this is not a legal operation in std::span, so we are no longer supporting it
+    // conversion from span<int, 4> to span<int, dynamic_extent> via call to `first`
+    // then convert from span<int, dynamic_extent> to span<int, 1>
+    // The dynamic to fixed extents are not supported in the standard
+    // to make this work, span<int, 1> would need to be span<int>.
+    static_assert(!ConversionCompilesFor<span<int, 1>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 1>, span<int>>");
 
     // initialization or assignment to static span that requires size INCREASE is not ok.
     int arr2[2] = {1, 2};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 4> s3 = arr2;
-    }
-    {
-        span<int, 2> s2 = arr2;
-        span<int, 4> s4a = s2;
-    }
-#endif
+    static_assert(!ConversionCompilesFor<span<int, 4>, int[2]>,
+                  "!ConversionCompilesFor<span<int, 4>, int[2]>");
+    static_assert(!ConversionCompilesFor<span<int, 2>, int[2]>,
+                  "!ConversionCompilesFor<span<int, 2>, int[2]>");
+    static_assert(!ConversionCompilesFor<span<int, 4>, span<int, 2>>,
+                  "!ConversionCompilesFor<span<int, 4>, span<int, 2>>");
+
     {
         auto f = [&]() {
             const span<int, 4> _s4{arr2, 2};
@@ -1214,16 +1170,11 @@ TEST(span_test, fixed_size_conversions)
         EXPECT_DEATH(f(), expected);
     }
 
-    /*
-     // This no longer compiles. There is no suitable conversion from dynamic span to a fixed size
-     span.
-     // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
-     span<int> av = arr2; auto f = [&]() {
-         const span<int, 4> _s4 = av;
-         static_cast<void>(_s4);
-     };
-     EXPECT_DEATH(f(), expected);
-     */
+    // This no longer compiles. There is no suitable conversion from dynamic span to a fixed size
+    // span.
+    // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
+    static_assert(!ConversionCompilesFor<span<int, 4>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 4>, span<int>>");
 }
 
 TEST(span_test, interop_with_std_regex)
@@ -1297,3 +1248,179 @@ TEST(span_test, front_back)
     EXPECT_DEATH(s2.front(), expected);
     EXPECT_DEATH(s2.back(), expected);
 }
+
+#if defined(FORCE_STD_SPAN_TESTS) || defined(__cpp_lib_span) && __cpp_lib_span >= 202002L
+TEST(span_test, std_span)
+{
+    // make sure std::span can be constructed from gsl::span
+    int arr[5] = {1, 2, 3, 4, 5};
+    gsl::span<int> gsl_span{arr};
+#if defined(__cpp_lib_ranges) || (defined(_MSVC_STL_VERSION) && defined(__cpp_lib_concepts))
+    EXPECT_TRUE(std::to_address(gsl_span.begin()) == gsl_span.data());
+    EXPECT_TRUE(std::to_address(gsl_span.end()) == gsl_span.data() + gsl_span.size());
+#endif // __cpp_lib_ranges
+
+    std::span<int> std_span = gsl_span;
+    EXPECT_TRUE(std_span.data() == gsl_span.data());
+    EXPECT_TRUE(std_span.size() == gsl_span.size());
+}
+#endif // defined(FORCE_STD_SPAN_TESTS) || defined(__cpp_lib_span) && __cpp_lib_span >= 202002L
+
+#if defined(__cpp_lib_span) && defined(__cpp_lib_ranges)
+// This test covers the changes in PR #1100
+TEST(span_test, msvc_compile_error_PR1100)
+{
+    int arr[]{1, 7, 2, 9};
+    gsl::span sp{arr, std::size(arr)};
+    std::ranges::sort(sp);
+    for (const auto& e : sp) { (void) e; }
+}
+#endif // defined(__cpp_lib_span) && defined(__cpp_lib_ranges)
+
+TEST(span_test, empty_span)
+{
+    span<int> s{};
+    EXPECT_TRUE(s.empty());
+    EXPECT_TRUE(s.size() == 0);
+    EXPECT_TRUE(s.data() == nullptr);
+
+    span<const int> cs{};
+    EXPECT_TRUE(cs.empty());
+    EXPECT_TRUE(cs.size() == 0);
+    EXPECT_TRUE(cs.data() == nullptr);
+}
+
+TEST(span_test, conversions)
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+
+#if defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201611L)
+    span s = arr;
+    span cs = s;
+#else
+    span<int, 5> s = arr;
+    span<int, 5> cs = s;
+#endif
+
+    EXPECT_TRUE(cs.size() == s.size());
+    EXPECT_TRUE(cs.data() == s.data());
+
+    span<int, 5> fs = s;
+    EXPECT_TRUE(fs.size() == s.size());
+    EXPECT_TRUE(fs.data() == s.data());
+
+    span<const int, 5> cfs = s;
+    EXPECT_TRUE(cfs.size() == s.size());
+    EXPECT_TRUE(cfs.data() == s.data());
+}
+
+TEST(span_test, comparison_operators)
+{
+    int arr1[3] = {1, 2, 3};
+    int arr2[3] = {1, 2, 3};
+    int arr3[3] = {4, 5, 6};
+
+    span<int> s1 = arr1;
+    span<int> s2 = arr2;
+    span<int> s3 = arr3;
+
+    EXPECT_TRUE(s1 == s2);
+    EXPECT_FALSE(s1 != s2);
+    EXPECT_FALSE(s1 == s3);
+    EXPECT_TRUE(s1 != s3);
+    EXPECT_TRUE(s1 < s3);
+    EXPECT_FALSE(s3 < s1);
+    EXPECT_TRUE(s1 <= s2);
+    EXPECT_TRUE(s1 <= s3);
+    EXPECT_FALSE(s3 <= s1);
+    EXPECT_TRUE(s3 > s1);
+    EXPECT_FALSE(s1 > s3);
+    EXPECT_TRUE(s3 >= s1);
+    EXPECT_TRUE(s1 >= s2);
+    EXPECT_FALSE(s1 >= s3);
+}
+
+// ...existing code...
+
+#if defined(__cpp_lib_span) && __cpp_lib_span >= 202002L
+
+#include <span> // for std::span
+
+TEST(span_test, compare_empty_span)
+{
+    gsl::span<int> gsl_s{};
+    std::span<int> std_s{};
+
+    EXPECT_TRUE(gsl_s.empty());
+    EXPECT_TRUE(std_s.empty());
+    EXPECT_EQ(gsl_s.size(), std_s.size());
+    EXPECT_EQ(gsl_s.data(), std_s.data());
+}
+
+TEST(span_test, compare_subspan)
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+    gsl::span gsl_s = arr;
+    std::span std_s = arr;
+
+    auto gsl_sub1 = gsl_s.subspan(1);
+    auto std_sub1 = std_s.subspan(1);
+    EXPECT_EQ(gsl_sub1.size(), std_sub1.size());
+    EXPECT_EQ(gsl_sub1.data(), std_sub1.data());
+
+    auto gsl_sub2 = gsl_s.subspan(1, 2);
+    auto std_sub2 = std_s.subspan(1, 2);
+    EXPECT_EQ(gsl_sub2.size(), std_sub2.size());
+    EXPECT_EQ(gsl_sub2.data(), std_sub2.data());
+}
+
+TEST(span_test, compare_conversions)
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+    gsl::span gsl_s = arr;
+    std::span std_s = arr;
+
+    gsl::span gsl_cs = gsl_s;
+    std::span std_cs = std_s;
+    EXPECT_EQ(gsl_cs.size(), std_cs.size());
+    EXPECT_EQ(gsl_cs.data(), std_cs.data());
+
+    gsl::span<int, 5> gsl_fs = gsl_s;
+    std::span<int, 5> std_fs = std_s;
+    EXPECT_EQ(gsl_fs.size(), std_fs.size());
+    EXPECT_EQ(gsl_fs.data(), std_fs.data());
+
+    gsl::span<const int, 5> gsl_cfs = gsl_s;
+    std::span<const int, 5> std_cfs = std_s;
+    EXPECT_EQ(gsl_cfs.size(), std_cfs.size());
+    EXPECT_EQ(gsl_cfs.data(), std_cfs.data());
+}
+
+TEST(span_test, deduction_guides)
+{
+    int arr[5] = {1, 2, 3, 4, 5};
+    std::array<int, 5> std_arr = {1, 2, 3, 4, 5};
+    std::vector<int> vec = {1, 2, 3, 4, 5};
+
+    // Test deduction guides for gsl::span
+    gsl::span gsl_s1 = arr;
+    gsl::span gsl_s2 = std_arr;
+    gsl::span gsl_s3 = vec;
+
+    // Test deduction guides for std::span (for sanity checks)
+    std::span std_s1 = arr;
+    std::span std_s2 = std_arr;
+    std::span std_s3 = vec;
+
+    // Compare sizes
+    EXPECT_EQ(gsl_s1.size(), std_s1.size());
+    EXPECT_EQ(gsl_s2.size(), std_s2.size());
+    EXPECT_EQ(gsl_s3.size(), std_s3.size());
+
+    // Compare data pointers
+    EXPECT_EQ(gsl_s1.data(), std_s1.data());
+    EXPECT_EQ(gsl_s2.data(), std_s2.data());
+    EXPECT_EQ(gsl_s3.data(), std_s3.data());
+}
+
+#endif // defined(__cpp_lib_span) && __cpp_lib_span >= 202002L

tests/strict_notnull_tests.cpp

@@ -17,10 +17,28 @@
 #include <gsl/pointers> // for not_null, operator<, operator<=, operator>
 #include <gtest/gtest.h>
 
+#include <type_traits> // for declval
+
 #include "deathTestCommon.h"
 
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
+// stand-in for a user-defined ref-counted class
+template <typename T>
+struct RefCounted
+{
+    RefCounted(T* p) : p_(p) {}
+    operator T*() { return p_; }
+    T* p_;
+};
+
 namespace
 {
 // clang-format off
@@ -43,12 +61,154 @@ GSL_SUPPRESS(f.4) // NO-FORMAT: attribute
 // clang-format on
 bool strict_helper_const(strict_not_null<const int*> p) { return *p == 12; }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
 int* return_pointer() { return nullptr; }
-const int* return_pointer_const() { return nullptr; }
-#endif
 } // namespace
 
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_A = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_A<U, void_t<decltype(gsl::strict_not_null<void*>{std::declval<U>()})>> = true;
+
+template <typename U, int N, typename = void>
+static constexpr bool CtorCompilesFor_B = false;
+template <typename U, int N>
+static constexpr bool CtorCompilesFor_B<U, N, void_t<decltype(gsl::strict_not_null<U>{N})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool DefaultCtorCompilesFor = false;
+template <typename U>
+static constexpr bool DefaultCtorCompilesFor<U, void_t<decltype(gsl::strict_not_null<U>{})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_C = false;
+template <typename U>
+static constexpr bool CtorCompilesFor_C<
+    U, void_t<decltype(gsl::strict_not_null<U*>{std::declval<std::unique_ptr<U>>()})>> = true;
+
+TEST(strict_notnull_tests, TestStrictNotNullConstructors)
+{
+    {
+        static_assert(CtorCompilesFor_A<void*>, "CtorCompilesFor_A<void*>");
+        static_assert(!CtorCompilesFor_A<std::nullptr_t>, "!CtorCompilesFor_A<std::nullptr_t>");
+        static_assert(!CtorCompilesFor_B<void*, 0>, "!CtorCompilesFor_B<void*, 0>");
+        static_assert(!DefaultCtorCompilesFor<void*>, "!DefaultCtorCompilesFor<void*>");
+        static_assert(!CtorCompilesFor_C<int>, "CtorCompilesFor_C<int>");
+#ifdef CONFIRM_COMPILATION_ERRORS
+        // Forbid non-nullptr assignable types
+        strict_not_null<std::vector<int>> f(std::vector<int>{1});
+        strict_not_null<int> z(10);
+        strict_not_null<std::vector<int>> y({1, 2});
+#endif
+    }
+
+    const auto terminateHandler = std::set_terminate([] {
+        std::cerr << "Expected Death. TestNotNullConstructors";
+        std::abort();
+    });
+    const auto expected = GetExpectedDeathString(terminateHandler);
+
+    {
+        // from shared pointer
+        int i = 12;
+        auto rp = RefCounted<int>(&i);
+        strict_not_null<int*> p(rp);
+        EXPECT_TRUE(p.get() == &i);
+
+        strict_not_null<std::shared_ptr<int>> x(
+            std::make_shared<int>(10)); // shared_ptr<int> is nullptr assignable
+
+        int* pi = nullptr;
+        EXPECT_DEATH((strict_not_null<decltype(pi)>(pi)), expected);
+    }
+
+    {
+        // from unique pointer
+        strict_not_null<std::unique_ptr<int>> x(
+            std::make_unique<int>(10)); // unique_ptr<int> is nullptr assignable
+
+        EXPECT_DEATH((strict_not_null<std::unique_ptr<int>>(std::unique_ptr<int>{})), expected);
+    }
+
+    {
+        // from pointer to local
+        int t = 42;
+
+        strict_not_null<int*> x{&t};
+        helper(&t);
+        helper_const(&t);
+
+        EXPECT_TRUE(*x == 42);
+    }
+
+    {
+        // from raw pointer
+        // from strict_not_null pointer
+
+        int t = 42;
+        int* p = &t;
+
+        strict_not_null<int*> x{p};
+        helper(p);
+        helper_const(p);
+        helper(x);
+        helper_const(x);
+
+        EXPECT_TRUE(*x == 42);
+    }
+
+    {
+        // from raw const pointer
+        // from strict_not_null const pointer
+
+        int t = 42;
+        const int* cp = &t;
+
+        strict_not_null<const int*> x{cp};
+        helper_const(cp);
+        helper_const(x);
+
+        EXPECT_TRUE(*x == 42);
+    }
+
+    {
+        // from strict_not_null const pointer, using auto
+        int t = 42;
+        const int* cp = &t;
+
+        auto x = strict_not_null<const int*>{cp};
+
+        EXPECT_TRUE(*x == 42);
+    }
+
+    {
+        // from returned pointer
+
+        EXPECT_DEATH(helper(return_pointer()), expected);
+        EXPECT_DEATH(helper_const(return_pointer()), expected);
+    }
+}
+
+template <typename U, typename = void>
+static constexpr bool StrictHelperCompilesFor = false;
+template <typename U>
+static constexpr bool
+    StrictHelperCompilesFor<U, void_t<decltype(strict_helper(std::declval<U>()))>> = true;
+
+
+template <typename U, typename = void>
+static constexpr bool StrictHelperConstCompilesFor = false;
+template <typename U>
+static constexpr bool
+    StrictHelperConstCompilesFor<U, void_t<decltype(strict_helper_const(std::declval<U>()))>> =
+        true;
+
+
+template <typename U, typename = void>
+static constexpr bool HelperCompilesFor = false;
+template <typename U>
+static constexpr bool HelperCompilesFor<U, void_t<decltype(helper(std::declval<U>()))>> = true;
+
 TEST(strict_notnull_tests, TestStrictNotNull)
 {
     {
@@ -57,20 +217,43 @@ TEST(strict_notnull_tests, TestStrictNotNull)
 
 #ifdef CONFIRM_COMPILATION_ERRORS
         strict_not_null<int*> snn = &x;
-        strict_helper(&x);
-        strict_helper_const(&x);
-        strict_helper(return_pointer());
-        strict_helper_const(return_pointer_const());
 #endif
+        static_assert(!StrictHelperCompilesFor<int*>, "!StrictHelperCompilesFor<int*>");
+        static_assert(!StrictHelperConstCompilesFor<int*>,
+                      "!StrictHelperCompilesFor<int*>");
 
         const strict_not_null<int*> snn1{&x};
 
+        static_assert(StrictHelperCompilesFor<const strict_not_null<int*>>,
+                          "StrictHelperCompilesFor<const strict_not_null<int*>>");
         helper(snn1);
         helper_const(snn1);
 
         EXPECT_TRUE(*snn1 == 42);
     }
 
+    {
+        // raw ptr <-> strict_not_null
+        const int x = 42;
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+        strict_not_null<int*> snn = &x;
+#endif
+        static_assert(!StrictHelperCompilesFor<const int*>, "!StrictHelperFor<const int*>");
+        static_assert(!StrictHelperConstCompilesFor<const int*>,
+                      "!StrictHelperCompilesFor<const int*>");
+
+        const strict_not_null<const int*> snn1{&x};
+
+        static_assert(!HelperCompilesFor<const strict_not_null<const int*>>,
+                      "!HelperCompilesFor<const strict_not_null<const int*>>");
+        static_assert(StrictHelperConstCompilesFor<const strict_not_null<const int*>>,
+                      "StrictHelperCompilesFor<const strict_not_null<const int*>>");
+        helper_const(snn1);
+
+        EXPECT_TRUE(*snn1 == 42);
+    }
+
     {
         // strict_not_null -> strict_not_null
         int x = 42;
@@ -85,6 +268,21 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         EXPECT_TRUE(snn1 == snn2);
     }
 
+    {
+        // strict_not_null -> strict_not_null
+        const int x = 42;
+
+        strict_not_null<const int*> snn1{&x};
+        const strict_not_null<const int*> snn2{&x};
+
+        static_assert(!StrictHelperCompilesFor<strict_not_null<const int*>>,
+                      "!StrictHelperCompilesFor<strict_not_null<const int*>>");
+        strict_helper_const(snn1);
+        strict_helper_const(snn2);
+
+        EXPECT_TRUE(snn1 == snn2);
+    }
+
     {
         // strict_not_null -> not_null
         int x = 42;
@@ -101,6 +299,23 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         EXPECT_TRUE(snn == nn2);
     }
 
+    {
+        // strict_not_null -> not_null
+        const int x = 42;
+
+        strict_not_null<const int*> snn{&x};
+
+        const not_null<const int*> nn1 = snn;
+        const not_null<const int*> nn2{snn};
+
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
+        helper_const(snn);
+
+        EXPECT_TRUE(snn == nn1);
+        EXPECT_TRUE(snn == nn2);
+    }
+
     {
         // not_null -> strict_not_null
         int x = 42;
@@ -125,11 +340,37 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         EXPECT_TRUE(hash_snn(snn1) == hash_snn(nn));
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
     {
-        strict_not_null<int*> p{nullptr};
+        // not_null -> strict_not_null
+        const int x = 42;
+
+        not_null<const int*> nn{&x};
+
+        const strict_not_null<const int*> snn1{nn};
+        const strict_not_null<const int*> snn2{nn};
+
+        static_assert(!StrictHelperCompilesFor<not_null<const int*>>,
+                      "!StrictHelperCompilesFor<not_null<const int*>>");
+        strict_helper_const(nn);
+
+        EXPECT_TRUE(snn1 == nn);
+        EXPECT_TRUE(snn2 == nn);
+
+        std::hash<strict_not_null<const int*>> hash_snn;
+        std::hash<not_null<const int*>> hash_nn;
+
+        EXPECT_TRUE(hash_nn(snn1) == hash_nn(nn));
+        EXPECT_TRUE(hash_snn(snn1) == hash_nn(nn));
+        EXPECT_TRUE(hash_nn(snn1) == hash_nn(snn2));
+        EXPECT_TRUE(hash_snn(snn1) == hash_snn(nn));
     }
-#endif
+}
+
+TEST(pointers_test, member_types)
+{
+    // make sure `element_type` is inherited from `gsl::not_null`
+    static_assert(std::is_same<gsl::strict_not_null<int*>::element_type, int*>::value,
+                  "check member type: element_type");
 }
 
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
@@ -152,6 +393,17 @@ TEST(strict_notnull_tests, TestStrictNotNullConstructorTypeDeduction)
         EXPECT_TRUE(*x == 42);
     }
 
+    {
+        const int i = 42;
+
+        strict_not_null x{&i};
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
+        helper_const(strict_not_null{&i});
+
+        EXPECT_TRUE(*x == 42);
+    }
+
     {
         int i = 42;
         int* p = &i;
@@ -163,6 +415,18 @@ TEST(strict_notnull_tests, TestStrictNotNullConstructorTypeDeduction)
         EXPECT_TRUE(*x == 42);
     }
 
+    {
+        const int i = 42;
+        const int* p = &i;
+
+        strict_not_null x{p};
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
+        helper_const(strict_not_null{p});
+
+        EXPECT_TRUE(*x == 42);
+    }
+
     {
         auto workaround_macro = []() {
             int* p1 = nullptr;

tests/utils_tests.cpp

@@ -19,11 +19,11 @@
 #include <algorithm>   // for move
 #include <complex>
 #include <cstddef>     // for std::ptrdiff_t
+#include <cstdint>     // for uint32_t, int32_t
 #include <functional>  // for reference_wrapper, _Bind_helper<>::type
 #include <gsl/narrow>  // for narrow, narrowing_error
 #include <gsl/util>    // finally, narrow_cast
 #include <limits>      // for numeric_limits
-#include <stdint.h>    // for uint32_t, int32_t
 #include <type_traits> // for is_same
 
 using namespace gsl;
@@ -96,7 +96,7 @@ TEST(utils_tests, finally_function_with_bind)
 {
     int i = 0;
     {
-        auto _ = finally(std::bind(&f, std::ref(i)));
+        auto _ = finally([&i] { return f(i); });
         EXPECT_TRUE(i == 0);
     }
     EXPECT_TRUE(i == 1);
@@ -112,6 +112,16 @@ TEST(utils_tests, finally_function_ptr)
     EXPECT_TRUE(j == 1);
 }
 
+TEST(utils_tests, finally_function)
+{
+    j = 0;
+    {
+        auto _ = finally(g);
+        EXPECT_TRUE(j == 0);
+    }
+    EXPECT_TRUE(j == 1);
+}
+
 TEST(utils_tests, narrow_cast)
 {
     int n = 120;
@@ -149,5 +159,7 @@ TEST(utils_tests, narrow)
     EXPECT_TRUE(
         narrow<std::complex<float>>(std::complex<double>(4, 2)) == std::complex<float>(4, 2));
     EXPECT_THROW(narrow<std::complex<float>>(std::complex<double>(4.2)), narrowing_error);
+
+    EXPECT_TRUE(narrow<int>(float(1)) == 1);
 }
 #endif // GSL_KERNEL_MODE