From cdd9ee5a6ac673fbf632f6b914821bb2cfaf7f05 Mon Sep 17 00:00:00 2001
From: Jeroen Akershoek <jeroen.akershoek@gmail.com>
Date: Tue, 5 Dec 2017 23:42:12 +0200
Subject: [PATCH] Improved conformance of gsl::span for compile-time errors

as per 23.7.2.1:
- span() and span(nullptr) can only be used if extent is dynamic or 0
- span( element_type(&arr)[N] ) can only be used if extent is dynamic or N
- std::array can be used for initialization if extent is dynamic or size of the array
- Containers can be used for initialization if pointer type is convertible

as per 23.7.2.2
- first<N> and last<N> may no longer exceed the span extents
---
 include/gsl/span     | 35 +++++++++++++++++++++++------------
 tests/span_tests.cpp |  1 +
 2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/include/gsl/span b/include/gsl/span
index bd9a21d..7cfdadd 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -342,6 +342,7 @@ public:
     {
     }
 
+    template <class = std::enable_if_t<Extent == dynamic_extent || Extent == 0>>
     constexpr span(std::nullptr_t) GSL_NOEXCEPT : span() {}
 
     constexpr span(pointer ptr, index_type count) : storage_(ptr, count) {}
@@ -351,19 +352,28 @@ public:
     {
     }
 
-    template <std::size_t N>
+    template <std::size_t N, class = std::enable_if_t<extent == dynamic_extent || N == extent>>
     constexpr span(element_type (&arr)[N]) GSL_NOEXCEPT
         : storage_(&arr[0], details::extent_type<N>())
     {
     }
 
-    template <std::size_t N, class ArrayElementType = std::remove_const_t<element_type>>
-    constexpr span(std::array<ArrayElementType, N>& arr) GSL_NOEXCEPT
+    template <std::size_t N, class ArrayElementType = std::remove_const_t<element_type>,
+              class = std::enable_if_t<
+                  ( extent == dynamic_extent || N == extent ) &&
+                  std::is_convertible_v<std::add_pointer_t<ArrayElementType>, pointer>
+    >>
+    constexpr span( std::array<ArrayElementType, N>& arr )
         : storage_(&arr[0], details::extent_type<N>())
     {
     }
 
-    template <std::size_t N>
+    template <std::size_t N,
+              class = std::enable_if_t<
+                  ( extent == dynamic_extent || N == extent ) &&
+                  std::is_const_v<element_type> &&
+                  std::is_convertible_v<std::add_pointer_t<element_type>, pointer>
+    >>
     constexpr span(const std::array<std::remove_const_t<element_type>, N>& arr) GSL_NOEXCEPT
         : storage_(&arr[0], details::extent_type<N>())
     {
@@ -387,9 +397,7 @@ public:
     template <class Container,
               class = std::enable_if_t<
                   !details::is_span<Container>::value && !details::is_std_array<Container>::value &&
-                  std::is_convertible<typename Container::pointer, pointer>::value &&
-                  std::is_convertible<typename Container::pointer,
-                                      decltype(std::declval<Container>().data())>::value>>
+                  std::is_convertible<decltype(std::declval<Container>().data()), pointer>::value>>
     constexpr span(Container& cont) : span(cont.data(), narrow<index_type>(cont.size()))
     {
     }
@@ -397,9 +405,8 @@ public:
     template <class Container,
               class = std::enable_if_t<
                   std::is_const<element_type>::value && !details::is_span<Container>::value &&
-                  std::is_convertible<typename Container::pointer, pointer>::value &&
-                  std::is_convertible<typename Container::pointer,
-                                      decltype(std::declval<Container>().data())>::value>>
+                  !details::is_std_array<Container>::value &&
+                  std::is_convertible<decltype(std::declval<Container>().data()), pointer>::value>>
     constexpr span(const Container& cont) : span(cont.data(), narrow<index_type>(cont.size()))
     {
     }
@@ -433,14 +440,18 @@ public:
     constexpr span& operator=(span&& other) GSL_NOEXCEPT = default;
 
     // [span.sub], span subviews
-    template <std::ptrdiff_t Count>
+    template <std::ptrdiff_t Count,
+        class = std::enable_if_t<
+            Count >= 0 && ( extent == dynamic_extent || Count <= extent )>>
     constexpr span<element_type, Count> first() const
     {
         Expects(Count >= 0 && Count <= size());
         return {data(), Count};
     }
 
-    template <std::ptrdiff_t Count>
+    template <std::ptrdiff_t Count,
+        class = std::enable_if_t<
+            Count >= 0 && ( extent == dynamic_extent || Count <= extent )>>
     constexpr span<element_type, Count> last() const
     {
         Expects(Count >= 0 && size() - Count >= 0);
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index b14ddf0..e0d16c9 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -23,6 +23,7 @@
 #include <array>       // for array
 #include <iostream>    // for ptrdiff_t
 #include <iterator>    // for reverse_iterator, operator-, operator==
+#include <map>         // for std::map
 #include <memory>      // for unique_ptr, shared_ptr, make_unique, allo...
 #include <regex>       // for match_results, sub_match, match_results<>...
 #include <stddef.h>    // for ptrdiff_t