diff --git a/include/gsl/util b/include/gsl/util
index 330b996..a215bad 100644
--- a/include/gsl/util
+++ b/include/gsl/util
@@ -143,10 +143,10 @@ GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
 
 #if defined(__cpp_lib_span) && __cpp_lib_span >= 202002L
 template <class T, size_t extent = std::dynamic_extent>
-constexpr auto at(std::span<T, extent> sp, const index i)
+constexpr auto at(std::span<T, extent> sp, const index i) -> decltype(sp[sp.size()])
 {
-    Expects(i >= 0 && i < narrow_cast<i>(sp.size()));
-    return sp[i];
+    Expects(i >= 0 && i < narrow_cast<index>(sp.size()));
+    return sp[gsl::narrow_cast<size_t>(i)];
 }
 #endif // __cpp_lib_span >= 202002L
 } // namespace gsl
diff --git a/pipelines/jobs.yml b/pipelines/jobs.yml
index dad2414..3025fdc 100644
--- a/pipelines/jobs.yml
+++ b/pipelines/jobs.yml
@@ -11,16 +11,24 @@ jobs:
     matrix:
       14_debug:
         GSL_CXX_STANDARD: '14'
+        CMAKE_CXX_STANDARD: '14'
         BUILD_TYPE: 'Debug'
       14_release:
         GSL_CXX_STANDARD: '14'
+        CMAKE_CXX_STANDARD: '14'
         BUILD_TYPE: 'Release'
       17_debug:
         GSL_CXX_STANDARD: '17'
+        CMAKE_CXX_STANDARD: '17'
         BUILD_TYPE: 'Debug'
       17_release:
         GSL_CXX_STANDARD: '17'
+        CMAKE_CXX_STANDARD: '17'
         BUILD_TYPE: 'Release'
+      20_debug:
+        GSL_CXX_STANDARD: '17'
+        CMAKE_CXX_STANDARD: '20'
+        BUILD_TYPE: 'Debug'
   continueOnError: false
   steps:
   - template: ./steps.yml
diff --git a/pipelines/steps.yml b/pipelines/steps.yml
index 41a7fca..6cc089b 100644
--- a/pipelines/steps.yml
+++ b/pipelines/steps.yml
@@ -3,7 +3,7 @@ steps:
     name: Configure
     inputs:
       workingDirectory: build
-      cmakeArgs: '-DCMAKE_CXX_STANDARD=$(GSL_CXX_STANDARD) -DCMAKE_BUILD_TYPE=$(BUILD_TYPE) -Werror=dev .. '
+      cmakeArgs: '-DCMAKE_CXX_STANDARD=$(CMAKE_CXX_STANDARD) -DCMAKE_BUILD_TYPE=$(BUILD_TYPE) -Werror=dev .. '
 
   - task: CMake@1
     name: Build
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index c05aaa7..dbd36d3 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -130,6 +130,12 @@ else()
             -Wno-unused-member-function
             -Wno-unused-variable
           >
+          $<$<BOOL:${CMAKE_CXX_STANDARD}>:
+            $<$<EQUAL:${CMAKE_CXX_STANDARD},20>:
+                -Wno-zero-as-null-pointer-constant # Very noisy, minor, and just so
+                -Wno-sign-conversion                                                                   # happens to trigger on googletest
+          >
+        >
         >
         $<$<CXX_COMPILER_ID:Clang>:
           $<$<AND:$<VERSION_GREATER:$<CXX_COMPILER_VERSION>,4.99>,$<VERSION_LESS:$<CXX_COMPILER_VERSION>,6>>:
@@ -232,6 +238,11 @@ else()
         -Wpedantic
         -Wshadow
         -Wsign-conversion
+        $<$<BOOL:${CMAKE_CXX_STANDARD}>:                                                           # happens to trigger on googletest
+          $<$<EQUAL:${CMAKE_CXX_STANDARD},20>:
+              -Wno-sign-conversion                                                                   # happens to trigger on googletest
+          >
+        >
         $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
           -Weverything
           -Wno-c++98-compat
diff --git a/tests/algorithm_tests.cpp b/tests/algorithm_tests.cpp
index 16746b4..e369be0 100644
--- a/tests/algorithm_tests.cpp
+++ b/tests/algorithm_tests.cpp
@@ -27,7 +27,6 @@ namespace gsl
 struct fail_fast;
 } // namespace gsl
 
-using namespace std;
 using namespace gsl;
 
 TEST(algorithm_tests, same_type)
@@ -73,8 +72,8 @@ TEST(algorithm_tests, same_type)
         std::array<int, 5> src{1, 2, 3, 4, 5};
         std::array<int, 10> dst{};
 
-        const span<int> src_span(src);
-        const span<int, 10> dst_span(dst);
+        const gsl::span<int> src_span(src);
+        const gsl::span<int, 10> dst_span(dst);
 
         copy(src_span, dst_span);
         copy(src_span, dst_span.subspan(src_span.size()));
diff --git a/tests/at_tests.cpp b/tests/at_tests.cpp
index 92a8e4d..381ab61 100644
--- a/tests/at_tests.cpp
+++ b/tests/at_tests.cpp
@@ -128,16 +128,22 @@ TEST(at_tests, std_span)
     std::vector<int> cvec{1, 2, 3, 4, 5};
     std::span csp{cvec};
 
-    for (size_t i = 0, i < vec.size(); ++i)
+    for (gsl::index i = 0; i < gsl::narrow_cast<gsl::index>(vec.size()); ++i)
     {
-        EXPECT_TRUE(&gsl::at(sp, i) == &vec[i]);
-        EXPECT_TRUE(&gsl::at(csp, i) == &cvec[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. std_span";
+        std::abort();
+    });
+    const auto expected = GetExpectedDeathString(terminateHandler);
+
     EXPECT_DEATH(gsl::at(sp, -1), expected);
-    EXPECT_DEATH(gsl::at(sp, sp.size()), 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, sp.size()), expected);
+    EXPECT_DEATH(gsl::at(csp, gsl::narrow_cast<gsl::index>(sp.size())), expected);
 }
 #endif // __cplusplus >= 202002L
 
diff --git a/tests/span_compatibility_tests.cpp b/tests/span_compatibility_tests.cpp
index 56bd385..95d8223 100644
--- a/tests/span_compatibility_tests.cpp
+++ b/tests/span_compatibility_tests.cpp
@@ -55,14 +55,14 @@ void ArrayConvertibilityCheck()
         EXPECT_TRUE(sp_const_nullptr_1.data() == stl_nullptr.data());
         EXPECT_TRUE(sp_const_nullptr_1.size() == 3);
 
-        span<const T* const> sp_const_nullptr_2{std::as_const(stl_nullptr)};
+        gsl::span<const T* const> sp_const_nullptr_2{std::as_const(stl_nullptr)};
         EXPECT_TRUE(sp_const_nullptr_2.data() == stl_nullptr.data());
         EXPECT_TRUE(sp_const_nullptr_2.size() == 3);
 
-        static_assert(std::is_same<decltype(span{stl_nullptr}), span<T*, 3>>::value,
+        static_assert(std::is_same<decltype(gsl::span{stl_nullptr}), gsl::span<T*, 3>>::value,
                       "std::is_same< decltype(span{stl_nullptr}), span<T*, 3>>::value");
         static_assert(
-            std::is_same<decltype(span{std::as_const(stl_nullptr)}), span<T* const, 3>>::value,
+            std::is_same<decltype(gsl::span{std::as_const(stl_nullptr)}), gsl::span<T* const, 3>>::value,
             "std::is_same< decltype(span{std::as_const(stl_nullptr)}), span<T* const, "
             "3>>::value");
     }
diff --git a/tests/span_ext_tests.cpp b/tests/span_ext_tests.cpp
index 7ce4e51..3d35f8f 100644
--- a/tests/span_ext_tests.cpp
+++ b/tests/span_ext_tests.cpp
@@ -48,7 +48,7 @@ TEST(span_ext_test, make_span_from_pointer_length_constructor)
 
     {
         int* p = nullptr;
-        auto s = make_span(p, narrow_cast<span<int>::size_type>(0));
+        auto s = make_span(p, narrow_cast<gsl::span<int>::size_type>(0));
         EXPECT_TRUE(s.size() == 0);
         EXPECT_TRUE(s.data() == nullptr);
     }
@@ -136,9 +136,9 @@ TEST(span_ext_test, make_span_from_std_array_constructor)
 
     // This test checks for the bug found in gcc 6.1, 6.2, 6.3, 6.4, 6.5 7.1, 7.2, 7.3 - issue #590
     {
-        span<int> s1 = make_span(arr);
+        gsl::span<int> s1 = make_span(arr);
 
-        static span<int> s2;
+        static gsl::span<int> s2;
         s2 = s1;
 
 #if defined(__GNUC__) && __GNUC__ == 6 && (__GNUC_MINOR__ == 4 || __GNUC_MINOR__ == 5) &&          \
@@ -194,7 +194,7 @@ TEST(span_ext_test, make_span_from_container_constructor)
 TEST(span_test, interop_with_gsl_at)
 {
     int arr[5] = {1, 2, 3, 4, 5};
-    span<int> s{arr};
+    gsl::span<int> s{arr};
     EXPECT_TRUE(at(s, 0) == 1);
     EXPECT_TRUE(at(s, 1) == 2);
 }
@@ -202,7 +202,7 @@ TEST(span_test, interop_with_gsl_at)
 TEST(span_ext_test, iterator_free_functions)
 {
     int a[] = {1, 2, 3, 4};
-    span<int> s{a};
+    gsl::span<int> s{a};
 
     EXPECT_TRUE((std::is_same<decltype(s.begin()), decltype(begin(s))>::value));
     EXPECT_TRUE((std::is_same<decltype(s.end()), decltype(end(s))>::value));
@@ -232,7 +232,7 @@ TEST(span_ext_test, iterator_free_functions)
 TEST(span_ext_test, ssize_free_function)
 {
     int a[] = {1, 2, 3, 4};
-    span<int> s{a};
+    gsl::span<int> s{a};
 
     EXPECT_FALSE((std::is_same<decltype(s.size()), decltype(ssize(s))>::value));
     EXPECT_TRUE(s.size() == static_cast<std::size_t>(ssize(s)));
@@ -242,8 +242,8 @@ TEST(span_ext_test, ssize_free_function)
 TEST(span_ext_test, comparison_operators)
 {
     {
-        span<int> s1;
-        span<int> s2;
+        gsl::span<int> s1;
+        gsl::span<int> s2;
         EXPECT_TRUE(s1 == s2);
         EXPECT_FALSE(s1 != s2);
         EXPECT_FALSE(s1 < s2);
@@ -260,8 +260,8 @@ TEST(span_ext_test, comparison_operators)
 
     {
         int arr[] = {2, 1};
-        span<int> s1 = arr;
-        span<int> s2 = arr;
+        gsl::span<int> s1 = arr;
+        gsl::span<int> s2 = arr;
 
         EXPECT_TRUE(s1 == s2);
         EXPECT_FALSE(s1 != s2);
@@ -280,8 +280,8 @@ TEST(span_ext_test, comparison_operators)
     {
         int arr[] = {2, 1}; // bigger
 
-        span<int> s1;
-        span<int> s2 = arr;
+        gsl::span<int> s1;
+        gsl::span<int> s2 = arr;
 
         EXPECT_TRUE(s1 != s2);
         EXPECT_TRUE(s2 != s1);
@@ -300,8 +300,8 @@ TEST(span_ext_test, comparison_operators)
     {
         int arr1[] = {1, 2};
         int arr2[] = {1, 2};
-        span<int> s1 = arr1;
-        span<int> s2 = arr2;
+        gsl::span<int> s1 = arr1;
+        gsl::span<int> s2 = arr2;
 
         EXPECT_TRUE(s1 == s2);
         EXPECT_FALSE(s1 != s2);
@@ -320,8 +320,8 @@ TEST(span_ext_test, comparison_operators)
     {
         int arr[] = {1, 2, 3};
 
-        span<int> s1 = {&arr[0], 2}; // shorter
-        span<int> s2 = arr;          // longer
+        gsl::span<int> s1 = {&arr[0], 2}; // shorter
+        gsl::span<int> s2 = arr;          // longer
 
         EXPECT_TRUE(s1 != s2);
         EXPECT_TRUE(s2 != s1);
@@ -341,8 +341,8 @@ TEST(span_ext_test, comparison_operators)
         int arr1[] = {1, 2}; // smaller
         int arr2[] = {2, 1}; // bigger
 
-        span<int> s1 = arr1;
-        span<int> s2 = arr2;
+        gsl::span<int> s1 = arr1;
+        gsl::span<int> s2 = arr2;
 
         EXPECT_TRUE(s1 != s2);
         EXPECT_TRUE(s2 != s1);
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index 1e0222e..33ccf56 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -43,7 +43,6 @@
 
 #include "deathTestCommon.h"
 
-using namespace std;
 using namespace gsl;
 
 namespace
@@ -1085,7 +1084,7 @@ TEST(span_test, as_bytes)
 
     int b[5] = {1, 2, 3, 4, 5};
     {
-        span<int> sp(begin(b), static_cast<size_t>(-2));
+        span<int> sp(std::begin(b), static_cast<size_t>(-2));
         EXPECT_DEATH((void) sp.size_bytes(), expected);
     }
 }
diff --git a/tests/string_span_tests.cpp b/tests/string_span_tests.cpp
index ed42b38..45a67d9 100644
--- a/tests/string_span_tests.cpp
+++ b/tests/string_span_tests.cpp
@@ -30,7 +30,6 @@
 
 #include "deathTestCommon.h"
 
-using namespace std;
 using namespace gsl;
 
 // Generic string functions