Make gsl::span's iterators use the contiguous_iterator concept (#1035)

Resolves #1016

Co-authored-by: Casey Carter <Casey@Carter.net>

include/gsl/span

@@ -25,8 +25,13 @@
 #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)
 
@@ -110,6 +115,9 @@ 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;
@@ -329,8 +337,24 @@ namespace details
         pointer begin_ = nullptr;
         pointer end_ = nullptr;
         pointer current_ = nullptr;
-    };
 
+        template <typename Ptr>
+        friend struct std::pointer_traits;
+    };
+}} // namespace gsl::details
+
+template <class Type>
+struct std::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 gsl { namespace details {
     template <std::size_t Ext>
     class extent_type
     {

tests/span_tests.cpp

@@ -40,6 +40,9 @@
 #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"
 
@@ -1297,3 +1300,20 @@ 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