Make gsl::span's iterators use the contiguous_iterator concept

[dmitrykobets-msft]

Resolves #1016

The only missing part to satisfy the contiguous_iterator concept was to use the contiguous_iterator_tag.
Used c++ - In C++20, how do I write a contiguous iterator? - Stack Overflow as a reference.

[sunnychatterjee]

Looks good, thanks for the fix Dmitry!

[CaseyCarter]

We should use __cpp_lib_ranges to detect support for std::contiguous_iterator_tag, __cpp_lib_concepts only indicates that the contents of <concepts> are available. Note that we have no portable guarantee that __cpp_lib_ranges is defined unless we've included the C++20 header <ranges> or <version>; I suggest conditionally including <version> on line 29ish:

#ifdef __has_include
#if __has_include(<version>)
#include <version>
#endif
#endif

and changing this line (and line 115) to instead refer to __cpp_lib_ranges.

[dmitrykobets-msft]

In STL span, their usage of std::contiguous_iterator_tag is guarded by __cpp_lib_concepts

template <class _Ty>
struct _Span_iterator {
#ifdef __cpp_lib_concepts
    using iterator_concept = contiguous_iterator_tag;
#endif // __cpp_lib_concepts

In VS 2019 it looks like __cpp_lib_ranges is guarded by CXX23, in yvals_core.h, and this condition is not met. So the changes in this PR do not compile in VS2019.

#if _HAS_CXX23 && defined(__cpp_lib_concepts) // TRANSITION, GH-395 and GH-1814
#define __cpp_lib_ranges 201911L
#endif // _HAS_CXX23 && defined(__cpp_lib_concepts)

I see that your STL PR microsoft/STL#2518 addresses that, but those changes are only available in VS2022.

[CaseyCarter]

If you want to support VS 2019 before we backport the final C++20 changes (which will be another month or two) you should still have the portable test for __cpp_lib_ranges but also specifically detect MSVCSTL in "close enough" mode:

#if defined(__cpp_lib_ranges) || (defined(_MSVC_STL_VERSION) && defined(__cpp_lib_concepts))

[CaseyCarter]

I recommend:

Suggested change

	gsl::span<int> gsl_span;
	std::span<int> std_span = gsl_span;
	(void)std_span; // suppress unused variable warning
	int arr[5] = {1, 2, 3, 4, 5};
	gsl::span<int> gsl_span{arr};
	#ifdef __cpp_lib_ranges
	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());

which will fail at EXPECT_TRUE(std::to_address(gsl_span.end()) == gsl_span.data() + gsl_span.size()); since operator-> isn't well-defined for the end iterator of a span. (contiguous_iterator requires std::to_address(meow) to work, which defaults to returning operator->().)

I assume you don't want operator-> to work for past-the-end iterators any more than we do, so you'll need to specialize std::pointer_traits and implement to_address to tell std::to_address how to convert your iterators into pointers.

Actually, it would be easier to make a PR into your branch than type this up. I've submitted dmitrykobets-msft#1.

[CaseyCarter]

The VS 2019 clang-cl tests are failing because that old version of clang doesn't recognize the reserved-identifier warning suppression. You'll need to either figure out how to avoid using it with older clangs, or add -Wno-unknown-warning-option to tell clang not to complain about it.