Fix std::variant, std::filesystem::path tests on GCC-8, Clang-7,8.

[phprus]

libstdc++ from GCC-8 support std::variant.
libstdc++ from GCC-8 has no <version> file.
libstdc++ from GCC-8 defines the macro __cpp_lib_variant in <variant>.

fmt/test/gtest/gtest/gtest.h

Lines 2621 to 2627 in 6a775e9

	#ifdef __has_include
	#if __has_include(<variant>) && __cplusplus >= 201703L
	// Otherwise for C++17 and higher use std::variant for UniversalPrinter<>
	// specializations.
	#define GTEST_INTERNAL_HAS_VARIANT 1
	#include <variant>
	namespace testing {

Because gtest.h include <variant>, then the macro __cpp_lib_variant is defined in the test, but is not defined in "fmt/std.h".

[vitaut]

Thanks for the PR. Unfortunately it only masks the issue so I suggest including <variant> in fmt/std.h when the header exists instead. This can be checked with FMT_HAS_INCLUDE.

[phprus]

@vitaut, Done.
I have added two new CI configurations to detect such errors.

[vitaut]

What is this check for? Shouldn't FMT_HAS_INCLUDE and feature test macro checks be enough?

[phprus]

Without FMT_CPLUSPLUS check:
libstdc++: ok (Checking the version of C++ standard is present)
libcxx: ok (Checking the version of C++ standard is absent)
MSVC message:

#if !_HAS_CXX17
#pragma message("The contents of <filesystem> are available only with C++17 or later.")
#else // ^^^ !_HAS_CXX17 / _HAS_CXX17 vvv

https://github.com/microsoft/STL/blob/c34f24920e463a71791c2ee3d2bed14926518965/stl/inc/filesystem#L12-L14
https://github.com/microsoft/STL/blob/c34f24920e463a71791c2ee3d2bed14926518965/stl/inc/variant#L12-L14
https://godbolt.org/z/1MM9Kee8n

For this reason, a check is required.

[vitaut]

Could you add a short comment clarifying that this check is for MSVC?

[phprus]

@vitaut Done

[vitaut]

Is it possible to simplify this?

[phprus]

The FS-library name is determined by the version of the C++ Standard Library.
CMake only determine the compiler version.
Clang supports libstdc++ and libcxx.
Intel C++ supports libstdc++ on Linux, libcxx on macOS, and Microsoft STL on Windows.
Therefore, the only way to determine which standard library is actually used is by compiling the code.

The code I wrote is the easiest way I know.

[vitaut]

Can this information be derived from CMAKE_CXX_STANDARD_LIBRARIES or CMAKE_CXX_IMPLICIT_LINK_LIBRARIES?

[phprus]

The variable CMAKE_CXX_IMPLICIT_LINK_LIBRARIES allows to detect the standard library name, but not its version.

Example:

GCC-8:
    CMAKE_CXX_IMPLICIT_LINK_LIBRARIES: stdc++;m;gcc_s;gcc;c;gcc_s;gcc

GCC-9:
    CMAKE_CXX_IMPLICIT_LINK_LIBRARIES: stdc++;m;gcc_s;gcc;c;gcc_s;gcc

Clang 13 + libstdc++ (from GCC-11):
    CMAKE_CXX_IMPLICIT_LINK_LIBRARIES: stdc++;m;gcc_s;gcc;c;gcc_s;gcc

Clang 13 + libc++:
    CMAKE_CXX_IMPLICIT_LINK_LIBRARIES: c++;m;gcc_s;gcc;c;gcc_s;gcc

CMAKE_CXX_STANDARD_LIBRARIES - CMake 3.6+ and empty in most configurations

[vitaut]

This include looks unused.

[phprus]

The macros _GLIBCXX_RELEASE and _LIBCPP_VERSION is defined in standard library header files.

<exception> is the smallest of the standard library header files.

[vitaut]

I see. Maybe include <initializer_list> which is slightly cheaper according to https://artificial-mind.net/projects/compile-health/ and add a comment that this is for _GLIBCXX_RELEASE & _LIBCPP_VERSION?

[phprus]

<initializer_list> is C++11.
Specifying the version of the standard will complicate the call to try_compile.

Comment added.

[vitaut]

What is this for?

[phprus]

Compiling detect-stdfs.cc and read compiler output to get fslib info from error message.

[vitaut]

I mean specifically the above line that forwards CMAKE_OSX_ARCHITECTURES.

[phprus]

CMAKE_OSX_ARCHITECTURES affects the compiler but I'm not sure if it affects the standard library as well.
Removed it.

[vitaut]

Merged, thanks!