Should we consistently use __cpp_char8_t or __cpp_lib_char8_t?

[frederick-vs-ja]

MSVC STL supports C++20/23 without char8_t and C++14/17 with char8_t dialects (see #2748). As a result, availability of some components is controlled by feature-test macros rather than standard revisions.

However, there doesn't seem a general whether the predefined feature-test macro __cpp_char8_t or the library feature-test macro __cpp_lib_char8_t should be used for controlling, and both of them are currently used.

STL/stl/inc/xstring

Lines 482 to 485 in c53ac59

	#ifdef __cpp_char8_t
	template <>
	struct char_traits<char8_t> : _Narrow_char_traits<char8_t, unsigned int> {};
	#endif // defined(__cpp_char8_t)

STL/stl/inc/xstring

Lines 1789 to 1791 in c53ac59

	#ifdef __cpp_lib_char8_t
	_EXPORT_STD using u8string_view = basic_string_view<char8_t>;
	#endif // defined(__cpp_lib_char8_t)

Presumably we should consistently use one for controlling.

[StephanTLavavej]

There was a rationale behind this scheme. I'll get more details for you.

[StephanTLavavej]

The rationale is that /Zc:char8_t is (perhaps surprisingly) available as an opt-in mode in C++14/17. In the STL, we enable minimal recognition of C++14/17 with char8_t by inspecting the compiler macro __cpp_char8_t. This is usually in always-available components that need to react to char8_t's presence, e.g. the type traits like is_integral_v:

STL/stl/inc/xtr1common

Lines 187 to 193 in c53ac59

	_EXPORT_STD template <class _Ty>
	_INLINE_VAR constexpr bool is_integral_v = _Is_any_of_v<remove_cv_t<_Ty>, bool, char, signed char, unsigned char,
	wchar_t,
	#ifdef __cpp_char8_t
	char8_t,
	#endif // defined(__cpp_char8_t)
	char16_t, char32_t, short, unsigned short, int, unsigned int, long, unsigned long, long long, unsigned long long>;

However, we still don't want to enable C++20 library tech downlevel, so that's what __cpp_lib_char8_t guards. We would usually use plain _HAS_CXX20 guards, except that /Zc:char8_t- acts as an opt-out here. So that's why library tech like u8string_view is guarded this way:

STL/stl/inc/xstring

Lines 1789 to 1791 in c53ac59

	#ifdef __cpp_lib_char8_t
	_EXPORT_STD using u8string_view = basic_string_view<char8_t>;
	#endif // defined(__cpp_lib_char8_t)