Silence Clang -Wc++20-extensions

[StephanTLavavej]

In pair, tuple, and optional, we use C++20 conditional explicit (aka explicit(bool)) to simplify constructors and improve compiler throughput, when compiler support is available:

STL/stl/inc/optional

Lines 182 to 193 in 0cbd1b2

	#if _HAS_CONDITIONAL_EXPLICIT
	template <class _Ty2 = _Ty, enable_if_t<_AllowDirectConversion<_Ty2>::value, int> = 0>
	constexpr explicit(!is_convertible_v<_Ty2, _Ty>) optional(_Ty2&& _Right)
	: _Mybase(in_place, _STD forward<_Ty2>(_Right)) {}
	#else // ^^^ _HAS_CONDITIONAL_EXPLICIT ^^^ / vvv !_HAS_CONDITIONAL_EXPLICIT vvv
	template <class _Ty2 = _Ty,
	enable_if_t<conjunction_v<_AllowDirectConversion<_Ty2>, is_convertible<_Ty2, _Ty>>, int> = 0>
	constexpr optional(_Ty2&& _Right) : _Mybase(in_place, _STD forward<_Ty2>(_Right)) {}
	template <class _Ty2 = _Ty,
	enable_if_t<conjunction_v<_AllowDirectConversion<_Ty2>, negation<is_convertible<_Ty2, _Ty>>>, int> = 0>
	constexpr explicit optional(_Ty2&& _Right) : _Mybase(in_place, _STD forward<_Ty2>(_Right)) {}
	#endif // ^^^ !_HAS_CONDITIONAL_EXPLICIT ^^^

Like C++17 if constexpr, MSVC supports C++20 explicit(bool) in older Standard modes, with a suppressible warning. (This is because the feature requires novel syntax, so it can't be accidentally used, and it's extremely useful to library developers.) We suppress both warnings in STL headers:

STL/stl/inc/yvals_core.h

Lines 368 to 380 in 0cbd1b2

	// warning C4984: 'if constexpr' is a C++17 language extension
	#if !_HAS_CXX17 && _HAS_IF_CONSTEXPR
	#define _STL_DISABLED_WARNING_C4984 4984
	#else // !_HAS_CXX17 && _HAS_IF_CONSTEXPR
	#define _STL_DISABLED_WARNING_C4984
	#endif // !_HAS_CXX17 && _HAS_IF_CONSTEXPR
	// warning C5053: support for 'explicit(<expr>)' in C++17 and earlier is a vendor extension
	#if !_HAS_CXX20 && _HAS_CONDITIONAL_EXPLICIT
	#define _STL_DISABLED_WARNING_C5053 5053
	#else // !_HAS_CXX20 && _HAS_CONDITIONAL_EXPLICIT
	#define _STL_DISABLED_WARNING_C5053
	#endif // !_HAS_CXX20 && _HAS_CONDITIONAL_EXPLICIT

Clang 9 supported explicit(bool) in C++20 mode, while Clang 10 added "downlevel" support similar to MSVC. When #708 changed the STL to require Clang 10, it also began using Clang 10's downlevel support for explicit(bool).

@cbezault discovered that we forgot to suppress the warning that Clang emits for explicit(bool) in older Standard modes. We missed this because Clang (unlike MSVC) ordinarily suppresses warnings from "system headers". (-Wsystem-headers activates warnings in system headers, although this is rarely used because users are rarely interested in such warnings, and UCRT/WinSDK headers are still being cleaned up.)

This is a practical issue because anything on the INCLUDE path is considered to be a system header for warning purposes, while /I directories aren't (despite #include <meow> searching both INCLUDE and /I). During STL development, it is very convenient to be able to compile with /I S:\GitHub\STL\stl\inc and immediately consume updated STL headers without rebuilding (this is not possible when separately compiled changes are involved, of course), which will emit Clang warnings.

After this lengthy backstory, the fix is delightfully simple.

[BillyONeal]

:sigh: I really wish clang had a 'there are no system headers' option :(

[CaseyCarter]

Thanks for the cleanup!

[miscco]

If we allow C++17 if constexpr in older code why are there still so many #ifdefs?
Is it solely because of EDG or could one get rid of all that old tag dispatch?

[CaseyCarter]

If we allow C++17 if constexpr in older code why are there still so many #ifdefs?

Not all of our front ends allow if constexpr in C++14 mode: CUDA 9.2 is now the remaining problem child. Once we're done updating our required CUDA version to 10.1 update 2 (#639) we can remove the #ifdefs (#189).