gsl_byte should inspect __cpp_lib_byte in addition to _HAS_STD_BYTE

[StephanTLavavej]

GSL/include/gsl/gsl_byte

Lines 43 to 54 in 3d56ba9

	#ifndef GSL_USE_STD_BYTE
	// this tests if we are under MSVC and the standard lib has std::byte and it is enabled
	#if defined(_HAS_STD_BYTE) && _HAS_STD_BYTE
	#define GSL_USE_STD_BYTE 1
	#else // defined(_HAS_STD_BYTE) && _HAS_STD_BYTE
	#define GSL_USE_STD_BYTE 0
	#endif // defined(_HAS_STD_BYTE) && _HAS_STD_BYTE
	#endif // GSL_USE_STD_BYTE

inspects an implementation-specific macro _HAS_STD_BYTE defined by MSVC's STL:

https://github.com/microsoft/STL/blob/447f879b136950baf3ca35dfb54c359494fa2a77/stl/inc/yvals_core.h#L475-L477

instead of the Standard feature-test macro __cpp_lib_byte:

https://github.com/microsoft/STL/blob/447f879b136950baf3ca35dfb54c359494fa2a77/stl/inc/yvals_core.h#L888-L890

GSL could inspect both, which would make it compatible with:

• VS 2017 15.3 (which added std::byte and _HAS_STD_BYTE) through 15.7
• VS 2017 15.8 (which added __cpp_lib_byte) through VS 2019 today
• Future versions of VS (when GSL stops inspecting _HAS_STD_BYTE alone, VS can stop defining it).

[JordanMaples]

Stephan T. Lavavej (@StephanTLavavej) The changes to gsl_byte are merged.