P1032R1 Miscellaneous constexpr

[StephanTLavavej]

P1032R1 Miscellaneous constexpr
P1424R1 __cpp_lib_constexpr

WG21-P1902 "Missing Feature-Test Macros 2017-2019" further revised this, saying:

[P1032R1] (Misc constexpr bits): This paper originally added a single feature test macro, __cpp_lib_constexpr_misc, that was since renamed by [P1424R1] to __cpp_lib_constexpr. Per the policy laid out in this paper, this paper proposes to remove __cpp_lib_constexpr and instead add separate macros for each header: __cpp_lib_constexpr_functional, __cpp_lib_constexpr_iterator, __cpp_lib_constexpr_string_view, __cpp_lib_constexpr_tuple, and __cpp_lib_constexpr_utility and to bump __cpp_lib_array_constexpr.

Feature-test macros as of WG21-N4842:

#define __cpp_lib_constexpr_iterator 201811L
#define __cpp_lib_constexpr_string_view 201811L
#define __cpp_lib_constexpr_tuple 201811L
#define __cpp_lib_constexpr_utility 201811L

The following added value is unusual (it was further increased for #51 WG21-P1065 "constexpr INVOKE" to 201907L, see SD-6):
#define __cpp_lib_constexpr_functional 201811L

The following increased value is unusual (it was originally increased for #49 WG21-P1023 "constexpr For std::array Comparisons" to 201806L, see SD-6):
#define __cpp_lib_array_constexpr 201811L

[crackedmind]

Just for notice. I'm starting implementing this.

[crackedmind]

Decided to continue conversation from #203 here.
I started with _CONSTEXPR20 define.

And i just realized, that i can't fully finish without compiler support. Because we need char_traits copy/move/assign specialization mark constexpr. But how i can do this without __builtin_memcpy/__builtin_memmove/__buitlin_memset ?

[StephanTLavavej]

So this is yet another feature that's blocked on #156 is_constant_evaluated().

[StephanTLavavej]

is_constant_evaluated() is now available for Clang, and will be available for VS 2019 16.5 Preview 2 in the future, so work on this feature is unblocked (if properly guarded with __cpp_lib_is_constant_evaluated).