C++17: std::char_traits<>::{compare,length} is constexpr - v2

[phprus]

Fixed my mistake in the previous patch.

Fix for Microsoft Visual Studio and clang/libc++ < 4.0.

Discussion: #2246 (comment)

Links:
https://docs.microsoft.com/en-us/cpp/overview/visual-cpp-language-conformance?view=msvc-150
https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-150
https://libcxx.llvm.org/docs/Cxx1zStatus.html

[vitaut]

Thank you!

[vitaut]

BTW Jonathan Wakely suggested using __cpp_lib_constexpr_char_traits to detect this in libstdc++: https://twitter.com/wakomeup/status/1387422935590260738.

[phprus]

https://github.com/gcc-mirror/gcc/blob/releases/gcc-7.1.0/libstdc++-v3/include/bits/char_traits.h:

// #define __cpp_lib_constexpr_char_traits 201611

Macro __cpp_lib_constexpr_char_traits defined only in gcc-8 and newer, but constexpr char_traits is presented in gcc-7.

Tweet not found ("Hmm...this page doesn’t exist. Try searching for something else.") :(

[vitaut]

On further investigation it turned out that __cpp_lib_constexpr_char_traits is not defined consistently.

[vitaut]

Tweet not found ("Hmm...this page doesn’t exist

It was retracted =)