some libc++ 19.1.0 headers no longer compile with C++11

[edfvogel]

I note that the libc++ 19.1.0 headers contain code that is not valid C++11 code.

For example __type_traits/desugars_to.h contains:

template <class _CanonicalTag, class _Operation, class... _Args>
inline const bool __desugars_to_v = false;

variable templates were introduced in C++14.

If I compile with -Werror -Wstd=c++11 the compiler I use will give an error.

I believe this code (and its uses) should be guarded with _LIBCPP_STD_VERSION >= 14 conditionals.

Thank you for your time in this matter,

Ed Vogel

[frederick-vs-ja]

Actually it's only valid since C++17 given inline is used, but Clang & GCC make these features in old modes as extensions. Also, note that we're not using constexpr because it was not reserved in C++03.

Since libc++ suppresses many pedantic errors/warnings within headers, the headers can be accepted as expected. Godbolt link.

[edfvogel]

Thanks for the quick reply!

So you only support compilation by compilers enable these extensions?

[frederick-vs-ja]

IIUC yes. libc++ only officially supports very recent versions of Clang and GCC. See also #79532.

[edfvogel]

Very good! Again thanks for the quick response! Please close this.