[libcxx-dev] How to check for a feature-test macro

[Marshall Clow via libcxx-dev]

In the tests, we have the following usage:

#if TEST_STD_VER > 14
# if !defined(__cpp_lib_filesystem)
#  error "__cpp_lib_filesystem is not defined"
# elif __cpp_lib_filesystem < 201703L
#  error "__cpp_lib_filesystem has an invalid value"
# endif
#endif

I submit that that's non-portable, because some standard libraries may not
implement all the features (that's the point of the feature-test macro),
and this test should not fail.

In D55308, I am using the following form:

#if TEST_STD_VER > 17
  LIBCPP_ASSERT(IS_DEFINED(__cpp_lib_char8_t)); // libc++ implements this
# if defined(__cpp_lib_char8_t)
#  if __cpp_lib_char8_t < 201811L
#   error "__cpp_lib_char8_t has an invalid value"
#  endif
# endif
#endif

Basically it (unconditionally) checks that if the macro is defined, then it
has a sane value.
Additionally, since we know that libc++ defines this, we check that.

An alternate formulation - w/o the `IS_DEFINED` trickery.

#if TEST_STD_VER > 17
# if !defined(__cpp_lib_char8_t)
  LIBCPP_STATIC_ASSERT(false, "__cpp_lib_char8_t is not defined");
# else
#  if __cpp_lib_char8_t < 201811L
#   error "__cpp_lib_char8_t has an invalid value"
#  endif
# endif
#endif

Comments welcome.

-- Marshall
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