We already have the attribute on std::__is_constant_evaluated() but for
some reason not on std::is_constant_evaluated().
libstdc++-v3/ChangeLog:
* include/std/type_traits (is_constant_evaluated): Add
always_inline attribute.
#ifdef __cpp_lib_is_constant_evaluated // C++ >= 20 && HAVE_IS_CONST_EVAL
/// Returns true only when called during constant evaluation.
/// @since C++20
- constexpr inline bool
+ [[__gnu__::__always_inline__]]
+ constexpr bool
is_constant_evaluated() noexcept
{
#if __cpp_if_consteval >= 202106L