Fails to compile with Clang in c++20 mode when constexpr is disabled #4545
Description
I need to disable constexpr because I am using a custom string class:
#define FMT_USE_CONSTEXPR 0
#define FMT_USE_CONSTEVAL 0
The problem now is that this does not disable FMT_USE_CONSTEVAL. The fix for this is is to not define FMT_USE_CONSTEVAL if it is already defined:
#ifndef FMT_USE_CONSTEVAL <------------------ this change
#if !defined(__cpp_lib_is_constant_evaluated)
# define FMT_USE_CONSTEVAL 0
#elif FMT_CPLUSPLUS < 201709L
# define FMT_USE_CONSTEVAL 0
#elif FMT_GLIBCXX_RELEASE && FMT_GLIBCXX_RELEASE < 10
# define FMT_USE_CONSTEVAL 0
#elif FMT_LIBCPP_VERSION && FMT_LIBCPP_VERSION < 10000
# define FMT_USE_CONSTEVAL 0
#elif defined(__apple_build_version__) && __apple_build_version__ < 14000029L
# define FMT_USE_CONSTEVAL 0 // consteval is broken in Apple clang < 14.
#elif FMT_MSC_VERSION && FMT_MSC_VERSION < 1929
# define FMT_USE_CONSTEVAL 0 // consteval is broken in MSVC VS2019 < 16.10.
#elif defined(__cpp_consteval)
# define FMT_USE_CONSTEVAL 1
#elif FMT_GCC_VERSION >= 1002 || FMT_CLANG_VERSION >= 1101
# define FMT_USE_CONSTEVAL 1
#else
# define FMT_USE_CONSTEVAL 0
#endif
#endif
Finally is_constant_evaluated ignores the value of FMT_USE_CONSTEXPR. This can be fixed by short circuiting it:
constexpr auto is_constant_evaluated(bool default_value = false) noexcept
-> bool {
// is_constant_evaluated is never true if we're not using constexpr
#if !FMT_USE_CONSTEXPR
return false;
#endif
// Workaround for incompatibility between clang 14 and libstdc++ consteval-based
// std::is_constant_evaluated: https://github.com/fmtlib/fmt/issues/3247.
#if FMT_CPLUSPLUS >= 202002L && FMT_GLIBCXX_RELEASE >= 12 && \
(FMT_CLANG_VERSION >= 1400 && FMT_CLANG_VERSION < 1500)
ignore_unused(default_value);
return __builtin_is_constant_evaluated();
#elif defined(__cpp_lib_is_constant_evaluated)
ignore_unused(default_value);
return std::is_constant_evaluated();
#else
return default_value;
#endif
}
This allows it to compile and run