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C++17: std::char_traits<>::{compare,length} is constexpr - v2 #2257

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Apr 28, 2021
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C++17: std::char_traits<>::{compare,length} is constexpr - v2 #2257

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23 changes: 18 additions & 5 deletions include/fmt/core.h
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Original file line number Diff line number Diff line change
Expand Up @@ -44,14 +44,27 @@
#endif

// Check if constexpr std::char_traits<>::compare,length is supported.
// libstdc++: present on GCC 7 and newer and __cplusplus >= 201703L
// MSVC, libc++: always if __cplusplus >= 201703L
//
// libstdc++: GCC 7 and newer and __cplusplus >= 201703L
// MSVC : VS 2017 15.7 and newer and /std:c++17
// https://docs.microsoft.com/en-us/cpp/overview/visual-cpp-language-conformance
// https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
// libc++ : 4.0 and newer and if __cplusplus >= 201703L
// https://libcxx.llvm.org/docs/Cxx1zStatus.html
//
// NOTE: FMT_GCC_VERSION - is not libstdc++ version.
// _GLIBCXX_RELEASE - is present in GCC 7 libstdc++ and newer.
#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
# ifndef __GLIBCXX__
#if defined(_MSC_VER)
# if _MSVC_LANG >= 201703L && _MSC_VER >= 1914
# define FMT_CONSTEXPR_CHAR_TRAITS constexpr
# elif defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE >= 7
# endif
#elif defined(_LIBCPP_VERSION)
# if __cplusplus >= 201703L && _LIBCPP_VERSION >= 4000
# define FMT_CONSTEXPR_CHAR_TRAITS constexpr
# endif
#elif defined(__GLIBCXX__)
# if __cplusplus >= 201703L && defined(_GLIBCXX_RELEASE) && \
_GLIBCXX_RELEASE >= 7
# define FMT_CONSTEXPR_CHAR_TRAITS constexpr
# endif
#endif
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