fmt::v8: bigint build failure with clang-10

[jcelerier]

#define FMT_HEADER_ONLY
#include <fmt/format.h>
int main() 
{
  fmt::detail::bigint b;
  b.square();
}

causes

In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:2983:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format-inl.h:557:49: error: variable of non-literal type 'basic_memory_buffer<fmt::v8::detail::bigint::bigit, bigits_capacity>' (aka 'basic_memory_buffer<unsigned int, bigits_capacity>') cannot be defined in a constexpr function
    basic_memory_buffer<bigit, bigits_capacity> n(std::move(bigits_));
                                                ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:651:13: note: 'basic_memory_buffer<unsigned int, 32, std::allocator<unsigned int> >' is not literal because it has data member 'alloc_' of non-literal type 'std::allocator<unsigned int>'
  Allocator alloc_;
            ^

repro: https://godbolt.org/z/nT1TP4Wof ; maybe FMT_CONSTEXPR20 needs to start from clang-11 (where the above code compiles without issues)

[jcelerier]

e.g. something like

#if ((__cplusplus >= 202002L) && \
     (!FMT_CLANG_VERSION || FMT_CLANG_VERSION >= 1100)) || \
    (__cplusplus >= 201709L && FMT_GCC_VERSION >= 1002)
#  define FMT_CONSTEXPR20 constexpr
#else
#  define FMT_CONSTEXPR20
#endif

(can make a pr if you want and that sounds ok)

[alexezeder]

The error is actually caused by libstdc++ (GCC standard library) headers, not the Clang or libc++ (LLVM standard library) - https://godbolt.org/z/dbqre1TaW. So Clang 10 config has libstdc++ with major version 9, while Clang 11 config has major version 10. (thanks to @phprus for _GLIBCXX_RELEASE macro)

[phprus]

libstdc++ defined macro _GLIBCXX_RELEASE with major libstdc++ version.

[jcelerier]

I think that the libstdc++ version comes from the gcc version installed on the docker image used, which is what clang will use by default ; what would be interesting is to see whether clang-10 with libstdc++-11 would work. hmm..

[jcelerier]

and indeed, g++-9 in -std=c++2a mode does define __cplusplus to 201709 so this case is not encountered with GCC, only with Clang ✕ libstdc++

[jcelerier]

#if ((__cplusplus >= 202002L) && \
     (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE > 9)) || \
    (__cplusplus >= 201709L && FMT_GCC_VERSION >= 1002)
#  define FMT_CONSTEXPR20 constexpr
#else
#  define FMT_CONSTEXPR20
#endif

[vitaut]

PR is welcome!

[alexezeder]

#if ((__cplusplus >= 202002L) && \
     (!defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE > 9)) || \
    (__cplusplus >= 201709L && FMT_GCC_VERSION >= 1002)
#  define FMT_CONSTEXPR20 constexpr
#else
#  define FMT_CONSTEXPR20
#endif

Yeah, that should work - https://godbolt.org/z/vYP7MWfz9