consteval error with FreeBSD 13.1 and Clang13

[phire]

/usr/home/buildbot/freebsd/pr-freebsd-x64/build/Source/UnitTests/Common/BitFieldTest.cpp:234:27: error: call to consteval function 'fmt::basic_format_string<char, BitField<0, 64, unsigned long, unsigned long> &>::basic_format_string<char [7], 0>' is not a constant expression
    EXPECT_EQ(fmt::format("{:02x}", object.full_u64),
                          ^
/usr/home/buildbot/freebsd/pr-freebsd-x64/build/Source/UnitTests/Common/BitFieldTest.cpp:234:27: note: undefined constructor 'basic_format_string<char [7], 0>' cannot be used in a constant expression
/usr/home/buildbot/freebsd/pr-freebsd-x64/build/Externals/fmt/include/fmt/core.h:3115:28: note: declared here
  FMT_CONSTEVAL FMT_INLINE basic_format_string(const S& s) : str_(s) {
                           ^
/usr/home/buildbot/freebsd/pr-freebsd-x64/build/Source/UnitTests/Common/BitFieldTest.cpp:236:27: error: call to consteval function 'fmt::basic_format_string<char, BitField<0, 64, long, long> &>::basic_format_string<char [7], 0>' is not a constant expression
    EXPECT_EQ(fmt::format("{:02x}", object.full_s64),

I could try and make a minimal reproduction and/or test on master, but I don't have a freebsd system setup, and this is just #2455 again. If you need one, just ask.

Looks like last time you worked around this issue by disabling consteval for all AppleClang. But it seems the bug isn't in AppleClang, just when you have both Regular Clang and libc++. And now we have a second OS with libc++ plus a new enough version of Clang.

Our testing showed that upgrading to Clang14 fixed the issue. Also forcing FMT_CONSTEVAL off on FreeBSD + Clang13 fixed it dolphin-emu/dolphin#10973, which is the solution we will go with.

But maybe you should update master to check for _LIBCPP_VERSION instead of __apple_build_version__. Also, since this seems to be fixed in later versions of Clang, maybe you should limit that check to just Clang 13?

[mpiatka]

I run into the same or similar issue on both GCC and clang on c++20. Reproduces on current master. Minimal reproduction:

#include <string_view>
#include <fmt/core.h>

extern int get_number();

template<typename... Args>
inline void log_msg(std::string_view msg, Args... args){
    fmt::print(msg, args...);
}

int main(){
    log_msg("{}\n", get_number());

    return 0;
}

compiler explorer link

[vitaut]

@mpiatka, the error in your example is unrelated and expected because the format string should be known at compile time: https://fmt.dev/latest/api.html#core-api.

[vitaut]

@phire, could you provide a repro (ideally on godbolt)?

[phire]

Godbolt doesn't seem to have freebsd, but I'll download a VM and get you a repro.

[vitaut]

But maybe you should update master to check for _LIBCPP_VERSION instead of apple_build_version.

I don't think it's a good idea because this config works on some systems, e.g. https://godbolt.org/z/rMcYPj66P.

[vitaut]

I wasn't able to repro the issue on the following test case:

test.cc:

#include <fmt/core.h>

int main() {
  uint64_t full_u64;
  auto s = fmt::format("{:02x}", full_u64);
}

% uname -a
FreeBSD freebsd 13.1-RELEASE-p1 FreeBSD 13.1-RELEASE-p1 GENERIC amd64
% clang++ --version
FreeBSD clang version 13.0.0 ([email protected]:llvm/llvm-project.git llvmorg-13.0.0-0-gd7b669b3a303)
Target: x86_64-unknown-freebsd13.1
Thread model: posix
InstalledDir: /usr/bin

Compile command: c++ -c test.cc -I include -std=c++20.

Please provide a self-contained repro.

[vitaut]

Closing as I wasn't able to reproduce the issue in the described configuration. Feel free to reopen with more repro details.