fmtlib
Constexpr formatted_size
Minimal changes to make fmt::formatted_size constexpr in C++20. Adjust the existing unit test accordingly.
CI is failing: https://github.com/fmtlib/fmt/runs/7734651761?check_suite_focus=true
Seems Clang-11 has some issue with memcpy not being constexpr. Trying to repro but for me clang 11 actually crashes...
OK got a repro. I was missing that I needed to use libc++. Will investigate.
Seems the problem is here:
https://github.com/fmtlib/fmt/blob/master/include/fmt/format.h#L295-L305
I think Clang 11 supports -std=c++20, but the included libc++ doesn't have __cpp_lib_bit_cast, so we end up trying to use memcpy in a constexpr function.
For C++20 constexpr fmt needs a bit_cast.
bit_cast requires (https://en.cppreference.com/w/cpp/compiler_support):
GCC libstdc++: 11 (_GLIBCXX_RELEASE >= 11, based on: __builtin_bit_cast)
Clang libc++: 14 (_LIBCPP_VERSION >= 14000, based on: __builtin_bit_cast)
MSVC STL: 19.27
I suggest checking these conditions or __builtin_bit_cast when defining FMT_CONSTEXPR20.
cc @vitaut
@marksantaniello, You can enable Github Actions for your fmt fork
@phprus
You can enable Github Actions for your fmt fork
Are you saying that there's a way to get all the same test workflows running on my fork?
Thanks. I'll have to figure do that if I make PRs in the future. The test matrix is pretty large.
Anyway, it looks like the current version using __builtin_bit_cast is passing the checks.
Current test matrix does not check:
gcc-10 + C++20 (defined FMT_CONSTEXPR20, but not support __builtin_bit_cast)
Clang libc++ < 14 + C++20
Ah, OK. Maybe I should figure out the workflows now then. I enabled "Actions" but I'm not sure how to clone the checks from here. And then I guess I'd need to add at least two more?
Strategies are defined in the .github/workflows/linux.yml file:
https://github.com/fmtlib/fmt/blob/8bd02e93b2d38ce3d69f117623e64e28c6f331cd/.github/workflows/linux.yml#L11-L61
I think for the test you need to add: gcc-9,gcc-10 + C++20
FMT_CONSTEXPR20 should definitely not be changed. I suggest making the test conditional on whether __cpp_lib_bit_cast is defined, removing the bit_cast changes and new CI configs (we already have way too many).
Then everything that used to work will continue to work and if bit_cast is constexpr and there is C++20 constexpr support then formatted_size will usable in constexpr.