Error compiling with -std=c++20 using CUDA/Clang

[andrewcorrigan]

I'm encountering errors compiling with clang++ -x cuda using -std=c++20. I observed this on the current commit on master and bisected to the commit where the errors arose to be [fd34a3d].

A simple reproducer is:

#include <iostream>
#include <fmt/ostream.h>

int main(int argc, char** argv)
{
    std::cout << fmt::format("hello {}", "world") << std::endl;

    return 0;
}

The errors are triggered using Clang 10, 11, 12 (perhaps 13 as well, but I can't test that immediately) with the following command-line arguments:

% clang++ -x cuda -std=c++20 -c --cuda-gpu-arch=sm_35 -Ifmt/include test_fmt.cpp

The first of ten error messages is:

fmt/include/fmt/core.h:957:27: error: non-virtual member function marked 'override' hides virtual member function
  void grow(size_t) final FMT_OVERRIDE {
                          ^
fmt/include/fmt/core.h:129:26: note: expanded from macro 'FMT_OVERRIDE'
#    define FMT_OVERRIDE override
                         ^
fmt/include/fmt/core.h:3002:14: note: in instantiation of template class 'fmt::v8::detail::counting_buffer<char>' requested here
  auto buf = detail::counting_buffer<>();
             ^
fmt/include/fmt/core.h:798:32: note: hidden overloaded virtual function 'fmt::v8::detail::buffer<char>::grow' declared here
  virtual FMT_CONSTEXPR20 void grow(size_t capacity) = 0;
                               ^

Note, the following builds fine:

1. CUDA in C++17 mode:

% clang++ -x cuda -std=c++17 -c --cuda-gpu-arch=sm_35 -Ifmt/include test_fmt.cpp

2. C++20 mode without CUDA:

% clang++ -std=c++20 -c -Ifmt/include test_fmt.cpp

[andrewcorrigan]

It looks like Clang/CUDA wants constexpr to be consistent between the base class and the override.

There are two ways to fix the errors:

1. Decorate all overrides of grow with FMT_CONSTEXPR20, or,
2. Eliminate FMT_CONSTEXPR20 from the abstract declaration of grow in class buffer.

I'd be happy to prepare a PR pending feedback on which approach would be preferred.

[vitaut]

I think we should go with option 1 because option 2 would break compile-time formatting. A PR is welcome.