On GCC C++20, formatting std::string using core.h, then including format.h, causes partial specialization error

[nyanpasu64]

After upgrading from fmtlib 7.1.3 to 8.1.1, I started getting a mysterious compilation error on Arch Linux GCC 11.2.0 C++20 (but no error on Clang 13.0.1):

In file included from /home/nyanpasu64/code/exotracker-cpp/src/serialize.cpp:4:
/home/nyanpasu64/code/exotracker-cpp/3rdparty/fmt_/include/fmt/format.h:2582:10: error: partial specialization of ‘struct fmt::v8::formatter<std::__cxx11::basic_string<_CharT>, Char>’ after instantiation of ‘struct fmt::v8::formatter<std::__cxx11::basic_string<char>, char, void>’ [-fpermissive]
 2582 |   struct formatter<Type, Char> : formatter<Base, Char> {                 \
      |          ^~~~~~~~~~~~~~~~~~~~~
/home/nyanpasu64/code/exotracker-cpp/3rdparty/fmt_/include/fmt/format.h:2597:1: note: in expansion of macro ‘FMT_FORMAT_AS’
 2597 | FMT_FORMAT_AS(std::basic_string<Char>, basic_string_view<Char>);
      | ^~~~~~~~~~~~~

Annoyingly, this error doesn't point to where the concrete formatter was instantiated.

A minimal test case for this behavior is as follows:

#include <string>
#include <fmt/core.h>
inline void f() {
    (void) fmt::format("{}", std::string());
}
#include <fmt/format.h>
int main() {}

The error can be reproduced using g++ -std=gnu++20 fmt-err.cpp and c++20, but not gnu++17 and below, and not using clang++ with gnu++20 or gnu++17.

Is this intended to function properly? (I should probably stop using fmtlib in my headers... it's rather sloppy on my part.)

[vitaut]

Repro: https://godbolt.org/z/35WPeoM6P

[vitaut]

Fixed in a1ea3e0, thanks for reporting.

[nyanpasu64]

Out of curiosity, why does the error only occur on GCC C++20 mode?

[vitaut]

I'm not sure why it only happens in C++20.