Documentation: API example for user-defined formatter doesn't compile

[frank-weinberg]

The example for defining a custom formatter via specializing the formatterstruct in https://fmt.dev/latest/api.html#formatting-user-defined-types fails to compile when the definition of the format function is in a different compilation unit than its usage.

Using color.h and color.cc as in the API doc and a simple main.cc:

#include "color.h"

int main() { fmt::print("{}\n", color::red); }

gives the error (gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04) ):

 /workspaces/frawen/temp/fmt-10.2.1/include/fmt/core.h:1328:28: error: use of ‘auto fmt::v10::formatter<color>::format(color, fmt::v10::format_context&) const’ before deduction of ‘auto’

After amending the signature of format to

auto format(color c, format_context &ctx) const -> format_context::iterator

in both color.h and color.cc, the code works as expected.

(No godbolt because I couldn't get its multifile mode to work.)

[vitaut]

Good catch, thanks. Could you by any chance submit a PR with the signature change?