When compiling with libfmt-11.1.0 and newer the following compile errors occur:
In file included from ../src/decoder/DecoderPrint.cxx:23:
../src/client/Response.hxx: In instantiation of 'bool Response::Fmt(const S&, Args&& ...) [with S = decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING; Args = {const char* const&}]':
../src/decoder/DecoderPrint.cxx:38:7: required from here
38 | r.Fmt(FMT_STRING("plugin: {}\n"), plugin.name);
| ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/client/Response.hxx:86:28: error: cannot convert 'const decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING' to 'fmt::v11::string_view' {aka 'fmt::v11::basic_string_view<char>'}
86 | return VFmt(format_str,
| ~~~~^~~~~~~~~~~~
87 | fmt::make_format_args(args...));
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/client/Response.hxx:81:36: note: initializing argument 1 of 'bool Response::VFmt(fmt::v11::string_view, fmt::v11::format_args)'
81 | bool VFmt(fmt::string_view format_str, fmt::format_args args) noexcept;
| ~~~~~~~~~~~~~~~~~^~~~~~~~~~
../src/client/Response.hxx: In instantiation of 'bool Response::Fmt(const S&, Args&& ...) [with S = decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING; Args = {const char* const&}]':
The error is due to the use of FMT_STRING. The libfmt team shared the following:
The correct way of using FMT_STRING is to wrap a format string when passing to a
function with compile-time checks (i.e. that takes format_string) as documented
in https://fmt.dev/11.1/api/#legacy-compile-time-checks.
Noting that FMT_STRING is a legacy API and has been superseded by consteval-based
API starting from version 8: https://github.com/fmtlib/fmt/releases/tag/8.0.0. It
looks like MPD is trying to emulate {fmt}'s old way of implementing compile-time
checks which was never properly documented because it was basically a hack. So the
correct fix is to switch to format_string and, possibly, remove usage of FMT_STRING.
The old way of doing compile-time checks (fmt::make_args_checked) was documented
in https://fmt.dev/7.1/api.html#argument-lists but it looks like MPD is not using
that API so the problematic uses of FMT_STRING have no effect and can just be removed.
The FMT_STRING has been removed in this change based on the fmt-7.1 API and now MPD is
successfully compile against the current libfmt-11.1.0 which highlighted the issue that
had been present in the codebase as it is now triggering the error, is legacy and was
not using the API for which FMT_STRING was aligned with.
Since we switched from autotools to Meson in commit
94592c1406, we don't need to include
`config.h` early to properly enable large file support. Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.
This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.
