formatting a std::optional with format_as function fails to compile using clang

[cptFracassa]

clang reports compiler error for code that formats a variable of type std::optional, where T is an enum with a format_as custom formatter. The same code compiles fine with gcc or msvc.
gobolt example

The compiler error is due to the optional formatter (in fmt/std.h) calling u.set_debug_format(set); on line 170, and this function being private due to private inheritance (fmt/format.h - line 4055.

Adding the following line to fmt/format line 4058 solves the problem: using base::set_debug_format;. Alternatively, changing from private to public inheritance on line 4055 fixes it too.

[ecatmur]

caused by llvm/llvm-project#68849