[Bug libstdc++/121765] `std::formatter` specialization for user-defined type inheriting from `std::formatter` fails concept `std::formattable` check unexpectedly

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121765

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2025-09-03

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
r15-9517-g473dde525248a6 said:

As in the case of previous commits, the signatures of the user-facing parse
and format methods of the provided formatters deviate from the standard by
constraining types of parameters:
* _CharT is constrained __formatter::__char
* basic_format_parse_context<_CharT> for parse argument
* basic_format_context<_Out, _CharT> for format second argument
The standard specifies last three of above as unconstrained types.

But the problem seems to be that the formattable concept instantiates
__format_padded which doesn't work with arbitrary output iterators (as required
by the concept).