[Bug libstdc++/126731] `std::format("{}", std::float16_t)` does not agree with `std::to_chars`

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126731

--- Comment #1 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tomasz Kaminski <[email protected]>:

https://gcc.gnu.org/g:4b6ce501c4f1db06c8084170a68796622266b671

commit r17-3473-g4b6ce501c4f1db06c8084170a68796622266b671
Author: Tomasz KamiÅski <[email protected]>
Date:   Thu Aug 13 09:51:56 2026 +0200

    libstdc++: Format std::float16_t and std::bfloat16_t using respective
std::to_chars overloads [PR126731]

    This patch removes the partial specialization for float16_t and bfloat16_t
    that formatted the values by casting to float. In consequence an implicit
    specialization of generic floating-point specialization is used, and
    to_chars overloads for their types are used.
    As explained in r13-3591-g0ae26533b3e268 (that adds corresponding to_chars
    overload), the shortest string (produced when no precision is specified)
    is shorter than float for above.

    As formatter specializations were defined under the same condition as
    corresponding to_chars overloads, the __formattable_float constrain on
    formatter floating-point specialization makes it enable in same cases.

    This reverted parts of r14-3305-g6cf214b4fc97f5 (revereted partially
    in r14-3329-g27d0cfcb2b33de), leading to (necessary for correctness)
    increase of the number template specializations. It also depends on
    preserving values of above types (instead of float) in basic_format_arg
    introduced r16-616-g9c9a7316adb996.

    The specializations for float32_t and float64_t are left unchanged,
    as their to_chars overloads are also implemented in terms of casting
    to float/double respectivelly.

    libstdc++-v3/ChangeLog:

            PR libstdc++/126731
            * include/std/format (std::formatter<_Float16, _CharT>)
            (std::formatter<__format::__bflt16_t, _CharT>): Remove
            partial specializations.
            * testsuite/std/format/formatter/ext_float.cc: Test output
            for closest value to 1/10.

    Reviewed-by: Jonathan Wakely <[email protected]>
    Signed-off-by: Tomasz KamiÅski <[email protected]>