[Bug libstdc++/122032] The size of bind_front/bind_back<fn>(...) is not optimized

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122032

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tomasz Kaminski <tkaminsk@gcc.gnu.org>:

https://gcc.gnu.org/g:71004c2414337bd781e1221acbcca2031bb6b614

commit r16-5276-g71004c2414337bd781e1221acbcca2031bb6b614
Author: Tomasz KamiÅski <tkaminsk@redhat.com>
Date:   Fri Oct 24 10:24:26 2025 +0200

    libstdc++: Use _Bind_front_t/_Bind_back_t in bind_front<f>/bind_back<f>
[PR122032]

    This patch changes the implementation of bind_front<f> and bind_back<f> to
    return a _Bind_front_t<_Bind_fn_t<f>, ...> and _Bind_back_t<_Bind_fn_t<f>,
...>
    respectively, replacing the previous lambda-based implementation. The prior
use
    of a lambda caused non-conforming behavior with respect to C++23
[func.require]
    p8, which requires that bind_front<f>(s), bind_front<f>(move(s)), and
    bind_front<f>(as_const(s)) produce the same type.

    Additionally, using specialized structs reduces the size of the resulting
functor
    in certain scenarios (see PR).

    For the zero-argument case, the function still returns a _Bind_fn_t<f>.
Since this
    type is already a perfect forwarding call wrapper, it yields the same
result as
    _Bind_front_t<_Bind_fn_t<f>>.

    A consequence of this change is that the types returned by
bind_front<f>(args...)
    and bind_back<f>(args...) are no longer structural - they are not required
to be
    structural by the standard.

            PR libstdc++/122032

    libstdc++-v3/ChangeLog:

            * include/std/functional (std::bind_front<f>, std::bind_back<f>):
            Define in terms of _Bind_front_t/_Bind_back_t.
            * testsuite/20_util/function_objects/bind_back/nttp.cc: New tests.
            * testsuite/20_util/function_objects/bind_front/nttp.cc: New tests.

    Reviewed-by: Patrick Palka <ppalka@redhat.com>
    Signed-off-by: Tomasz KamiÅski <tkaminsk@redhat.com>