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 Kamiński <tkaminsk@redhat.com>

0 files changed, 0 insertions, 0 deletions