libstdc++: Reuse _Bind_back_t functor in ranges::_PartialHEADtrunkmaster

This patch refactors ranges::_Partial to be implemented using _Bind_back_t.
This allows it to benefit from the changes in r16-3398-g250dd5b5604fbc,
specifically making the closure trivially copyable. Since _Bind_back_t
already provides an optimized implementation for a single bound argument,
specializations for _Partial with a single argument are now removed.

We still preserve a specialization of _Partial for trivially copy-constructible
arguments that define only a const overload of operator(). To avoid
re-checking invocability constraints, this specialization calls the now-public,
unconstrained _Binder::_S_call static method instead of the constrained
_Binder::operator().

The primary specialization of _Partial retains its operator(), which
uses a simpler __adaptor_invocable constraint that does not consider
member pointers, as they are not relevant here. This implementation also
calls _Binder::_S_call to avoid re-performing overload resolution and
invocability checks for _Binder::operator().

Finally, the _M_binder member (_Bind_back_t) is now marked
[[no_unique_address]]. This is beneficial as ranges::_Partial is used with
ranges::to, which commonly has zero or empty bound arguments (e.g., stateless
allocators, comparators, or hash functions).

libstdc++-v3/ChangeLog:

	* include/bits/binders.h (_Binder::_S_call): Make public.
	* include/std/ranges (ranges::_Partial<_Adaptor, _Args...>):
	Replace tuple<_Args...> with _Bind_back_t<_Adaptor, _Args...>.
	(ranges::_Partial<_Adaptor, _Arg>): Remove.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

0 files changed, 0 insertions, 0 deletions