libstdc++: fix uses of explicit object parameter [PR116038]

libstdc++: fix uses of explicit object parameter [PR116038]

The type of an implicit object parameter is always the current class.
For an explicit object parameter however, its deduced type can be a
derived class of the current class.  So when combining multiple
implicit-object overloads into a single explicit-object overload we need
to account for this possibility.  For example when accessing a member of
the current class through an explicit object parameter, it may now be a
derived class from which the member is not accessible, as in the below
testcases.

This pitfall is discussed[1] in the deducing this paper.  The general
solution is to cast the explicit object parameter to (a reference to)
the current class rather than e.g. using std::forward which preserves
the deduced type.

This patch corrects the existing problematic uses of explicit object
parameters in the library, all of which forward the parameter via
std::forward, to instead cast the parameter to the current class via
our __like_t alias template.  Note that unlike the paper's like_t,
ours always returns a reference so we can just write

  __like_t<Self, B>(self)

instead of

  (_like_t<Self, B>&&)self

as the paper does.

[1]: https://wg21.link/P0847#name-lookup-within-member-functions (and the
section after that)

	PR libstdc++/116038

libstdc++-v3/ChangeLog:

	* include/std/functional (_Bind_front::operator()): Use __like_t
	instead of std::forward when forwarding __self.
	(_Bind_back::operator()): Likewise.
	* include/std/ranges (_Partial::operator()): Likewise.
	(_Pipe::operator()): Likewise.
	* testsuite/20_util/function_objects/bind_back/116038.cc: New test.
	* testsuite/20_util/function_objects/bind_front/116038.cc: New test.
	* testsuite/std/ranges/adaptors/116038.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>