c++: memfn lookup consistency and using-decls [PR104432]

c++: memfn lookup consistency and using-decls [PR104432]

In filter_memfn_lookup, we weren't correctly recognizing and matching up
member functions introduced via a non-dependent using-decl.  This caused
us to crash in the below testcases in which we correctly pruned the
overload set for the non-dependent call ahead of time, but then at
instantiation time filter_memfn_lookup failed to match the selected
function (introduced in each case by a non-dependent using-decl) to the
corresponding function from the new lookup set.  Such member functions
need special handling in filter_memfn_lookup because they look exactly
the same in the old and new lookup sets, whereas ordinary member
functions that're defined in the (dependent) current class become more
specialized in the new lookup set.

This patch reworks the matching logic in filter_memfn_lookup so that it
handles (member functions introduced by) non-dependent using-decls
correctly, and is hopefully simpler overall.

	PR c++/104432

gcc/cp/ChangeLog:

	* call.cc (build_new_method_call): When a non-dependent call
	resolves to a specialization of a member template, always build
	the pruned overload set using the member template, not the
	specialization.
	* pt.cc (filter_memfn_lookup): New parameter newtype.  Simplify
	and correct how members from the new lookup set are matched to
	those from the old one.
	(tsubst_baselink): Pass binfo_type as newtype to
	filter_memfn_lookup.

gcc/testsuite/ChangeLog:

	* g++.dg/template/non-dependent19.C: New test.
	* g++.dg/template/non-dependent19a.C: New test.
	* g++.dg/template/non-dependent20.C: New test.