c++: unifying FUNCTION_DECLs [PR93740]

c++: unifying FUNCTION_DECLs [PR93740]

unify currently always returns success when unifying two FUNCTION_DECLs
(due to the is_overloaded_fn deferment within the default case), which
means for the below testcase we incorrectly unify &A::foo and &A::bar
leading to deduction failure for the index_of calls due to a bogus base
class ambiguity.

This patch makes unify handle FUNCTION_DECL naturally like other decls.

	PR c++/93740

gcc/cp/ChangeLog:

	* pt.cc (unify) <case FUNCTION_DECL>: Handle it like FIELD_DECL
	and TEMPLATE_DECL.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ptrmem34.C: New test.