Fix PR c++/47172

gcc/cp/

	PR c++/47172
	* pt.c (finish_call_expr): Consider a call expression that has a
	dependent "this" pointer as being dependent.  Add comments.
	(dependent_type_p, type_dependent_expression_p): Update comments.

gcc/testsuite/

	* g++.dg/template/inherit6.C: New test.

From-SVN: r170240