c++: using operator= [PR105223]

c++: using operator= [PR105223]

In a template class A we normally add an implicit using A::operator= as a
placeholder for the implicitly declared operator whose signature we don't
know yet.  In my patch for PR92918 I stopped doing that if the class has an
explicit operator=, but that was wrong; an operator= taking an unrelated
type doesn't prevent the implicit declaration.

When I was working on that patch, the change was necessary to avoid another
regression, but apparently it is no longer needed.

	PR c++/105223
	PR c++/92918

gcc/cp/ChangeLog:

	* class.cc (finish_struct): Always using op=.

gcc/testsuite/ChangeLog:

	* g++.dg/template/using31.C: New test.