c++: variadic using-decl with parm pack in terminal name [PR102104]

c++: variadic using-decl with parm pack in terminal name [PR102104]

There's a curious corner case with variadic member using-decls: the
terminal name can also contain a parameter pack, and only through naming
a conversion function, e.g.

  using A<Ts>::operator Ts...;

We currently only handle parameter packs appearing in the qualifying
scope of a variadic using-decl; this patch adds support for the above
case as well, representing such a using-decl via two pack expansions,
one for the qualifying scope and one for the terminal name (despite
logically there being just one).  Then at instantiation time we manually
merge them.

	PR c++/102104
	PR c++/108090

gcc/cp/ChangeLog:

	* error.cc (dump_decl) <case USING_DECL>: Look through a
	pack expansion in the name as well.
	* parser.cc (cp_parser_using_declaration): Handle a parameter
	pack appearing in the terminal name of a variadic using-decl.
	* pt.cc (tsubst_decl) <case USING_DECL>: Likewise.  Combine the
	handling of variadic and non-variadic using-decls.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/using-variadic1.C: New test.
	* g++.dg/cpp1z/using-variadic1a.C: New test.
	* g++.dg/cpp1z/using-variadic1b.C: New test.
	* g++.dg/cpp1z/using-variadic1c.C: New test.
	* g++.dg/cpp1z/using-variadic2.C: New test.
	* g++.dg/cpp1z/using-variadic3.C: New test.