c++: non-template alias with dependent attributes [PR115897]

c++: non-template alias with dependent attributes [PR115897]

This patch generalizes our support for dependent attributes on alias
templates to also support them on non-template aliases.  The main
addition is a new predicate dependent_opaque_alias_p controlling whether
we can treat an alias (template or non-template) as type-equivalent to
its expansion.

	PR c++/115897

gcc/cp/ChangeLog:

	* cp-tree.h (dependent_opaque_alias_p): Declare.
	* pt.cc (push_template_decl): Manually mark a dependent opaque
	alias or dependent alias template specialization as dependent,
	and use structural equality for them.
	(dependent_opaque_alias_p): Define.
	(alias_template_specialization_p): Don't look through an
	opaque alias.
	(complex_alias_template_p): Use dependent_opaque_alias_p instead of
	any_dependent_template_arguments_p directly.
	(dependent_alias_template_spec_p): Don't look through an
	opaque alias.
	(get_underlying_template): Use dependent_opaque_alias_p instead of
	any_dependent_template_arguments_p.
	(instantiate_alias_template): Mention same logic in
	push_template_decl.
	(dependent_type_p_r): Remove dependent_alias_template_spec_p check.
	(any_template_arguments_need_structural_equality_p): Return true
	for a dependent opaque alias.
	(alias_ctad_tweaks): Use template_args_equal instead of same_type_p
	followed by dependent_alias_template_spec_p.
	* tree.cc (strip_typedefs): Don't strip an opaque alias.
	* typeck.cc (structural_comptypes): Compare declaration attributes
	for an opaque alias.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-79.C: Remove xfails.
	* g++.dg/cpp0x/alias-decl-79a.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>