c++: alias in qualified-id in template arg [PR98570]

c++: alias in qualified-id in template arg [PR98570]

template_args_equal has handled dependent alias specializations for a while,
but in this testcase the actual template argument is a SCOPE_REF, so we
called cp_tree_equal, which doesn't handle aliases specially when we get to
them.

This patch generalizes this by setting a flag so structural_comptypes will
check for template alias equivalence (if we aren't doing partial ordering).
The existing flag, comparing_specializations, was too broad; in particular,
when we're doing decls_match, we want to treat corresponding parameters as
equivalent, so we need to separate that from alias comparison.  So I
introduce the comparing_dependent_aliases flag.

From looking at other uses of comparing_specializations, it seems to me that
the new flag is what modules wants, as well.

The other use of comparing_specializations in structural_comptypes is a hack
to deal with spec_hasher::equal not calling push_to_top_level, which we
also don't want to tie to the alias comparison semantics.

This patch also changes how we get to structural comparison of aliases from
checking TYPE_CANONICAL in comptypes to marking the aliases as getting
structural comparison when they are built, which is more consistent with how
e.g. typename is handled.

As I mention in the comment for comparing_dependent_aliases, I think the
default should be to treat different dependent aliases for the same type as
distinct, only treating them as equal during deduction (particularly partial
ordering).  But that's a matter for the C++ committee, to try in stage 1.

gcc/cp/ChangeLog:

	PR c++/98570
	* cp-tree.h: Declare it.
	* pt.c (comparing_dependent_aliases): New flag.
	(template_args_equal, spec_hasher::equal): Set it.
	(dependent_alias_template_spec_p): Assert that we don't
	get non-types other than error_mark_node.
	(instantiate_alias_template): SET_TYPE_STRUCTURAL_EQUALITY
	on complex alias specializations.  Set TYPE_DEPENDENT_P here.
	(tsubst_decl): Not here.
	* module.cc (module_state::read_cluster): Set
	comparing_dependent_aliases instead of
	comparing_specializations.
	* tree.c (cp_tree_equal): Remove comparing_specializations
	module handling.
	* typeck.c (structural_comptypes): Adjust.
	(comptypes): Remove comparing_specializations handling.

gcc/testsuite/ChangeLog:

	PR c++/98570
	* g++.dg/cpp0x/alias-decl-targ1.C: New test.