c++: maybe_dependent_member_ref and stripped alias [PR118626]

c++: maybe_dependent_member_ref and stripped alias [PR118626]

Here during maybe_dependent_member_ref (as part of CTAD rewriting
for the variant constructor) for __accepted_type<_Up> we strip this
alias all the way to type _Nth_type<__accepted_index<_Up>>, for which
we return NULL since _Nth_type is at namespace scope and so no
longer needs rewriting.

Note that however the template argument __accepted_index<_Up> of this
stripped type _does_ need rewriting (since it specializes a variable
template from the current instantiation).  We end up not rewriting this
variable template reference at any point however because upon returning
NULL, the caller (tsubst) proceeds to substitute the original form of
the type __accepted_type<_Up>, which doesn't directly refer to
__accepted_index.  This later leads to an ICE during subsequent alias
CTAD rewriting of this guide that contains a non-rewritten reference
to __accepted_index.

So when maybe_dependent_member_ref decides to not rewrite a class-scope
alias that's been stripped, the caller needs to commit to substituting
the stripped type rather than the original type.  This patch essentially
implements that by making maybe_dependent_member_ref call tsubst itself
in that case.

	PR c++/118626

gcc/cp/ChangeLog:

	* pt.cc (maybe_dependent_member_ref): Substitute and return the
	stripped type if we decided to not rewrite it directly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias25.C: New test.

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