c++: current inst w/ indirect dependent bases [PR117993]

c++: current inst w/ indirect dependent bases [PR117993]

In the first testcase we're overeagerly diagnosing qualified name lookup
failure for f from the current instantiation B<T>::C ahead of time
because we (correctly) deem C to not have any direct dependent bases:
its direct base B<T> is part of the current instantiation and therefore
not a dependent base, and we decide it's safe to diagnose name lookup
failure ahead of time.

But this testcase demonstrates it's not enough to consider only direct
dependent bases: f is defined in A<T> which is a dependent base of
B<T>, so qualified name lookup from C won't search it ahead of time and
in turn won't be exhaustive, and so it's wrong to diagnose lookup
failure ahead of time.  This ultimately suggests that
any_dependent_bases_p needs to consider indirect bases as well.

To that end it seems sufficient to make the predicate recurse into any
!BINFO_DEPENDENT_BASE_P base since the recursive call will exit early
for non-dependent types.  So effectively we'll only recurse into bases
belonging to the current instantiation.

I considered more narrowly making only dependentish_scope_p consider
indirect dependent bases, but it seems other any_dependent_bases_p
callers also want this behavior, e.g. build_new_method_call for benefit
of the second testcase (which is an even older regression since GCC 7).

	PR c++/117993

gcc/cp/ChangeLog:

	* search.cc (any_dependent_bases_p): Recurse into bases (of
	dependent type) that are not BINFO_DEPENDENT_BASE_P.  Document
	default argument.

gcc/testsuite/ChangeLog:

	* g++.dg/template/dependent-base4.C: New test.
	* g++.dg/template/dependent-base5.C: New test.

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