https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95468
--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The reason we fail to compile the testcase when 'condition' is at class scope
instead of at namespace scope is because in the former case, the template
argument 'c::condition<bool(true)>' is a CALL_EXPR to a BASELINK, and when
doing instantiate_non_dependent_expr_internal on this CALL_EXPR, we don't
semantically process the template argument 'bool(true)' within BASELINK because
we never call tsubst_baselink because tsubst_copy exits early when args is
NULL_TREE.
We don't have this problem in the latter case where 'condition' is at namespace
scope, because then the template argument 'condition<bool(true)>' is a
CALL_EXPR to a TEMPLATE_ID_EXPR, and TEMPLATE_ID_EXPRs are handled directly
from tsubst_copy_and_build which doesn't do an early exit when args is
NULL_TREE.
A potential untested fix therefore is to also handle BASELINK directly from
tsubst_copy_and_build:
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index d7f61289621..e15fca54823 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -19472,6 +19472,11 @@ tsubst_copy_and_build (tree t,
case SCOPE_REF:
RETURN (tsubst_qualified_id (t, args, complain, in_decl, /*done=*/true,
/*address_p=*/false));
+
+ case BASELINK:
+ RETURN (tsubst_baselink (t, current_nonlambda_class_type (),
+ args, complain, in_decl));
+
case ARRAY_REF:
op1 = tsubst_non_call_postfix_expression (TREE_OPERAND (t, 0),
args, complain, in_decl);
Ideally we might want two versions of tsubst_baselink, one that does template
argument substitution and another than additionally performs semantic
processing?
