c++: Fix folding of non-dependent BASELINKs [PR95468]

Here, the problem ultimately seems to be that tsubst_copy_and_build,
when called with empty args as we do during non-dependent expression
folding, doesn't touch BASELINKs at all: it delegates to tsubst_copy
which then immediately exits early due to the empty args.  This means
that the CAST_EXPR int(1) in the BASELINK A::condition<int(1)> never
gets folded (as part of folding of the overall CALL_EXPR), which later
causes us to crash when performing overload resolution of the rebuilt
CALL_EXPR (which is still in terms of this templated BASELINK).

This doesn't happen when condition() is a namespace-scope function
because then condition<int(1)> is represented by a TEMPLATE_ID_EXPR
rather than by a BASELINK, which does get handled directly from
tsubst_copy_and_build.

This patch fixes this issue by having tsubst_copy_and_build handle
BASELINK directly rather than delegating to tsubst_copy, so that it
processes BASELINKs even when args is empty.

gcc/cp/ChangeLog:

	PR c++/95468
	* pt.c (tsubst_copy_and_build) <case BASELINK>: New case, copied
	over from tsubst_copy.

gcc/testsuite/ChangeLog:

	PR c++/95468
	* g++.dg/template/non-dependent15.C: New test.

2 files changed, 17 insertions, 0 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 0393531..3576e0e 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -19856,6 +19856,11 @@ 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);
diff --git a/gcc/testsuite/g++.dg/template/non-dependent15.C b/gcc/testsuite/g++.dg/template/non-dependent15.C
new file mode 100644
index 0000000..00dfe26
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent15.C
@@ -0,0 +1,12 @@
+// PR c++/95468
+// { dg-do compile { target c++11 } }
+
+struct A {
+  template <int N>
+  static constexpr int condition() { return N; }
+};
+
+template <int> struct B {};
+
+template <class>
+using T = B<A::condition<int(1)>()>;