c++: Tolerate empty initial args during normalization [PR97412]

When normalizing the constraint-expression of a nested-requirement, we
pass NULL_TREE as the initial template arguments for normalization, but
tsubst_argument_pack is not prepared to handle a NULL_TREE args vector.
This causes us to ICE when normalizing a variadic concept as part of a
nested-requirement.

This patch fixes the ICE by guarding the call to tsubst_template_args in
normalize_concept_check appropriately.  This will also enable us to
simplify many of the normalization routines to just pass NULL_TREE
(instead of a set of generic template arguments) as the initial template
arguments.

gcc/cp/ChangeLog:

	PR c++/97412
	* constraint.cc (normalize_concept_check): Don't call
	tsubst_template_args when 'args' is NULL.

gcc/testsuite/ChangeLog:

	PR c++/97412
	* g++.dg/cpp2a/concepts-variadic2.C: New test.

2 files changed, 14 insertions, 1 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index f4f5174..75457a2 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -686,7 +686,8 @@ normalize_concept_check (tree check, tree args, norm_info info)
     }
 
   /* Substitute through the arguments of the concept check. */
-  targs = tsubst_template_args (targs, args, info.complain, info.in_decl);
+  if (args)
+    targs = tsubst_template_args (targs, args, info.complain, info.in_decl);
   if (targs == error_mark_node)
     return error_mark_node;
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-variadic2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-variadic2.C
new file mode 100644
index 0000000..ce61aef
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-variadic2.C
@@ -0,0 +1,12 @@
+// PR c++/97412
+// { dg-do compile { target c++20 } }
+
+template <class T, class... TArgs>
+concept call_bar_with = requires(T t, TArgs... args) {
+  t.bar(args...);
+};
+
+template <class T, class... TArgs>
+concept foo = requires {
+  requires call_bar_with<T, TArgs...>;
+};