c++: non-dep decltype folding of concept-id C<Ts...> [PR123676]

Here since the expression within the decltype C<Ts...> is not instantiation
dependent (we know its type is bool, and don't care about its value)
finish_decltype_type instantiates it immediately via the usual tsubst_expr
with NULL_TREE args.  During which however tsubst_pack_expansion isn't
prepared to handle such a substitution due to an overly strict assert.
This patch relaxes the assert accordingly.

	PR c++/123676

gcc/cp/ChangeLog:

	* pt.cc (tsubst_pack_expansion): Relax unsubsituted_packs
	assert to allow !processing_template_decl when args is
	NULL_TREE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-decltype5.C: New test.

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

gcc/cp/pt.cc

@@ -14146,7 +14146,8 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 	  /* We can't substitute for this parameter pack.  We use a flag as
 	     well as the missing_level counter because function parameter
 	     packs don't have a level.  */
-	  gcc_assert (processing_template_decl || is_auto (parm_pack));
+	  gcc_assert (processing_template_decl || is_auto (parm_pack)
+		      || args == NULL_TREE);
 	  unsubstituted_packs = true;
 	}
     }

gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C

@@ -0,0 +1,10 @@
+// PR c++/123676
+// { dg-do compile { target c++20 } }
+
+template<class...>
+concept C = true;
+
+template<class... Ts>
+auto f() -> decltype(C<Ts...>) {
+  return true;
+}