[PATCH] c++: reject concept w/ multiple tparm lists [PR105067]

[Patrick Palka via Gcc-patches]

We weren't rejecting a concept declared with multiple template
parameter lists.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

        PR c++/105067

gcc/cp/ChangeLog:

        * pt.cc (finish_concept_definition): Require that a concept is
        declared with exactly one template parameter list.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-err4.C: New test.
---
 gcc/cp/pt.cc                               | 6 ++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-err4.C | 6 ++++++
 2 files changed, 12 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-err4.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 173bc3a8c7f..bd937499934 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -28775,6 +28775,12 @@ finish_concept_definition (cp_expr id, tree init)
       return error_mark_node;
     }
 
+  if (current_template_depth > 1)
+    {
+      error_at (loc, "concept %qE has multiple template parameter lists", *id);
+      return error_mark_node;
+    }
+
   /* Initially build the concept declaration; its type is bool.  */
   tree decl = build_lang_decl_loc (loc, CONCEPT_DECL, *id, boolean_type_node);
   DECL_CONTEXT (decl) = current_scope ();
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-err4.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-err4.C
new file mode 100644
index 00000000000..57a96a095c7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-err4.C
@@ -0,0 +1,6 @@
+// PR c++/105067
+// { dg-do compile { target c++20 } }
+
+template<class>
+template<class>
+concept C = true; // { dg-error "parameter list" }
-- 
2.35.1.677.gabf474a5dd