c++: ICE with concept defined in function [PR97536]

This is an ICE-on-invalid, but I keep seeing it when reducing code so
I'd like to fix it.  We crash on

  template <typename> void forward() {
    concept C = true;
  }

which breaks two requirements:
[temp.concept]/1: A concept is a template ...
[temp.concept]/3: A concept-definition shall inhabit a namespace scope.

This patch adds a test that exercises broken code and fixes the ICE
by checking that a concept-definition is defined at namespace scope.

gcc/cp/ChangeLog:

	PR c++/97536
	* decl.c (grokvardecl): Given an error when a concept is not defined
	at namespace scope.

gcc/testsuite/ChangeLog:

	PR c++/97536
	* g++.dg/concepts/diagnostic16.C: New test.

gcc/cp/decl.c

@@ -10365,6 +10365,12 @@ grokvardecl (tree type,
 		    "a non-template variable cannot be %<concept%>");
           return NULL_TREE;
         }
+      else if (!at_namespace_scope_p ())
+	{
+	  error_at (declspecs->locations[ds_concept],
+		    "concept must be defined at namespace scope");
+	  return NULL_TREE;
+	}
       else
         DECL_DECLARED_CONCEPT_P (decl) = true;
       if (!same_type_ignoring_top_level_qualifiers_p (type, boolean_type_node))

gcc/testsuite/g++.dg/concepts/diagnostic16.C

@@ -0,0 +1,45 @@
+// PR c++/97536
+// { dg-do compile { target concepts } }
+
+template<typename>
+concept C1 = true;
+
+concept C2 = true; // { dg-error "non-template variable cannot be .concept." }
+// { dg-error "concept definition syntax is" "" { target *-*-* } .-1 }
+
+template<typename>
+void fn1 ()
+{
+  concept bar = true; // { dg-error "concept must be defined at namespace scope" }
+// { dg-error "concept definition syntax is" "" { target *-*-* } .-1 }
+}
+
+void fn2 ()
+{
+  concept bar = true; // { dg-error "non-template variable cannot be .concept." }
+// { dg-error "concept definition syntax is" "" { target *-*-* } .-1 }
+}
+
+template<typename>
+void fn3 ()
+{
+  template<typename> // { dg-error "template declaration cannot appear at block scope" }
+  concept bar = true;
+}
+
+void fn4 ()
+{
+  template<typename> // { dg-error "template declaration cannot appear at block scope" }
+  concept bar = true;
+}
+
+void fn5 ()
+{
+  C1 auto x = 42;
+}
+
+template<typename>
+void fn6 ()
+{
+  C1 auto x = 42;
+}