c++: Some additional tests

I created a few tests on the modules branch that are not actually
module-related.  Here they are.

	gcc/testsuite/
	* g++.dg/concepts/pack-1.C: New.
	* g++.dg/lookup/using53.C: Add an enum.
	* g++.dg/template/error25.C: Relax 'export' error check.

gcc/testsuite/g++.dg/concepts/pack-1.C

@@ -0,0 +1,31 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-fconcepts" }
+
+// distilled from <concepts>, via header units
+
+template<typename _ArgTypes>
+struct is_invocable;
+
+template<typename... _Args>
+concept invocable = is_invocable<_Args...>::value;
+
+template<typename _Is>
+requires invocable<_Is>
+class BUG;
+
+template<typename _Is>
+requires invocable<_Is>
+class BUG {}; // { dg-bogus "different constraints" }
+
+template<int> struct is_invocable_NT;
+
+template<int... Ints>
+concept invocable_NT = is_invocable_NT<Ints...>::value;
+
+template<int _Is>
+requires invocable_NT<_Is>
+class BUG_NT;
+
+template<int _Is>
+requires invocable_NT<_Is>
+class BUG_NT {};

gcc/testsuite/g++.dg/lookup/using53.C

@@ -43,6 +43,7 @@ template class DT<int>;
 namespace N
 {
   int i;
+  enum bob {Q};
 }
 
 void

gcc/testsuite/g++.dg/template/error25.C

@@ -12,5 +12,5 @@ extern void f2 ();
 template<>
 extern void f2<void> ();  // { dg-error "explicit template specialization cannot have a storage class" }
 
-export template<class T>  // { dg-warning "keyword 'export' not implemented" }
+export template<class T>  // { dg-warning "keyword 'export'" }
 static void* f3 ();