Reject class template placeholder as non-type template parm type in C++17.

	* pt.c (invalid_nontype_parm_type_p): Reject class placeholder in
	C++17.

From-SVN: r279870

gcc/cp/ChangeLog

+2020-01-02  Jason Merrill  <jason@redhat.com>
+
+	* pt.c (invalid_nontype_parm_type_p): Reject class placeholder in
+	C++17.
+
 2020-01-02  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c/90677

gcc/cp/pt.c

@@ -25811,7 +25811,16 @@ invalid_nontype_parm_type_p (tree type, tsubst_flags_t complain)
   else if (TYPE_PTRMEM_P (type))
     return false;
   else if (TREE_CODE (type) == TEMPLATE_TYPE_PARM)
-    return false;
+    {
+      if (CLASS_PLACEHOLDER_TEMPLATE (type) && cxx_dialect < cxx2a)
+	{
+	  if (complain & tf_error)
+	    error ("non-type template parameters of deduced class type only "
+		   "available with %<-std=c++2a%> or %<-std=gnu++2a%>");
+	  return true;
+	}
+      return false;
+    }
   else if (TREE_CODE (type) == TYPENAME_TYPE)
     return false;
   else if (TREE_CODE (type) == DECLTYPE_TYPE)

gcc/testsuite/g++.dg/cpp1z/class-deduction64.C

@@ -3,7 +3,7 @@
 
 template <int> struct S;
 
-template <S> struct W {
+template <S> struct W {	  // { dg-error "class type" "" { target c++17_only } }
   template <typename> static int foo();
   bool b = foo<int>();
 };