Index: cp/parser.c
===================================================================
--- cp/parser.c	(revision 200742)
+++ cp/parser.c	(working copy)
@@ -11009,11 +11009,19 @@ cp_parser_simple_declaration (cp_parser* parser,
 
   /* Issue an error message if no declarators are present, and the
      decl-specifier-seq does not itself declare a class or
-     enumeration.  */
+     enumeration: [dcl.dcl]/3.  */
   if (!saw_declarator)
     {
       if (cp_parser_declares_only_class_p (parser))
-	shadow_tag (&decl_specifiers);
+	{
+	  if (!declares_class_or_enum
+	      && decl_specifiers.type != error_mark_node)
+	    /* Ensure an error is issued anyway when finish_decltype_type,
+	       called via cp_parser_decl_specifier_seq, returns something
+	       sensible (c++/51786).  */
+	    decl_specifiers.type = NULL_TREE;
+	  shadow_tag (&decl_specifiers);
+	}
       /* Perform any deferred access checks.  */
       perform_deferred_access_checks (tf_warning_or_error);
     }
Index: testsuite/g++.dg/cpp0x/auto30.C
===================================================================
--- testsuite/g++.dg/cpp0x/auto30.C	(revision 200742)
+++ testsuite/g++.dg/cpp0x/auto30.C	(working copy)
@@ -6,4 +6,4 @@ struct A
     auto friend struct B; // { dg-error "multiple types|can only be specified|friend" }
 };
 
-auto int; // { dg-error "multiple types|can only be specified for variables" }
+auto int; // { dg-error "multiple types|does not declare anything" }
Index: testsuite/g++.dg/cpp0x/pr51786.C
===================================================================
--- testsuite/g++.dg/cpp0x/pr51786.C	(revision 0)
+++ testsuite/g++.dg/cpp0x/pr51786.C	(working copy)
@@ -0,0 +1,8 @@
+// PR c++/51786
+// { dg-do compile { target c++11 } }
+
+enum E {};
+struct A {};
+
+void foo() { decltype(E{}); }  // { dg-error "does not declare anything" }
+void bar() { decltype(A{}); }  // { dg-error "does not declare anything" }