diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 4cad303dc44..845f0b4ea6c 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,11 @@
+2012-08-25  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/51421
+	* decl2.c (mark_used): Consistently return false after errors
+	about uses before deduction of auto.
+	* semantics.c (finish_id_expression): Check mark_used return
+	value and return error_mark_node in case of failure.
+
 2012-08-24  Jason Merrill  <jason@redhat.com>
 
 	PR c++/51213 (again)
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index 281f6ff36f9..f76b59605ae 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -4238,7 +4238,10 @@ mark_used (tree decl)
       || DECL_THUNK_P (decl))
     {
       if (!processing_template_decl && type_uses_auto (TREE_TYPE (decl)))
-	error ("use of %qD before deduction of %<auto%>", decl);
+	{
+	  error ("use of %qD before deduction of %<auto%>", decl);
+	  return false;
+	}
       return true;
     }
 
@@ -4284,7 +4287,10 @@ mark_used (tree decl)
     }
 
   if (type_uses_auto (TREE_TYPE (decl)))
-    error ("use of %qD before deduction of %<auto%>", decl);
+    {
+      error ("use of %qD before deduction of %<auto%>", decl);
+      return false;
+    }
 
   /* If we don't need a value, then we don't need to synthesize DECL.  */
   if (cp_unevaluated_operand != 0)
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 4faca914716..d66accfa934 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -3221,11 +3221,12 @@ finish_id_expression (tree id_expression,
 
       /* Mark variable-like entities as used.  Functions are similarly
 	 marked either below or after overload resolution.  */
-      if (TREE_CODE (decl) == VAR_DECL
-	  || TREE_CODE (decl) == PARM_DECL
-	  || TREE_CODE (decl) == CONST_DECL
-	  || TREE_CODE (decl) == RESULT_DECL)
-	mark_used (decl);
+      if ((TREE_CODE (decl) == VAR_DECL
+	   || TREE_CODE (decl) == PARM_DECL
+	   || TREE_CODE (decl) == CONST_DECL
+	   || TREE_CODE (decl) == RESULT_DECL)
+	  && !mark_used (decl))
+	return error_mark_node;
 
       /* Only certain kinds of names are allowed in constant
 	 expression.  Template parameters have already
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index eeb62dc03a9..94b23a444f8 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2012-08-25  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/51421
+	* g++.dg/cpp0x/auto34.C: New.
+
 2012-08-24  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c/54363
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto34.C b/gcc/testsuite/g++.dg/cpp0x/auto34.C
new file mode 100644
index 00000000000..3682d60bf4b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/auto34.C
@@ -0,0 +1,18 @@
+// PR c++/51421
+// { dg-do compile { target c++11 } }
+
+int foo1(int);
+
+void bar1()
+{
+  auto i = foo1(i);   // { dg-error "before deduction" }
+}
+
+struct A {};
+
+A foo2(A);
+
+void bar2()
+{
+  auto a = foo2(a);   // { dg-error "before deduction" }
+}