Index: include/clang/Basic/DiagnosticParseKinds.td
===================================================================
--- include/clang/Basic/DiagnosticParseKinds.td
+++ include/clang/Basic/DiagnosticParseKinds.td
@@ -173,6 +173,9 @@
 def warn_gcc_attribute_location : Warning<
   "GCC does not allow an attribute in this position on a function declaration">, 
   InGroup<GccCompat>;
+def warn_gcc_variable_decl_in_for_loop : Warning<
+  "GCC does not allow variable declarations in for loop initializers before "
+  "C99">, InGroup<GccCompat>;
 def warn_attribute_no_decl : Warning<
   "attribute %0 ignored, because it is not attached to a declaration">, 
   InGroup<IgnoredAttributes>;
Index: lib/Parse/ParseStmt.cpp
===================================================================
--- lib/Parse/ParseStmt.cpp
+++ lib/Parse/ParseStmt.cpp
@@ -1622,8 +1622,10 @@
     ForRange = true;
   } else if (isForInitDeclaration()) {  // for (int X = 4;
     // Parse declaration, which eats the ';'.
-    if (!C99orCXXorObjC)   // Use of C99-style for loops in C90 mode?
+    if (!C99orCXXorObjC) {   // Use of C99-style for loops in C90 mode?
       Diag(Tok, diag::ext_c99_variable_decl_in_for_loop);
+      Diag(Tok, diag::warn_gcc_variable_decl_in_for_loop);
+    }
 
     // In C++0x, "for (T NS:a" might not be a typo for ::
     bool MightBeForRangeStmt = getLangOpts().CPlusPlus;
Index: test/Parser/gcc-for-loop-init-compatibility.c
===================================================================
--- /dev/null
+++ test/Parser/gcc-for-loop-init-compatibility.c
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c89 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=gnu89 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c99 -fsyntax-only -verify %s -DC99
+
+#ifdef C99
+// expected-no-diagnostics
+#endif
+
+void foo() {
+#ifndef C99
+  // expected-warning@+2{{GCC does not allow variable declarations in for loop initializers before C99}}
+#endif
+  for (int i = 0; i < 10; i++)
+    ;
+}