diff --git a/gcc/c-tree.h b/gcc/c-tree.h
index 4538f4b32ac..21d0b49d426 100644
--- a/gcc/c-tree.h
+++ b/gcc/c-tree.h
@@ -479,7 +479,7 @@ extern int warn_missing_braces;
 
 /* Warn about comparison of signed and unsigned values.  */
 
-int warn_sign_compare;
+extern int warn_sign_compare;
 
 /* Nonzero means this is a function to call to perform comptypes
    on two record types.  */