Wouldn't -Wshadow be more useful if it obeyed -Wno-system-headers?
For code like

        #include <stdlib.h>
        int foo (int atof);
        int foo (int atof) { return atof; }

we currently do not warn on the prototype, but do on the function
definition, leading to reports such as
http://sourceware.org/bugzilla/show_bug.cgi?id=13121

The following has been bootstrapped and regression tested
powerpc-linux.  OK to apply?

        * c-decl.c (warn_if_shadowing): Don't warn if shadowed
        identifier is from system header.

Index: gcc/c-decl.c
===================================================================
--- gcc/c-decl.c        (revision 178035)
+++ gcc/c-decl.c        (working copy)
@@ -2516,7 +2516,10 @@ warn_if_shadowing (tree new_decl)
 
   /* Is anything being shadowed?  Invisible decls do not count.  */
   for (b = I_SYMBOL_BINDING (DECL_NAME (new_decl)); b; b = b->shadowed)
-    if (b->decl && b->decl != new_decl && !b->invisible)
+    if (b->decl && b->decl != new_decl && !b->invisible
+       && (b->decl == error_mark_node
+           || diagnostic_report_warnings_p (global_dc,
+                                            DECL_SOURCE_LOCATION (b->decl))))
       {
        tree old_decl = b->decl;
 

-- 
Alan Modra
Australia Development Lab, IBM

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