mismatched decls w/ both builtin and explicit decl

[DJ Delorie]

> Please send a patch that adds a testcase to the testsuite to show the 
> diagnostics you get after the patch.

How's this?

	* c-decl.c (locate_old_decl): If a previous conflicting decl is
	both explicit and builtin, print the location of the explicit one.

	* gcc.dg/mismatch-decl-1.c: New test.

Index: c/c-decl.c
===================================================================
--- c/c-decl.c	(revision 204300)
+++ c/c-decl.c	(working copy)
@@ -1630,13 +1630,14 @@ validate_proto_after_old_defn (tree newd
 /* Subroutine of diagnose_mismatched_decls.  Report the location of DECL,
    first in a pair of mismatched declarations, using the diagnostic
    function DIAG.  */
 static void
 locate_old_decl (tree decl)
 {
-  if (TREE_CODE (decl) == FUNCTION_DECL && DECL_BUILT_IN (decl))
+  if (TREE_CODE (decl) == FUNCTION_DECL && DECL_BUILT_IN (decl)
+      && !C_DECL_DECLARED_BUILTIN (decl))
     ;
   else if (DECL_INITIAL (decl))
     inform (input_location, "previous definition of %q+D was here", decl);
   else if (C_DECL_IMPLICIT (decl))
     inform (input_location, "previous implicit declaration of %q+D was here", decl);
   else
Index: testsuite/gcc.dg/mismatch-decl-1.c
===================================================================
--- testsuite/gcc.dg/mismatch-decl-1.c	(revision 0)
+++ testsuite/gcc.dg/mismatch-decl-1.c	(revision 0)
@@ -0,0 +1,22 @@
+/* This test assumes -fbuiltin and not -fansi to get "index" and "memchr" builtin DECLs.  */
+/* { dg-do compile } */
+/* { dg-options "" } */
+
+/* The bug this is testing is that if a new decl conflicts with an
+   explicit decl, you don't get the "changes type of builtin" message,
+   but if there was *also* a builtin, you *also* don't get the
+   "previous declaration was here" message, leaving you with no clue
+   where the previous declaration came from.  */
+
+extern char foo(int,int); /* { dg-message "previous declaration of 'foo' was here" } */
+extern char *index(const char *,int); /* { dg-message "previous declaration of 'index' was here" } */
+
+/* This changes the type of "index", which is both a builtin and an
+   explicit decl.  */
+int index; /* { dg-error "redeclared as different kind of symbol" } */
+
+/* This changes the type of "memchr", which is only a builtin.  */
+int memchr; /* { dg-warning "built-in function 'memchr' declared as non-function" } */
+
+/* This changes the type of "foo", which is only an explicit decl.  */
+int foo; /* { dg-error "redeclared as different kind of symbol" } */