RFA: PATCH to c_strlen for c++/35652 (wrong source location for string bounds warning)

[Jason Merrill <jason at redhat dot com>]

Having source location information on most expressions makes this bug a 
lot easier to fix.

Tested x86_64-pc-linux-gnu.  OK for 4.5?
commit 3ca3ae93bd4b675ce3bc846ad6a5735d368b7bb6
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Feb 2 22:34:31 2010 -0500

    	PR c++/35652
    	* builtins.c (c_strlen): Use EXPR_LOCATION in diagnostics.

diff --git a/gcc/builtins.c b/gcc/builtins.c
index ac96934..b6fd1ea 100644
--- a/gcc/builtins.c
+++ b/gcc/builtins.c
@@ -433,6 +433,7 @@ c_strlen (tree src, int only_value)
   HOST_WIDE_INT offset;
   int max;
   const char *ptr;
+  location_t loc;
 
   STRIP_NOPS (src);
   if (TREE_CODE (src) == COND_EXPR
@@ -450,6 +451,11 @@ c_strlen (tree src, int only_value)
       && (only_value || !TREE_SIDE_EFFECTS (TREE_OPERAND (src, 0))))
     return c_strlen (TREE_OPERAND (src, 1), only_value);
 
+  if (EXPR_HAS_LOCATION (src))
+    loc = EXPR_LOCATION (src);
+  else
+    loc = input_location;
+
   src = string_constant (src, &offset_node);
   if (src == 0)
     return NULL_TREE;
@@ -475,7 +481,7 @@ c_strlen (tree src, int only_value)
 	 and return that.  This would perhaps not be valid if we were dealing
 	 with named arrays in addition to literal string constants.  */
 
-      return size_diffop_loc (input_location, size_int (max), offset_node);
+      return size_diffop_loc (loc, size_int (max), offset_node);
     }
 
   /* We have a known offset into the string.  Start searching there for
@@ -494,7 +500,7 @@ c_strlen (tree src, int only_value)
      /* Suppress multiple warnings for propagated constant strings.  */
       if (! TREE_NO_WARNING (src))
         {
-          warning (0, "offset outside bounds of constant string");
+          warning_at (loc, 0, "offset outside bounds of constant string");
           TREE_NO_WARNING (src) = 1;
         }
       return NULL_TREE;
diff --git a/gcc/testsuite/g++.dg/warn/string1.C b/gcc/testsuite/g++.dg/warn/string1.C
new file mode 100644
index 0000000..3027727
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/string1.C
@@ -0,0 +1,18 @@
+// PR c++/35652
+// { dg-options "-O" }
+
+#include <string>
+int main() {
+  // blank line padding, could also be code...
+  //
+  //
+  //
+  //
+  //
+  //
+  //
+  //
+  //
+  std::string s = "";
+  s += 'x' + "y";	      // { dg-warning "bounds of constant string" }
+}