re PR c/21159 ("no effect" warning despite cast to void*)

	PR c/21159
	* c-typeck.c (build_compound_expr): Don't warn for left-hand side
	being a compound expression whose right-hand side is cast to void.

testsuite:
	* gcc.dg/void-cast-1.c: New test.

From-SVN: r98886

gcc/ChangeLog

+2005-04-27  Joseph S. Myers  <joseph@codesourcery.com>
+
+	PR c/21159
+	* c-typeck.c (build_compound_expr): Don't warn for left-hand side
+	being a compound expression whose right-hand side is cast to void.
+
 2005-04-27  Caroline Tice  <ctice@apple.com>
 
         * bb-reorder.c (find_rarely_executed_basic_blocks_and_crossing_edges):

gcc/c-typeck.c

@@ -3109,9 +3109,16 @@ build_compound_expr (tree expr1, tree expr2)
          statement: with -Wextra or -Wunused, we should warn if it doesn't have
 	 any side-effects, unless it was explicitly cast to (void).  */
       if (warn_unused_value
-           && !(TREE_CODE (expr1) == CONVERT_EXPR
-                && VOID_TYPE_P (TREE_TYPE (expr1))))
-        warning (0, "left-hand operand of comma expression has no effect");
+	  && !VOID_TYPE_P (TREE_TYPE (expr1)))
+	{
+	  if (TREE_CODE (expr1) == CONVERT_EXPR)
+	    ; /* (void) a, b */
+	  else if (TREE_CODE (expr1) == COMPOUND_EXPR
+		   && TREE_CODE (TREE_OPERAND (expr1, 1)) == CONVERT_EXPR)
+	    ; /* (void) a, (void) b, c */
+	  else
+	    warning (0, "left-hand operand of comma expression has no effect");
+	}
     }
 
   /* With -Wunused, we should also warn if the left-hand operand does have

gcc/testsuite/ChangeLog

+2005-04-27  Joseph S. Myers  <joseph@codesourcery.com>
+
+	PR c/21159
+	* gcc.dg/void-cast-1.c: New test.
+
 2005-04-27  Paolo Bonzini  <bonzini@gnu.org>
 
 	* gcc.dg/tree-ssa/gen-vect-25.c: Make more portable.

gcc/testsuite/gcc.dg/void-cast-1.c

+/* Don't warn where the left-hand side of a comma expression is a
+   comma expression whose right-hand side is cast to void.  Bug
+   21159.  */
+/* Origin: Joseph Myers <joseph@codesourcery.com> */
+/* { dg-do compile } */
+/* { dg-options "-Wall" } */
+
+int a, b, c, d;
+int e(void) { return (void)a, b; }
+int f(void) { return (void)a, (void)b, c; }
+int g(void) { return (void)a, (void)b, (void)c, d; }