[C PATCH] fix column number in comma expression warning

[Prathamesh Kulkarni <bilbotheelffriend at gmail dot com>]

Show column number of left operand instead of comma operator
in the warning "left-hand operand of comma expression has no effect"

Example:
ax.c:4:6: warning: left-hand operand of comma expression has no effect
[-Wunused-value]
   x  ,  y;
      ^
Instead of comma operator, show location of left-operand:
ax.c:4:3: warning: left-hand operand of comma expression has no effect
[-Wunused-value]
   x  ,  y;
   ^
Bootstrapped on x86_64-unknown-linux-gnu.

[gcc/c]
* c-parser.c (c_parser_expression): Pass tloc instead of loc to
build_compound_expr.

[gcc/testsuite]
* gcc.dg/Wunused-value-4.c: New test case.

I am not able to figure out, how to write test-case that
raises multiple warnings. example: (void) x, y, z.
I tried this:
/* { dg-warning "8:left-hand operand of comma expression has no effect
                      |11:left-hand operand of comma expression has no
effect" } */
x has column number 8 and y has column number 11, but this doesn't
seem to work (the patch works correctly).

Thanks and Regards,
Prathamesh

Index: gcc/c/c-parser.c
===================================================================
--- gcc/c/c-parser.c	(revision 207916)
+++ gcc/c/c-parser.c	(working copy)
@@ -7838,7 +7838,6 @@ c_parser_expression (c_parser *parser)
     {
       struct c_expr next;
       tree lhsval;
-      location_t loc = c_parser_peek_token (parser)->location;
       location_t expr_loc;
       c_parser_consume_token (parser);
       expr_loc = c_parser_peek_token (parser)->location;
@@ -7849,9 +7848,10 @@ c_parser_expression (c_parser *parser)
 	mark_exp_read (lhsval);
       next = c_parser_expr_no_commas (parser, NULL);
       next = convert_lvalue_to_rvalue (expr_loc, next, true, false);
-      expr.value = build_compound_expr (loc, expr.value, next.value);
+      expr.value = build_compound_expr (tloc, expr.value, next.value);
       expr.original_code = COMPOUND_EXPR;
       expr.original_type = next.original_type;
+      tloc = expr_loc;
     }
   return expr;
 }
Index: gcc/testsuite/gcc.dg/Wunused-value-4.c
===================================================================
--- gcc/testsuite/gcc.dg/Wunused-value-4.c	(revision 0)
+++ gcc/testsuite/gcc.dg/Wunused-value-4.c	(working copy)
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-Wunused-value" } */
+
+void foo(int x);
+
+int a[10];
+
+void f(void)
+{
+  foo ((1, 2));                /* { dg-warning "9: left-hand operand of comma expression has no effect" } */ 
+  a[0x03, 004] = 1992;         /* { dg-warning "5: left-hand operand of comma expression has no effect" } */
+}