rest = internal_build_compound_expr (TREE_CHAIN (list), FALSE);
- /* When pedantic, a compound expression can be neither an lvalue
- nor an integer constant expression. */
- if (! TREE_SIDE_EFFECTS (TREE_VALUE (list)) && ! pedantic)
- return rest;
+ if (! TREE_SIDE_EFFECTS (TREE_VALUE (list)))
+ {
+ /* The left-hand operand of a comma expression is like an expression
+ statement: with -W or -Wunused, we should warn if it doesn't have
+ any side-effects, unless it was explicitly cast to (void). */
+ if ((extra_warnings || warn_unused)
+ && ! (TREE_CODE (TREE_VALUE (list)) == CONVERT_EXPR
+ && TREE_TYPE (TREE_VALUE (list)) == void_type_node))
+ warning ("left-hand operand of comma expression has no effect");
+
+ /* When pedantic, a compound expression can be neither an lvalue
+ nor an integer constant expression. */
+ if (! pedantic)
+ return rest;
+ }
+
+ /* With -Wunused, we should also warn if the left-hand operand does have
+ side-effects, but computes a value which is not used. For example, in
+ `foo() + bar(), baz()' the result of the `+' operator is not used,
+ so we should issue a warning. */
+ else if (warn_unused)
+ warn_if_unused_value (TREE_VALUE (list));
return build (COMPOUND_EXPR, TREE_TYPE (rest), TREE_VALUE (list), rest);
}