ISO/IEC 9899:201x Committee Draft (N1570)

6.3.2.2 void

1
The (nonexistent) value of a void expression (an expression that has type void) shall not　be used in any way, and implicit or explicit conversions (except to void) shall not be applied to such an expression.
If an expression of any other type is evaluated as a void expression, its value or designator is discarded. (A void expression is evaluated for its side effects.)

これ以上何か言うべきことある？