A type cv void is an incomplete type that cannot be completed;
 such a type has an empty set of values.
 It is used as the return type for functions that do not return a value.
 An expression of type cv void shall be used only as 
 - an expression statement ([stmt.expr]),
 - the expression in a return statement ([stmt.return])
   for a function with return type cv void
 - an operand of a comma expression ([expr.comma])
+- the operand of a parenthesized expression ([expr.prim.paren])
+- a requirement in a requires expression ([expr.prim.req.general])
 - the second or third operand of ?: ([expr.cond])
 - the operand of a typeid expression ([expr.typeid])
 - the operand of a noexcept operator ([expr.unary.noexcept])
 - the operand of a decltype specifier ([dcl.type.decltype]), or
 - the operand of an explicit conversion to type
   cv void ([expr.type.conv], [expr.static.cast], [expr.cast])