Super-types declaration

There are a RFC for union-types (https://wiki.php.net/rfc/union_types) that was declined, but I see that it is a recurring question here (today, for instance, David Eddy tried to start this discussion again with the same concept). Then, I don't know if the problem is the union methodology like string|int or if it could be useless. In my mind, I think that is very useful, mainly over scalar types (string|int, for instance) or when method could support mixed types of objects. For instance, I have a method called mergeWith($value) that allows $value to be instanceof array or self. Currently I am solving this problem by using assert(), something like assert(is_array($value) || $value instanceof self), but I see that is not the best option. In this case, I suggests to create a super-type that allows to define a super-type (typedef) or inline super-type (union-types). class { typedef MyType (self, array); public function mergeWith(MyType $value) { } } Or, following the union-types (as inline super-type): public function mergeWith(types(self, array) $value) { } public function anotherMethod(): types(self, array) { } -- David Rodrigues