Description:
------------
Hello,

When I declare a function in a class/interface with "mixed" return type, and than extend the class and apply covariant return type "iterable", I get compatibility error saying that functions are not compatible.

Code snippet bellow gives me:

Fatal error: Declaration of Child::getValue(): iterable must be compatible with SomeInterface::getValue(): mixed

I am not sure if "iterable" has any special treatment (it works correctly for other types, such as int, self, Child, ...) or is it just forgotten pseudo type.

Thank you.

Václav Pelíšek @peldax

Test script:
---------------
<?php

interface SomeInterface {
    public function getValue() : mixed;
}

class Child implements SomeInterface
{
    public function getValue() : iterable
    {
        return [];
    }
}

Expected result:
----------------
Script compiles successfully.

Actual result:
--------------
Fatal error: Declaration of Child::getValue(): iterable must be compatible with SomeInterface::getValue(): mixed