Phpstan complains that a method returns a child where it expected a parent

# Bug report

In the following code:
https://phpstan.org/r/934fcc4c-109b-4696-b1f4-4586a04cbcb9

PHPStan says:
`Method A::filterSubject() should return TSubject of Subject|null but returns Filterable|null.`

but Filterable is a child of Subject so it should be valid code

The error message isn't entirely true either, at line 19, we can return something that isn't a Filterable.

Thanks!