Narrowing-down the return type of an implemented method

Question

I\'m trying to specify two interfaces for class A that returns instances of class B, and for class B itself.

I\'m declaring return types on the interface.

Sa

Answer 1

Happy to answer my own question after a year or so:

This will be possible in PHP 7.4

This will be possible:

interface X {
  function m(Y $z): X;
}
interface Y extends X {
  // not permitted but type-safe
  function m(X $z): Y;
}

Check out this RFC.