Invariant Property Types
Covariant Returns and Contravariant Parameters — Levi Morrison
https://wiki.php.net/rfc/covariant-returns-and-contravariant-parameters —
typed properties are invariant
<?php
class 🌐sphere {}
class 🌏planet extends 🌐sphere {}
class Copernicus
{
public 🌐sphere $gravity;
}
class Galileo extends Copernicus
{
public 🌏planet $gravity;
}
?>
Fatal error: Type of Galileo::$gravity must be 🌐sphere (as in class Copernicus)
Property types are *in-variant*:
~they can *not* represent a more specific or a more broad type~