Emitted when a class inheriting from an immutable interface or class does not also have a @psalm-immutable declaration
<?php
/** @psalm-immutable */
interface SomethingImmutable {
public function someInteger() : int;
}
class MutableImplementation implements SomethingImmutable {
private int $counter = 0;
public function someInteger() : int {
return ++$this->counter;
}
}