Immutable object properties must be immutable objects

[svnldwg]

If a class has a property of type object which is declared immutable and accessible from outside (public or via getter), the object itself must be immutable as well.

Example:

/** @var immutable */
class Immutable
{
    public Cat $cat;
}

class Cat {
    public int $lala; // not allowed here, Cat must be immutable
}