If you look through the later items in the series, it seems to get more complex than this. In particular, a character can use either 0 or 1 weapons at a particular moment, and either class can also use the Dagger weapon. It's not that each character inherently only has, and uses, the only weapon appropriate to that character -- this would make things quite a bit simpler.

Well, maybe someday Dotty will be a mainstream language...

    trait Weapon
    class Sword extends Weapon
    class Staff extends Weapon
    class Dagger extends Weapon
    trait Player {
      type T <: Weapon
    }
    class Wizard extends Player {
      type T  = Dagger | Staff
      var weapon: T = null
    }
    class Warrior extends Player {
      type T = Dagger | Sword
      var weapon: T = new Sword
    }

Python already is a mainstream language:

    class Weapon: pass
    class Sword(Weapon): pass
    class Staff(Weapon): pass
    class Dagger(Weapon): pass

    class Player:
        weapon: Weapon
    class Wizard(Player):
        weapon: Optional[Union[Dagger, Staff]] = None
    class Warrior(Player):
        weapon: Optional[Union[Dagger, Sword]] = Sword()

    class Wizard extends Player {
      type T  = Dagger | Staff
      var weapon: T = null
    }

The billion-dollar mistake in a new language? What a waste.

Yeah that freaking blows. They keep it for compatibility with java. It's not idiomatic code though, you would use an Option.

Until you want to model a bar-fight and then a chair (a piece of furniture) becomes a weapon, i.e. "class Chair: Furniture{}" turns into "Class Chair: Weapon {}". Or when the winter gets tougher and longer and "class Chair: Furniture{}" changes into "class Chair: Firewood{}". I know that there's probably a "solution" involving object "composition", but that would only mask the problem, will not really solve it.

Honest question: does anyone have a clean solution to this problem? I've run into this often and always end up hacking something ugly to make it work.

You probably want to solve "the expression problem".

I'm not sure I understand your objection. Those are protocols, so Chair can adhere to all of them at once.

class Chair: Furniture, Weapon, Firewood {}

That's why protocol extensions were implemented.

That forces wizard to always have a staff and warrior to always have a sword?

warrior can also use a staff

both can use daggers