Closures don't recognize class hierarchy

I have this closure interface:

public interface ItemClosure
	function run(UnitInventory inventory, ItemInstance inst) 

I also have this assignment somewhere in code:

onUnequip = (UnitInventory inventory, ItemInstance inst) -> begin
    let armor = inst castTo ItemInstanceArmor
    inventory.getUnit().defense.add(armor.def)
end

ItemInstanceArmor is a subclass of ItemInstance, so this works, nothing weird there. What doesn’t work, however, is this:

onUnequip = (UnitInventory inventory, ItemInstanceArmor inst) -> begin
    inventory.getUnit().defense.add(inst.def)
end

I get a huge red line across the whole assignment because:

file: 'file:///x%3A/wc3/projects/TheRpg/wurst/equipment/classes/ItemClassArmor.wurst'
severity: 'Error'
message: 'Cannot assign (UnitInventory, ItemInstanceArmor) -> Void to ItemClosure'
at: '42,9'
source: 'Wurst'

[peq]

Downcasts are not safe, so they are not done automatically.

This is the same (or even more restrictive) in most languages. If you want to read more about it have a look at https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)

[IgorSamurovic]

Oh, I didn’t think about that at all. I’ve now spent some time reading up on the subject and I realized I didn’t actually understand the problematics of it. Thank you!