I wouldn't call it broken duck typing. The problem is that ShowAbstracts could just modify the array and put an instance of Abstract there that _isn't_ an Impl

I wouldn't call it broken duck typing. The problem is that ShowAbstracts could just modify the array and put an instance of Abstract there that _isn't_ an Impl, thus violating the original constraint of arr containing only instances of Impl. Once again mutability/invariance raises its ugly head!