I may be missing something, but your example doesn't typecheck?

  class Animal { }
  class Dog extends Animal{
    bark(){return 1}
  }
  class Cat extends Animal{
    bark(){return 1}
  }
  const dogs: Dog[] = []
  const animals: Animal[] = dogs
  animals.push(new Cat())
  animals[0].bark() <<<<< "Property 'bark' does not exist on type 'Animal'."

Should be `dogs[0].bark()`