Comment by catnaroek

> The abstract class defines which methods every class that inherits from it must define by using abstract methods.

In their example, the type checker can infer that Animal has a `talk` method, even if it's never explicitly defined:

    abstract class Animal
      # no talk method here!
    end
    
    class Dog < Animal
      def talk
        "Woof!"
      end
    end
    
    class Cat < Animal
      def talk
        "Miau"
      end
    end
    
    class Person
      getter pet
    
      def initialize(@name : String, @pet : Animal)
      end
    end
    
    john = Person.new "John", Dog.new

In their own words, “Now the code compiles:”

    john.pet.talk #=> "Woof!"

Now, what happens if, in a separate module, I define a Snake subclass of Animal, without a talk method? What happens in this corner case isn't documented anywhere.

Type system design is very serious business, and can't be done by mindless trial and error. When a type system has a safety hole, patching it is pretty much guaranteed to break other people's code.