Overriden class member annotated with a subtype could be a recipe for disaster (by design.. ;) )

TypeScript Version:
1.9.0-dev.20160505 with both strictNullChecks and noImplicitAny enabled.

Code:

class Animal {
    name: string = "Any animal";
}

class Dog extends Animal {
    name: string = "Dog";

    bark() {
        console.log("bark");
    }
}

class AnimalCage {
    member: Animal = new Animal();
}

class DogCage extends AnimalCage {
    member: Dog;
}

let dogCage = new DogCage();

if (dogCage.member != null) { // Yup, it is even checked for null or undefined, 
                              // but it turned out that wasn't enough..
    dogCage.member.bark(); // Bang!
}

Expected behavior:
Design a language where inheritance actually works.
(Edit: I was felling very 'passionate' when I wrote this.. sorry :) nothing personal)

Actual behavior:
By design:

    dogCage.member.bark();
                   ^

TypeError: dogCage.member.bark is not a function
    at Object.<anonymous> (C:\X\Documents\Drafts\TypeScript\Code\a.js:38:20)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:456:32)
    at tryModuleLoad (module.js:415:12)
    at Function.Module._load (module.js:407:3)
    at Function.Module.runMain (module.js:575:10)
    at startup (node.js:159:18)
    at node.js:444:3

Resolution:
tslint volunteers, for the rescue!

Commencez par compiler et exécuter l’exemple TypeScript dans l’issue, puis lisez la discussion sur le comportement d’héritage prévu. Une contribution viable nécessiterait une conception du langage approuvée, un plan d’implémentation et une couverture de régression montrant le comportement obtenu.