Typescript generic problems and fixes

generics.ts

class Animal {
    name: string;
}

class Dog extends Animal {
    bark() {};
}

class Cat extends Animal {
    meow() {};
}

class Box<V> {
    item: V;
}

// function f<T extends Box<Animal>>(box: T): T {
function f<T extends Box<Animal>>(box: T): Box<Animal> { // specify return type as Box<Animal> to expand generic return type
    box.item = new Cat(); // <-- This is perfectly valid and allowed by the constraint
    return box; // <-- The returned type is T
}

let result = f(new Box<Dog>()); // Type of 'result' is inferred as Box<Dog>

// Now error is thrown as expected
result.item.bark(); // <-- No compilation error. Run-time error...


let cats: Cat[] = [];
// let animals: Animal[] = cats;
let animals = cats;
// remove explicit type declaration and error is thrown

animals.push(new Dog())

cats.forEach(cat => cat.meow()) // <-- No compilation error. Run-time error...