Method overriding rules might be too permissive

[tobich]

Consider this example:

class T1 {
    prop1: string = 'T1/prop1';
}

class T2 extends T1 {
    prop2: string = 'T2/prop2';
}

class A {
    propA: string;
    checkValue(value: T1) {
        console.assert(!!value.prop1);
    }
}

class B extends A {
    propB: string;
    checkValue(value: T2) {
        console.assert(!!value.prop2);
    }
}

B::checkValue(T2) can override A::checkValue(T1) because T2 is assignable to T1 (as it is its subclass).

However, following code, while perfectly valid, will break the second assertion:

const a: A = new B();
const t1: T1 = new T1();
a.checkValue(t1); // B.prototype.checkValue(), expecting T2 but getting T1.

[mhegazy]

Please see https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-function-parameters-bivariant for relevant details.