Shouldn't allow a class instance passed to function if it is the wrong type

[eamodio]

TypeScript Version: 2.1.4

Code

Playground link

class Base {
    value: string;
    constructor(value: string) { this.value = value; }
}

class A extends Base {
    constructor(value: string) { super(value); }
}

class B extends Base {
    constructor(value: string) { super(value); }
}

function onlyA(o: A) { console.log("onlyA", o.value); }
function onlyB(o: B) { console.log("onlyB", o.value); }

let a = new A("a");
let b = new B("b");

onlyA(a); // good
onlyA(b); // bad
onlyB(b); // good
onlyB(a); // bad

Expected behavior:

I would expect the lines marked bad above to cause compile errors. I would expect this behavior when using interfaces, but not with classes.

Actual behavior:

Compiles without errors.

[zpdDG4gta8XKpMCd]

answer: https://github.com/Microsoft/TypeScript/wiki/FAQ#what-is-structural-typing

[eamodio]

OK, right that make sense. Although now that Class means something to javascript (a specific implementation) -- using structural typing when specifying a class seems "wrong". If I was using an interface, I totally agree.

But what actually sent me down this path was I originally tried this:

function onlyA(o: A) {
    if (o instanceof A) {
        console.log("onlyA - is A", o.value);
        return;    
    }
    console.log("onlyA - not A", o.value);
}

Playground link

But that doesn't compile, because it thinks o is the never type.

It feels like that is a bug right? @Aleksey-Bykov @RyanCavanaugh

[RyanCavanaugh]

See #10422, #10648, #7271, etc