mixin properties not checked

[ghost]

TypeScript Version: master (as of #26283)

Code

/**
 * @template {new (...args: any[]) => any} T
 * @param {T} SuperClass
 */
function myMixin(SuperClass) {
    return class extends SuperClass {
        method() { return 0; }
    }
};

class Base {}
const Mixed = myMixin(Base);

const instance = new Mixed();
instance.method(); // Correctly typed in signature help
instance.meshod(); // But not checked

Run with allowJs, checkJs, and strict.

Expected behavior:

Error at meshod.

Actual behavior:

No error.

[sandersn]

This is true of Typescript as well:

function mix<T extends new (...args: any[]) => any>(S: T) {
    return class extends S {
        f() { return 0; }
    }
};

class Base {}
const Mixed = mix(Base);

export const m = new Mixed();
m.f(); // Correctly typed in signature help
m.j(); // should error here, but doesn't

[ghost]

Looks like it has to do with the type T extends returning any (fixed if you change it to new (...args: any[]) => {}) -- though I think the actual T ought to (but isn't) used instead of its constraint when outside of mix?

[sandersn]

Well, the reason that you see the apparent type of T is that the any comes from class extends S. S has type T, whose apparent type has a constructor with return type any, which is equivalent to extending any. That’s what allows you to access any property on m.

[ghost]

I think the problem then is that we force the user to write any in that position rather than unknown, which is an error (Base constructor return type 'unknown' is not a class or interface type.)

[michaeljota]

I though about using InstanceType in the return, that actually makes sense, although the code does throw when expected, it also throws extending S

function mix<T extends new (...args: any[]) => InstanceType<T>>(S: T) {
    return class extends S { // Throws: Base constructor return type 'InstanceType<T>' is not a class or interface type.
        f() { return 0; }
    }
};

class Base {}
const Mixed = mix(Base);

export const m = new Mixed();
m.f(); // Correctly typed in signature help
m.j(); // Throws: Property 'j' does not exist on type 'mix<typeof Base>.(Anonymous class) & Base'.

Maybe I'm using InstanceType poorly, but I think this should be the way to go.