mixin properties not checked

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