Generator prototype ignored

TypeScript Version: 3.8.0-dev.20200107

Search Terms: generator, iterator, prototype, inherit

Code

function* gen() {
    while (true) {
        yield "something";
    }
}

gen.prototype.test = () => {
    console.log("OK");
};

gen().test();

Expected behavior: No errors

Actual behavior: Error TS2339: Property 'test' does not exist on type 'Generator<string, void, unknown>'

Playground Link: Provided

Start with the provided TypeScript Playground reproduction and compare the generator function's prototype type with the type returned by gen(). Trace how generator declarations and prototype members are represented by the type checker. Done means the example compiles without TS2339 while preserving correct generator and iterator typing.