Fail to get contextual type when an index signature exists

TypeScript Version: 3.2.0-dev.20181017

Code

interface Options {
    m(x: number): void;
    [key: string]: unknown;
}
declare function f(options: number | Options): void;
f({
    m(x) {},
});

Expected behavior:

No error.

Actual behavior:

src/a.ts:7:7 - error TS7006: Parameter 'x' implicitly has an 'any' type.

Was introduced by #27849. For some reason it only happens when options is a union. Discovered in jquery on DefinitelyTyped.