Inconsistent errors when assigning to property with index signature on object vs function

TypeScript Version: 3.1.0-dev.20180823

Code

interface Idx {
    [key: string]: number;
}

interface Obj {
    idx?: Idx;
}
const l: Obj = {};
l.idx = {}; // Works

interface Fn {
    (): void;
    idx?: Idx;
}
const fn: Fn = () => {}; // Error: `{}` not assignable to `Idx`
fn.idx = {};

Expected behavior:

No error.

Actual behavior:

src/a.ts:15:7 - error TS2322: Type '{ (): void; idx: {}; }' is not assignable to type 'Fn'.
  Types of property 'idx' are incompatible.
    Type '{}' is not assignable to type 'Idx'.
      Index signature is missing in type '{}'.

Discovered in koa-morgan on DefinitelyTyped. Probably due to #26368.

[Flarna]

I think this is the same as #26521.

Thanks, should have tested against master.