Inconsistency between type literal and interface assignability to index signature

[ghost]

TypeScript Version: 3.1.0-dev.20180721

Code

type T = { x: number };
interface I { x: number; }

let index: { [key: string]: number };
let t: T = { x: 0 };
let i: I = { x: 0 };

index = { x: 0 }; // OK
index = t; // OK
index = i; // Error

Expected behavior:

No error.

Actual behavior:

src/a.ts:10:1 - error TS2322: Type 'I' is not assignable to type '{ [key: string]: number; }'.
  Index signature is missing in type 'I'.

10 index = i; // Error

[mhegazy]

Duplicate of #15300

[ghost]

What's relevant to DefinitelyTyped/DefinitelyTyped#27465 is, how can one constrain a type such that all of the values of that type extend something?

type ValuesAreStringOrNumber = { [key: string]: string | number };
declare function f<T extends ValuesAreStringOrNumber>(t: T): T;
interface XNumber {
    x: number;
}
const o = f<XNumber>({ x: 0 });

The example works perfectly without the type annotation (and errors on { x: {} }), but fails when an interface type is provided.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.