Mapped type error inside but not outside generic function

[RafaelSalguero]

TypeScript Version: 2.3.4

Code

type MT<Keys extends string> = {
    [key in Keys]: number
}

type keys = "x" | "y";
interface Vector { x: number, y: number };

const vectorA: Vector = { x: 10, y: 20 };
const vectorB: MT<keyof Vector> = vectorA; //Correctly works
const vectorC: MT<string> = vectorB; //Correctly works
const vectorD: MT<keys> = vectorA; //Correctly works  

function MyFunc<TData extends MT<keyof TData>>(data: TData) {
    const a : TData = data;
    const b: MT<keyof TData> = a; //Correctly works
    //error!
    const c: MT<string> = b; //MT<keyof TData> is not assignable to MT<string>
}

Expected behavior:
Variable b should be assignable to c since keyof TData its a string union
Actual behavior:
Type MT<keyof TData> is not assignable to MT<string>

[ahejlsberg]

Duplicate of #14548 (since, in the example above, MT<string> is effectively the same as { [key: string]: number }).

[mhegazy]

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