Downcast leads to compiler error TS2352 #17338

interface Base { v2: string; }
interface A extends Base { v1: number; }
interface B extends Base { v3: object; }
const x: A = { v1: 5, v2: "I'm a c", v3: { hans: 'franz' } } as A;
const y: B = x as B;

error TS2352: Type 'A' cannot be converted to type 'B'.
  Property 'v3' is missing in type 'A'.