TypeScript inference not working for Array.map [duplicate]

In the following code, why does the function notWorking pass TypeScript compilation, but the function working fails with:

error TS2353: Object literal may only specify known properties, and 'x' does not exist in type 'AAA'.

(Incidentally, notWorking does break compilation if I remove the property AAA.A - why?)

interface AAA {
  A: number;
  B?: number;
}

function notWorking(vals: number[]): AAA[] {
  // No typescript error!
  return vals.map(v => ({A: v, x: 'bad!'}));
}

function working(vals: number[]): AAA[] {
  // error TS2353
  return vals.map((v): AAA => ({A: v, x: 'bad!'}));
}