Typescript This overload signature is not compatible with its implementation signature

function overloading

I wrote the 2 interfaces. Also I wrote 3 overloading functions. And I have implementation of functions. And here an error - This overload signature is not compatible with its implementation signature

interface MyPosition {
  x: number | undefined
  y: number | undefined
}

interface MyPositionWithDefault extends MyPosition {
  default: string
}

function position(): MyPosition
function position(a: number): MyPositionWithDefault // Here`s an 
//error
function position(a: number, b: number): MyPosition
function position(a?: number, b?: number) {
  if (!a && !b) {
    return {x: undefined, u: undefined}
  }

  if (a && !b) {
    return {x: a, y: undefined, default: a.toString}
  }

  return {x: a, y: b}
}

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The compiler for some reason cannot infer the correct return type, simply adding the annotation fixes the problem:

interface MyPosition {
  x: number | undefined
  y: number | undefined
}

interface MyPositionWithDefault extends MyPosition {
  default: string
}

function position(): MyPosition
function position(a: number): MyPositionWithDefault // No error
function position(a: number, b: number): MyPosition
function position(a?: number, b?: number): MyPosition | MyPositionWithDefault { // added return type annotation
  if (!a && !b) {
    return {x: undefined, y: undefined}
  }

  if (a && !b) {
    return {x: a, y: undefined, default: a.toString() }
  }

  return {x: a, y: b}
}

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I also cleaned up some typos.