Incorrect circularity detection

TypeScript Version: 3.6.2

Search Terms:

• 7022
• directly or indirectly in its own initializer

Code

function extent(nums: number[]) {
  let result: [number, number] | null = null;
  for (const num of nums) {
    if (!result) {
      result = [num, num];
    } else {
      const [oldMin, oldMax] = result;
      result = [Math.min(num, oldMin), Math.max(num, oldMax)];
    }
  }
  return result;
}

Expected behavior:

No error.

Actual behavior:

'oldMin' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.ts(7022)
'oldMax' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.ts(7022)

I’m really struggling to see where the circularity comes in. The type of result is shown as [number, number] in that branch, so I’d assume destructuring it would result in two number types.

Removing the destructuring and using array accesses is equivalent but produces no error:

function extent(nums: number[]) {
  let result: [number, number] | null = null;
  for (const num of nums) {
    if (!result) {
      result = [num, num];
    } else {
      result = [Math.min(num, result[0]), Math.max(num, result[1])];  // ok
    }
  }
  return result;
}

Playground Link: link

Related Issues:

• https://github.com/microsoft/TypeScript/issues/32950

[JoostK]

BTW, using @JoostK’s example, if you removes the asignation, error goes away… 😆

Removing the assignment removes the circular nature of currentNode, so that’s expected. It ends up being circular due to the involvement of flow typing.

[MichaelMitchell-at]

A new one in 4.7.0-beta https://github.com/microsoft/TypeScript/issues/48708