Using nullish coalescing with untyped field in constructor trigger TS7022

[6XGate]

Bug Report

Using nullish coalescing in a constructor to initialize a class member with no type annotation will result in a TS7022 error.

🔎 Search Terms

nullish, coalescing, 7022, ??, and ts7022

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about nullish coalescing and type inference.

⏯ Playground Link

Playground link with relevant code

💻 Code

export class Node {
  #parent

  #children = new Children(this)

  constructor (parent?: Node | null | undefined) {
    // Remove `??` and the code after and the code compiled. Now #parent is undefined which is not desired.
    this.#parent = parent ?? null
  }

  get parent () { return this.#parent }

  get children () { return this.#children }
}

export class Children {
  #parent
  #head
  #tail

  constructor (parent: Node) {
    this.#parent = parent
    this.#head = new Node(parent)
    this.#tail = new Node(parent)
  }
}

🙁 Actual behavior

Attempting to compile the example code results in a TS7022 error: '#parent' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.

🙂 Expected behavior

The code should compile and #parent should be typed as Node | null.

[Andarist]

It's a very similar problem to the one that I diagnosed here (the code paths that we hit here are almost identical at the end, only the origin of the problem is slightly different)

The problem is that this binary expression in the initializer creates a union type here (of the left type and the right type) and that union is a subject of subtype reduction. That in turn asks for properties of the parent's properties and that encounters a cycle.

This particular case here probably could be avoided since the right type is null and the left type is not nullish but the other mentioned issue shows us that there are other, similar, cases for which this wouldn't help at all. A smarter fix would be appreciated but I don't have ideas for it.