Circular reference misdetection

[falsandtru]

TypeScript Version: 3.4.0-dev.20190817

Search Terms:

Code

export function f<T extends HTMLOListElement | DocumentFragment>(target: T): T {
  if (target instanceof HTMLOListElement) {
    for (const el of target.querySelectorAll('a')) {
      if (el.closest('ol') !== target) continue;
    }
  }
  return target;
}

Expected behavior:
pass

Actual behavior:

const el: any
'el' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.

Playground Link: http://www.typescriptlang.org/play/index.html#code/KYDwDg9gTgLgBAMwK4DsDGMCWEWIDwAqcoMwKAJgM5wASBAsgDIDyjmlMAogDbAC2ZeAB84AEQhokAlDABiUAIYBzaTAB8AChgKoS4DABccAgEojRAN4AoOHEwI4WnXviYUHBemAQHdJq3YuXlUTOGtbWwRoRzQcDmJuOB84bV19ADoARyRgKABPAGVgXgxoAEFubg0AcgVqk1DwiNt7R2L0tG4ISmAOGohuergAQgBeUZTnfVDYmTccgG4bCIBfZbXbKH0kKFxUlyWVoA

Related Issues:

[RyanCavanaugh]

🤔🤔🤔 Usually there's some sneaky dependency here, but I can't find one. We might be incorrectly thinking that the back edge of the for loop somehow influences el, but it doesn't?

Simplifiable a bit

export function f(target: HTMLOListElement | DocumentFragment) {
  if (target instanceof HTMLOListElement) {
    for (const el of target.querySelectorAll('a')) {
      if (el === target) {
      }
    }
  }
}

[falsandtru]

This bug is found in writing a polyfill of :scope keyword for Edge browser.

// expect
if (target instanceof HTMLOListElement) {
  for (const el of target.querySelectorAll(':scope > li > a')) {
  }
}
// polyfill
if (target instanceof HTMLOListElement) {
  for (const el of target.querySelectorAll('a')) {
    if (el.closest('ol') !== target) continue;
  }
}

[ahejlsberg]

I think what's happening here is that in order to determine the type of el we have to determine the type of target.querySelectorAll('a'), which requires us determine the control flow type of target, which could be narrowed by the expression el.closest('ol') !== target, which requires us to determine the type of el, which triggers a circularity.

[thorn0]

Another instance (playground; reproduces in TS 3.6.2 too):

declare function f(): number | undefined;
const [a, b = a] = [10, f()];
// 'a' implicitly has type 'any' because it does not have a type annotation and 
// is referenced directly or indirectly in its own initializer.

Or is this #33191? Or are these two issues the same?

[falsandtru]

Not similar to the others. Should be filed as a separated issue.