TypeScript 4.6 regression

[thetutlage]

Bug Report

I have read the release notes and the code that is breaking for me is not mentioned in the breaking changes section.

🔎 Search Terms

None. As the code works with 4.5

🕗 Version & Regression Information

The typeof var === 'function' call narrows the variable to function vs one of the union.

• This changed between versions 4.5 and 4.6

⏯ Playground Link

Playground link with relevant code

💻 Code

type Context = Record<string, any>

class Foo<T extends Context> {  
  private contextAccumlator!: () => T | Promise<T>

  constructor(
    context: T | (() => T | Promise<T>)
  ) {
    if (typeof context === 'function') {
      this.contextAccumlator = context
    }
  }
}

🙁 Actual behavior

I expect this.contextAccumlator = context to report zero errors.

🙂 Expected behavior

Instead it says

Type '(() => T | Promise<T>) | (T & Function)' is not assignable to type '() => T | Promise<T>'.
  Type 'T & Function' is not assignable to type '() => T | Promise<T>'.
    Type 'Context & Function' provides no match for the signature '(): T | Promise<T>'.(2322)

[tristan00b]

@thetutlage I played around with this a bit and reduced your example:

function isFunction<T extends Function>(it: unknown): it is T
{
  return typeof it === 'function'
}

function isFn(it: unknown): it is Function
{
  return typeof it === 'function'
}

function f<T>(arg: T | (() => T))
{
  let x: () => T
 
  if (typeof arg === 'function')
    x = arg // (error) arg: (() => T) | (T & Function)

  if (isFunction(arg))
    x = arg // (works) arg: () => T

  if (isFn(arg))
    x = arg // (works) arg: () => T
}

This can be reproduced as far back as version 3.3.3 (play).

[thetutlage]

🤔 Did you mean there is no regression?

[jihndai]

Weirdly enough, it seems those are two different cases. This issue's sample code was indeed working by 4.5.5, broken in 4.6.2, and still broken in 4.7.0-dev.20220304. While the second example was never fixed as far as I can tell.

[tristan00b]

Maybe a recent change caused the old bug to surface in @thetutlage 's code.

[RyanCavanaugh]

This is a correct error that was not properly found before:

type Context = Record<string, any>

class Foo<T extends Context> {  
  private contextAccumlator!: () => (T | Promise<T>)

  constructor(context: T | (() => T | Promise<T>)
  ) {
    if (typeof context === 'function') {
      this.contextAccumlator = context
    }
  }

  invoke() {
    this.contextAccumlator();
  }
}

function isPromise(arg: any): arg is Promise<any> {
  return false;
}

const p = new Foo<(n: string) => void>(n => n.substring(0));
// Crashes
p.invoke();

[thetutlage]

So shouldn't this be under breaking changes?

[RyanCavanaugh]

Yes, that's why it's a Docs bug assigned to the blog post author

[thetutlage]

Okay! Thanks for the clarification :)

[jakebailey]

This was "fixed" in #49119; should it have been?