Uncalled Function Checks don't check negative condition

[DanielRosenwasser]

function foo() {
}

if (!foo) {
    let x = 10
}
else {
}

Expected: Error on !foo that the condition is always true because foo is defined.
Actual: No error.

Also, any work on this should include the following test:

function foo() {

}

if (!foo) {
}
else {
    foo()
}

[brandon-leapyear]

✨ This is an old work account. Please reference @brandonchinn178 for all future communication ✨

---

Very much agree with this. In our code, we just got bit by

if (!nock.isActive) {
  nock.activate()
}

In general, I would usually not intentionally want !foo, and if I did want to do something like that, I would do foo === undefined. Even more generally, I would personally disallow coercion from function to boolean completely. Forcing devs to write foo === undefined would make it clear what their intention is, and would force the dev to type foo as Function | undefined, if that's truly what they want.

[AlexMarkUSDS]

This bit me last week in one of the codebases I maintain. I also see that it's bitten a number of others:
There's relevant info in:

• Uncalled function checks don't work with negation #43096, which is a duplicate issue
• TS: no longer seeing an error when not calling function in check vscode#91144
• One attempted fix at Uncalled function checks don't work with negation #43097, which was almost accepted.

@DanielRosenwasser, would the TS team accept a contribution for this? I've optimistically opened a PR at #57114

[RyanCavanaugh]

There are many, many false positives turned up in that PR. Unless these were ~all good finds, I think this is appropriately left still in "awaiting more feedback" state

[AlexMarkUSDS]

The tool for finding false positives is super neat. It makes sense to me to not merge this -- I followed ~20 links and I think this was the only legitimate bug I found (it should have an await). The rest were all just undefined checks on things that the typings say should always be defined.

I'm happy to close out that PR (or have you close it). Whatever works for you.