Uncalled function checks don't work when the variable is referenced in the block

[jonhue]

Bug Report

🔎 Search Terms

uncalled function checks, conjunction, &&

🕗 Version & Regression Information

This is the behavior in every version I tried

⏯ Playground Link

Playground link with relevant code

💻 Code

// @strictNullChecks: true

declare function isFoo(): boolean;
declare function isBar(): boolean;

if (isFoo) {
    // uncalled function checks already work well here
}

if (isFoo && isBar()) {
    // uncalled function checks already work well here
}

if (isFoo && isFoo()) {
    // uncalled function checks should cover this case too
}

if (isFoo && isBar()) {
    isFoo;
    // uncalled function checks should cover this case too
}

🙁 Actual behavior

In the last two cases we didn't detect that the first occurrence of isFoo in the condition is uncalled.

🙂 Expected behavior

When isFoo cannot possibly be a falsy value (i.e. undefined) we should highlight this as an error even if isFoo occurs again later on. This is a follow-up to a conversation in #42835 (comment).
I would be happy to enable this check, then we could run this against some user tests and decide whether there are too many false positives 🙂

[RyanCavanaugh]

A reference should be sufficient here to not complain, since you might write something like

if (isFoo) {
  invokeThisPlease(isFoo);
}

[jonhue]

@RyanCavanaugh I'm probably misunderstanding your example, but if isFoo cannot possible be undefined, isn't the condition in the example you gave above redundant?

[RyanCavanaugh]

It's intentional that the rule is not ironclad. This is because there are some places in the language where TS thinks something is always not-undefined, but it's not necessarily the case.