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There should probably be a special case for if(!!expr) { so that there's an idiomatic non-casting workaround for checkJs scenarios

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

I don't think this is the scope of strictNullChecks. This change injures the certainty and reliance of strictNullChecks. This change should be enabled by a new flag.

This didn't go nearly as well as expected, unfortunately.

RWC turned up over a hundred examples of code of the form

if (someExpr) {
    someExpr();
}

where someExpr isn't really provably not-undefined because:

• It was a class property not covered by strictPropertyInitialization
• It was an array or map property lookup, this might be undefined even though we generally pretend this never happens
• It's a DOM property that isn't present in all browsers, so it's valid feature-detection code
• It comes from props that might have bad JS callers (this one is more questionable)

Some of this also probably happened as codebases started out without strictNullChecks on, wrote code correctly to defensively handle undefineds, then turned SNC on and it just so happened that there weren't identifiable undefineds manifest in those positions after all.

However, there were a few hits some code that didn't call someExpr where it did appear to be a bug:

                if (this.isComponentMounted) {
                    ~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2774: This condition will always return true since the function is always defined. Did you mean to call it instead?
                    this.setState({ isUnblocking: false });
                }

Further restricting the check to functions returning boolean might make the false positive : true positive ratio good enough to be acceptable.

Let's try with the "functions returning boolean only" approach and hope for good results

Added a commit for this 🤞

I imagine one way to achieve a far better signal-to-noise ratio would be checking syntax to see if and how the thing being tested is used inside the block body.

if (this.isComponentMounted) {
    // definitely suspicious because `isComponentMounted` isn't used in the block
    this.setState({ isUnblocking: false });
}

if (this.isComponentMounted) {
    // but probably ok if it's called in the block
    this.setState({ isUnblocking: this.isComponentMounted() });

    // or is passed to something else
    this.setState({ isUnblocking: someCheck(this.isComponentMounted) });
}

However... I'm guessing a brute-force syntax walk over arbitrarily deep children would be too expensive to perform here. Is there some way to attach information like this during an earlier syntax walk, like I imagine might be done for scope checking or CFA?

There should probably be a special case for if(!!expr) { so that there's an idiomatic non-casting workaround for checkJs scenarios

This happens to already work due to the way we're checking specific syntax kinds. Added a test for it

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Latest commit has one false positive (the event handler happened to return boolean) and three (!) instances of

if (this.isComponentMounted) {
  ...
}

which is pretty good IMO.

jwbay re: tree walks!

We talked about this more and actually think a tree walk of the body of an if could be a plausibly-cheap thing to do in this particular case. Observations:

• The check is extremely scoped, so wouldn't fire very often
• 98%+ of the time when an error wouldn't be issued, the subsequent call will be very early in a depth-first check, so the walk can bail before covering too much
• The other 1% of the time when we do end up erroring, well, erroring compilations are allowed to be slow
• The very small proportion of cases where we do walk the entire if body to find the call "late" in the tree is rare enough to not be a meaningful perf hit

This would allow TS to check this for any function, not just those returning boolean, and would eliminate one of the false positives we had.

So if it's OK with you, we'd like to compare-and-contrast the results from the boolean-test PR with maybe a fresh PR that does do the walk? Would you be game to implement both ways so we can see which has the best yield?

Thanks again for your work on this so far; I think this is going to be a high-value check either way.

I agree with the new approach #33178. This approach should be adopted. However, I still this is not null checks. This check should be called like strictCallChecks.

Closing as #33178 is now merged

can't under stand why if (true) will report TS2774, but if (false) reports nothing.

interface A {
        a: () => boolean;
    }

    class AImpl implements A {
        public a(): boolean {
            return true;
        }
    }

    it('should fail', function () {
        const aImpl = new AImpl();
        if (aImpl.a) { // TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?
            console.log("always true");
        }
    });

    it('why success', function () {
        const aImpl = new AImpl();
        if (!aImpl.a) { // nothing happens here
            console.log('always false');
        }
    });

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