Incorrect never type inference.

[h-joo]

Bug Report

The type inference does not consider(or be conservative on) side effects of function calls.
Might be a related issue the first failure report in #43819

🔎 Search Terms

does not exist on type never

🕗 Version & Regression Information

• This changed between versions 4.2.3 and 4.3
• Also confirmed with the release candidate 4.3.1-rc with git hash d805ce877ae6235565ee697c3c0ce8a95a9c58f6

⏯ Playground Link

Playground link

💻 Code

You need to enable the --strictNullChecks to trigger the bug
./bin/tsc test.ts --strictNullChecks

// test.ts
class A {
    constructor (readonly attrib : number) {}
}
function foo2(callback : any) {
    callback();
}

function foo() {
    let abc :A | null = null;
    
    foo2( () => {
        if( new Date().getTime() > 1000000) {
            abc = new A(123);
        }
    });
    return abc ? abc.attrib : null;
              // compiler error triggers on the abc.attrib
}

🙁 Actual behavior

tsc prints the following error :

test.ts:21:23 - error TS2339: Property 'attrib' does not exist on type 'never'.
21     return abc ? abc.attrib : null;
Found 1 error.

🙂 Expected behavior

Not report the error.

[RyanCavanaugh]

Duplicate #11498, #9998. I'm not sure why this ever worked; adding ! in that position clearly shouldn't do anything since it's already known to be not-falsy due to its position in the ? :

[ajafff]

Previously a non-null assertion on a variable with inferred never fell back to the declared type of that variable. This bug has been fixed recently.

[h-joo]

Duplicate #11498, #9998. I'm not sure why this ever worked; adding ! in that position clearly shouldn't do anything since it's already known to be not-falsy due to its position in the ? :

You have a point. It seems a bit weird as you say.

[h-joo]

I've updated the example from

    return abc ? abc!.attrib : null;

to

    return abc ? abc.attrib : null;

[h-joo]

Previously a non-null assertion on a variable with inferred never fell back to the declared type of that variable. This bug has been fixed recently.

Sorry for the confusion in the original example, maybe this was not related to the non-null assertion (!.), as it still triggers after I removed the exclamation mark?

[nmain]

I've updated the example from

To clarify, with the ! weirdness removed this is still a combination of #9998, #11498, and #8513.

It's also no longer a regression: the oldest version on the playground, TS 3.3.3, behaves the same.

[h-joo]

I've updated the example from

To clarify, with the ! weirdness removed this is still a combination of #9998, #11498, and #8513.

It's also no longer a regression: the oldest version on the playground, TS 3.3.3, behaves the same.

Great, thank you for the clarifying.