never return type not respected on function called on object with inferred type

[nwalters512]

🔎 Search Terms

"return never type inference", "return never control flow"

I came across #12825, but the code snippet there works as expected, and I have something similar working as expected in my reproduction.

I also saw #34955, but it's not clear to me if it's related to my issue or not (it seems separate).

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgDIHsDmnrIN4BQyyEAHkgA5jDogAUAtgM6YBcyTYUomAlOyAgA3aAG4CAXwIEYAVxAJqtEqQgJZkAFLoARnQBWu9nSFwANu07cQmANoBdZAB9kIWWbMAaZGaw4o7BjY0LzIALwAfMhC6MAAJqGExAi0nD5+0IEZUOH4RMQqlEr0zGwcXDyJ+QXIYAAWUOgA7q4QLQCiUI1QjCy84jUSnvkS4vmGem4e3r7BUP2S0mRqGhDaeibmM9mhkXnEwDDIdACEAIJdcACeAHTATBdQ15tmvFU1s-43ZEU09ADkkE4PH+C2IUnyKRATHQZggN1mLzuIDiZAA8jA6ICIMCbKCFhIFgRluotLo6C9tnMsnNdlEkshDsdzpdbvdHs9TK93gVPtBvuQIFQ-ligdQ8WDkBDkqlYfDEVzkajSBjRTjxZh8eJCeIgA

💻 Code

interface Logger {
  exception(msg: string): never;
}

function executeJob(job: (val: string[] | null, logger: Logger) => void) {
  const logger: Logger = {
    exception(msg: string) {
      throw new Error(msg);
    },
  };

  job(null, logger);
}

executeJob((val, logger) => {
  if (!Array.isArray(val)) {
    logger.exception('testing');
  }

  console.log(val.indexOf('testing'));
});

executeJob((val, logger: Logger) => {
  if (!Array.isArray(val)) {
    logger.exception('testing');
  }

  console.log(val.indexOf('testing'));
});

The example is kind of contrived, but it's a very simplified version of something from a real-world codebase. I jumped through hoops to get it to clearly show an error in the failing case.

🙁 Actual behavior

When the "job" function has the type of logger inferred as below, reachability/control flow analysis incorrectly determines that val could be of type null on the console.log(...) line:

executeJob((val, logger) => {
  if (!Array.isArray(val)) {
    logger.exception('testing');
  }

  console.log(val.indexOf('testing')); // 'val' is possibly 'null'. (18047)
});

However, when the type of Logger is explicitly annotated, things work as expected and the type of val is correctly narrowed to string[] after logger.exception(...) is called.

executeJob((val, logger: Logger) => {
  if (!Array.isArray(val)) {
    logger.exception('testing');
  }

  console.log(val.indexOf('testing')); // no error, as expected
});

🙂 Expected behavior

I would expect the first example above with an implicit type for logger to behave the same as when the type of logger is explicitly declared to be Logger.

Additional information about the issue

No response

[IllusionMH]

More like #55154 and linked issues.
As you've noticed it needs explicit annotation.

[nwalters512]

Good find! That didn't show up in my search. I'll close this as a duplicate, though it's a shame this can't be made to work.

This should probably make its way into the FAQ? From the "many many others" comment in the issue you linked, it sounds like this comes up regularly.