assert type refinements don't work in expressions

[Gozala]

🔎 Search Terms

• asserts

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABAZygJxmA5gCgG4CGANiAKYBci4A1mHAO5gCUlByypaUyihJpiGD1QZsiAN4AoRDMHBEOKAE8ADqTjy+ZRAEIAvIgBEIzFkNMJ02dagALNA0RhS9RABVVpAKJoHaHAAGXgAeatCkACaIBCjopgFMVjIAvpKpkhAIqIj0cGjUPAY4yHAgaBAUVGC0DMyIegB8ltYm2MWl5aSJ1mikUGVIJWUVAHRQcACqKmpoAMJspDiJ6Zlg2QBGvQQF9QpDnZQ0dIwWjc2yqyVEpCNEcLitDx0VTN2yvf1og88341Mz8w4SzSkgyWSg0SIJU2pG2hT2P0O1WOdTOAG0krFRE9hl0ADSY-ajP7TTiAxaJAC6QA

💻 Code

function string(value: unknown): asserts value is string {
    if (typeof value != "string") {
        throw new TypeError(`Expected a string`)
    }
}

const works = (source: unknown) => {
    string(source)
    return source.toUpperCase()
}

const breaks = (source: unknown) => {
    console.log(string(source))
    return source.toUpperCase()
}

const alsobreaks = (source: unknown) => [
    string(source),
    source.toUpperCase()
]```


### 🙁 Actual behavior

Functions `break` and `alsobreak` do not type check and produce following error: "'source' is of type 'unknown'.", because type refinement produced by function "string" don't seem to apply, if asserting function is not a statement.

### 🙂 Expected behavior

I would expect all three functions to type check, because `string` type refinement should apply to even if refinement is nested in an expression.

### Additional information about the issue

Provided example seems contrived because it was reduced to bare minimum, however there are legitimate uses for refinements like the above [for example](https://www.typescriptlang.org/play/?#code/MYGwhgzhAEBqYCcA8AVaBeaBXAdgaxwHsB3HAPmgG8AoaaAFwE8AHAUwC5ocsQQAfCPQQBLHAHNa0YIRyCEWYPUIIAFAEoqkuvQAWwiADombDFx4hJAX0kBbMPWA6VANzAgsHbPiKk1nV+6s0PrQaDR0dMIAZtAqxqyEMQEe0ACE6Ji6+kYsrBrhEdo6CCRcrMShuQCiCCWqAAbwgdAAJJTJrJbQACaErDBE9FIyUcIINgyEDLmtlFmG8Zb1alrQXawgEEEFhQis9FgIOAzyrKvWdNbW1PGhrOOoprgEJOSmaHxwiKhk1NSgkBgAEkcEMCoJ7MJgNA7A4nPR7jZOChEX5oID7vQYAjxsEYCiHtwbAAje4UHbBGJxXKJBiItIZaAAciJpIQTOgADJOdAAHJYEn3Az6EEIsT3OKo-KrCK6EoVHDlSpsGp1FRMqoADzYilY3XRZkF7JWhQua2o1wBUGgAAVCKIwZIIfQoTD7I4VBBCIdgJ5KNBNZx4MhWWSADTQRhB76hhAUSzSwoAeiT0AM6asFr+KegAEEoJiYMRlHh0cAStbnawbKxQRB-jJBAHTIqKsHqKKDLCPZqVnaHV33U5-ZqI1Hmwm-tQcwARPoDQhDYsIUuiaCsbV7KDCRsN2RDRgtpXt6SyQggVgGECEMQqSSd7tORhqMOSfugwdwlQjziMMe-tYVhWPcm1POQwAdGBMAAbXvD9HxUZ9Xzod96E-D0f0jf9I0A6gAF0-iiXBFB3Y5HAgnBPW9BBfU4Z4fBwRNoD2A4jikHQKKzEChgALyPNtEBA89L2vW8HyHFQeLUPt7XgiTMJ47C+ITIA) when composing set of constraints for later execution.

[MartinJohns]

This is working as intended. See #32695.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[Gozala]

This is working as intended. See #32695.

@MartinJohns mind clarifying a bit what the intended behavior is ? I do see liked PR says

the call occurs as a top-level expression statement, and

But then again in works function assertion works as I'd expect even though it's not a top level expression. It could be that behavior matches what PR intended, but I would argue it is all but intuitive, if string function throws on non-string wrapping the call in console.log or array should have not change anything, yet currently it does. So perhaps that was a flaw in intended behavior ?

@RyanCavanaugh Mind point what is this issue duplicate of so I can subscribe to updates there ? Thanks

[MartinJohns]

But then again in works function assertion works as I'd expect even though it's not a top level expression.

But it is a top-level expression:

string(source);

It's not a nested expression, like in breaks and alsobreaks.

Mind point what is this issue duplicate of so I can subscribe to updates there ?

The issue is #32695, which describes that this is as intended. Another related issue would be #53195.