never properties should not require the properties to be present

[yuhr]

Bug Report

🔎 Search Terms

• never
• properties
• "missing in type"

🕗 Version & Regression Information

• Tested in nightly, 4.2.3, and 3.3.3

⏯ Playground Link

Playground link with relevant code

💻 Code

const x: { foo: never } = {};
// Property 'foo' is missing in type '{}' but required in type '{ foo: never; }'.(2741)

🙁 Actual behavior

• Object literals which are expected to have never properties require initialization for those impossible properties

🙂 Expected behavior

• never properties should disallow any initialization

I see there existed a similar issue #16762, but I think this bug is not related to "optional" concept. I believe the current behavior is definitely weird.

[jcalz]

Well, it's not a bug... that's how object types and never work. If you have an object of type {bar: string}, then it must have a bar property of type string. If you have an object of type {foo: never}, then it must have a foo property of type never. Since one cannot get one's hands on a value of type never to place into that object, it implies that values of type {foo: never} are similarly elusive. But this is a feature of never, not a bug.

If you want to be able to leave out a property, then you need to make that property optional (here's that "optional" concept). So my suggestion would be to use something like {foo?: never} to mean "an object which cannot have a foo property" instead of {foo: never} which means "an object which must have an impossible foo property".

I can imagine changing this into a suggestion that properties of type never be interpreted the way you want, but then this suggestion would be a breaking change, and I don't know that anyone would think it's worth breaking existing code for it.

[RyanCavanaugh]

A type with a property of type never has zero legal values; { } is not one of them.

[yuhr]

Since one cannot get one's hands on a value of type never to place into that object, it implies that values of type {foo: never} are similarly elusive. But this is a feature of never, not a bug.

A type with a property of type never has zero legal values; { } is not one of them.

Okay, that sounds. Note that the suggested type { foo?: never } results in { foo?: undefined } which is not the desired type at this point, but it should be eventually resolved by #43947 (I've tested with the insider package and it worked), so once it has been merged successfully, I won't suggest any breaking change about this.

Thank you all! I'd open a suggestion anew in case the PR was declined.