narrowing to null via typeguards doesn't work

[zpdDG4gta8XKpMCd]

i am hacking my way around this limitation #11920

so my goal is to make sure that isNull only applied to T | null types and by this i mean that null must be a valid case of a given type (types that don't have | null must not be permitted), i am trying to use conditional types to enforce it, this is what i got

unfortunately it doesn't work and i am clueless what i am doing wrong, it looks like a bug

export function isNull<T extends (null extends T ? any : never)>(value: T | null): value is null {
    return value === null;
}

declare var x: null | number;
if (isNull(x)) { // <-- expect to work, actual: number is not assignable to null
}
declare var y: number;
if (isNull(y)) { // <-- expect a type error
}

@ahejlsberg you might want to look at it

[RyanCavanaugh]

You'd need #9252 to do this

[zpdDG4gta8XKpMCd]

jfc, no country for an old man

[zpdDG4gta8XKpMCd]

@RyanCavanaugh is there any way to accomplish this up to your knowledge?
i want to flag places that check for null and where null isn't a valid option

[jack-williams]

Maybe?

export function isNull<T extends (null extends T ? any : never)>(value: T): value is Extract<T,null> {
    return value === null;
}

[zpdDG4gta8XKpMCd]

Extract<T, null> is sure a very interesting way of saying null, but it seems to work, thanks

[ahejlsberg]

I was going to suggest this:

export function isNull<T extends (null extends T ? any : never)>(value: T): value is T & null {
    return value === null;
}

But Extract<T, null> seems to work too.

[zpdDG4gta8XKpMCd]

@ahejlsberg your way (value is T & null) works better since Except<T, null> only works for unions of primitives, but doesn't work on unions of objects, thank you for your help

[jack-williams]

@Aleksey-Bykov Can you post an example of one that doesn't work?

[zpdDG4gta8XKpMCd]

@jack-williams hm, i guess i am taking my words back, now it looks working just as well

not sure what happened, but when i applied it first time i didn't get narrowing that i expected

if i see it again i will let you know

[zpdDG4gta8XKpMCd]

broken now: #29662

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.