Narrowing works incorrectly when destructuring

[tatomyr]

Bug Report

🔎 Search Terms

destructure, union, narrowing

🕗 Version & Regression Information

v.4.7.4, v4.8.0-beta.

⏯ Playground Link

Playground link

💻 Code

const removeX = <T, X>({x, ...rest}: T & {x: X}): T => rest

🙁 Actual behavior

It produces the following error:

Type 'Omit<T & { x: X; }, "x">' is not assignable to type 'T'.
  'T' could be instantiated with an arbitrary type which could be unrelated to 'Omit<T & { x: X; }, "x">'.

🙂 Expected behavior

There should be no error since Omit<T & { x: X; }, "x"> is essentially equal to T.

[IllusionMH]

Not if T contains property x. Then rest wont match T exactly.

[tatomyr]

Well, that's the part I'd expect Typescript to figure out for me that T does not contain x as I'm extracting it from whatever type is coming into my function.

[fatcerberus]

The return value is guaranteed not to contain property x. This is not guaranteed of T, because you can supply type arguments explicitly at the call site. It's a common mistake to write generic functions assuming the type parameter(s) are always inferred, but the compiler knows better.

[jakebailey]

Perhaps this is sufficient for your use case?

function removeX<T extends Record<string, any>>({x, ...rest}: T): Omit<T, "x"> {
    return rest;
}

Playground Link

[tatomyr]

Thanks all for the explanations!
Now I see what the point is.