Type parameters with constraints should be reconstructable

[zach-waggoner]

TypeScript Version: 3.6.0-dev.20190619

Search Terms: type parameter generic constraint assignable instantiated subtype reconstruct

Code

interface AConstraint {
  x: boolean;
}

function fn<A extends AConstraint>(arg: A): A {
  const { x, ...rest } = arg;
  return { x, ...rest };
}

Expected behavior:

pass

Actual behavior:

test.ts:7:3 - error TS2322: Type '{ x: boolean; } & Pick<A, Exclude<keyof A, "x">>' is not assignable to type 'A'.
  '{ x: boolean; } & Pick<A, Exclude<keyof A, "x">>' is assignable to the constraint of type 'A', but 'A' could be instantiated with a different subtype of constraint 'AConstraint'.

It seems like this should work, since { x, ...rest } should be assignable to A regardless of how A is instantiated.

Playground Link:

https://www.typescriptlang.org/play/#src=interface%20AConstraint%20%7B%0D%0A%20%20x%3A%20boolean%3B%0D%0A%7D%0D%0A%0D%0Afunction%20fn%3CA%20extends%20AConstraint%3E(arg%3A%20A)%3A%20A%20%7B%0D%0A%20%20const%20%7B%20x%2C%20...rest%20%7D%20%3D%20arg%3B%0D%0A%20%20return%20%7B%20x%2C%20...rest%20%7D%3B%0D%0A%7D%0D%0A

Related Issues:

#29049

[fatcerberus]

The error seems correct: you have a constraint on A that it's at least AConstraint so you can work with the value you have as though it's an AConstraint, but you can't actually produce a value of type A because you don't know how specific A actually is. There's no lower bound, so it could be any subtype at all.

For example:

function fn<A extends Animal>(arg: A): A
{
    return new Giraffe("Jeff");  // Giraffe is assignable to Animal, but...
}

let cat = new Cat("Mittens");
let newCat = fn(cat);  // ...we just wanted a cat. :-(

As you point out, in your case you capture ...rest which should cover all the bases in practice--but I think TS is just assuming the worst: A might be never. Then nothing at all would be assignable to it.

[zach-waggoner]

That could be. After further digging, I suspect this might be related to #28884. I will close this and defer to that issue.