Destructure and spread on generics not typing

[saulshanabrook]

TypeScript Version: 3.7.0-dev.20190924

Search Terms: destructure spread generic

Code

function test<T extends {key: number}>({key, ...rest}: T): T {
  return {key: 10, ...rest}
}

Expected behavior:

No type errors

Actual behavior:

Type '{ key: number; } & Pick<T, Exclude<keyof T, "key">>' is not assignable to type 'T'.
  '{ key: number; } & Pick<T, Exclude<keyof T, "key">>' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{ key: number; }'.

Playground Link: https://www.typescriptlang.org/play/index.html?ts=Nightly#code/GYVwdgxgLglg9mABFApgZygHgCqJQD1TABM1EBvAaxQE8AuRMEAWwCMUAnAXwD4AKKrQA0iAHTiO6KFwbYAlLIoBYAFCJEkqCA5JB9RAEYADCPGjJGLqq5A

Related Issues: #26412 #10727 #28234 #28312 #18552

[Nathan-Fenner]

This is behaving as expected, since you don't know that 10 is a valid value of type T["key"]. In particular, consider:

const example: {key: 1 | 2, val: string} = {key: 1, val: "abc"};

// No type error! But it would be wrong:
const sameType: {key: 1|2, val: string} = test(example);

So you'd need to instead give a type like:

function test<T extends {key: number}>({key, ...rest}: T): Omit<T, "key"> & {key: number} {
  return {key: 10, ...rest};
}

or like (but this one TypeScript also rejects, which is a design limitation since its flow-sensitive typing is not smart enough):

function test<T extends {key: number}>({key, ...rest}: T): number extends T["key"] ? T : Omit<T, "key"> & {key: number} {
  return {key: 10, ...rest}
}

[typescript-bot]

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