Can't assign existing property to the rest of generic

[klimashkin]

TypeScript Version: 3.2.2 and 3.3.0-dev.20181208

Search Terms:

• generic
• pick
• exclude
• spread

Code

interface AB {
  a: string;
  b: string;
}

function excludeA<T extends AB>(obj: T) {
  const {a, ...rest} = obj;

  rest.b = '3';

  return rest;
}

excludeA({ a: '1', b: '2', c: 'test' });

Expected behavior:
No errors

Actual behavior:
Property 'b' does not exist on type 'Pick<T, Exclude<keyof T, "a">>'.

Playground Link:
https://www.typescriptlang.org/play/index.html#src=interface%20AB%20%7B%0D%0A%20%20a%3A%20string%3B%0D%0A%20%20b%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Afunction%20excludeA%3CT%20extends%20AB%3E(obj%3A%20T)%20%7B%0D%0A%20%20const%20%7Ba%2C%20...rest%7D%20%3D%20obj%3B%0D%0A%0D%0A%20%20rest.b%20%3D%20'3'%3B%0D%0A%0D%0A%20%20return%20rest%3B%0D%0A%7D%0D%0A%0D%0AexcludeA(%7B%20a%3A%20'1'%2C%20b%3A%20'2'%2C%20c%3A%20'test'%20%7D)%3B

Related
#28821

[weswigham]

@klimashkin - Consider:

interface AB {
  a: string;
  b: string;
}

function excludeA<T extends AB>(obj: T) {
  const {a, ...rest} = obj;

  rest.b = '3';

  return rest;
}

interface ExactlyAB extends AB {
  a: "a";
  b: "b"
}

declare var exactAB: ExactlyAB;

excludeA(exactAB).b; // claims to be of type "b", but will be the value "3" with your implementation 

[weswigham]

(But in general yes, #28821 is the reason why the assignment doesn't work, even when you correctly assign a T["b"] - I just wanted to let you know that that code is unsafe from a typesystem perspective, even once that bug is fixed)

[que-etc]

@weswigham
Does it mean that any reassignment of an object's property, which has a generic type, should be considered invalid?

const test = <T extends {foo: string}>(obj: T): T => {
    // No errors as for v3.2.2
    obj.foo = 'bar';

    return obj;
};

Playground.

[weswigham]

Yea - it's a known hole that gets closed when you use indexed access types (which get automatically made when you fetch properties off mapped types). It's quite troublesome, really, since the only reason we haven't fixed it, AFAIK, is for backwards compatibility (and to make mutating patterns "easier" to type).

[typescript-bot]

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed for automatic house-keeping purposes.

[klimashkin]

The issue has been fixed:

http://www.typescriptlang.org/play/?allowJs=true&target=6#code/JYOwLgpgTgZghgYwgAgJIHkBGArZBvAKGWOTgC5kQBXAW02gG4iTMLq7HniE3b6omAXwIEYVEAjDAA9iGSQAzmAA8AFWQQAHpBAATBWizYANMgDSG7RD0H1AMnzIAXrw4DkggHwAKaTgqqAJQU6DTAKmamAORwUcgAPshRmFGexIQkyAiySvhwppimAHQlUBBKgsgAvMh+uHAGZkxcyGVKRU7VyADMAEzNmWVgVFBybWBCIopg3njkyACMBRS9pjw9proUACyCgUW6DEA