Spread object type inference error

[wmzy]

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

TS version: 4.8.4

⏯ Playground Link

Playground link with relevant code

💻 Code

type T = {
  foo?: string;
};

const a: T = {foo: undefined};
const b = {foo: '', ...a};

🙁 Actual behavior

🙂 Expected behavior

The type of b should be:

{
  foo?: string;
}

[fatcerberus]

There's no reason to infer foo be optional in b - there will always be a foo property, even if a doesn't have one, because you've provided it upfront. The unsoundness here is that undefined is assignable to optional properties - you can enable exactOptionalPropertyTypes to prevent that.

[fatcerberus]

btw this is not destructuring assignment - it’s a spread. Destructuring is when you do

let { x, y } = point;

[MartinJohns]

@fatcerberus Object spread only skips properties that haven't been defined, but it does not skip properties that have been defined to be undefined. Confusing, I know. The object ends up being { foo: undefined }, so the type string is wrong.

But I'm certain there's an existing issue for this that I can't find right now.

edit: Just noticed that you mentioned exactOptionalPropertyTypes, which is probably what I was thinking of and shows that you know about what I just said.

edit: This is a duplicate of #44432

[fatcerberus]

@MartinJohns Yes, I was aware. Without exactOptionalPropertyTypes, optional properties can be set to undefined, but the type system still generally treats the property as if it’s either present-with-the-given-type or else non-present. This is indeed unsound, but the fix is to enable exactOptionalPropertyTypes. 😄