Object spreading produces wrong type with non-literal keys

[coolCucumber-cat]

🔎 Search Terms

object spread, object rest, object spreading

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about object spreading

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.2#code/CYUwxgNghgTiAEYD2A7AzgF0QLngb3gG0BrEAT10xgEsUBzAXVxQFcBbAIxBngF8BYAFDJ0WKLgIlylDDXpN4VWnT7wAvPiEBIcfADkegDTaAdGbBDeQA

💻 Code

declare const c: { [key: string]: number }
const a: { [key: string]: string } = {
	a: '',
	...c
}

🙁 Actual behavior

It allows numbers to be in the object, when they shouldn't be allowed.

🙂 Expected behavior

It shouldn't allow numbers.

Additional information about the issue

Because c has non-literal keys, in this case string, which could be any string, it doesn't care what's in it.

[fatcerberus]

Just to make the issue more concrete...

TS Playground

const b: { [key: string]: number } = {
	b: 777
}
const a: { [key: string]: string } = {
	a: "foo",
	...b
}
console.log(a);

prints:

[LOG]: {
  "a": "foo",
  "b": 777
}

which is a clear type violation.

[MartinJohns]

I have a feeling that this is due to #27273.

[coolCucumber-cat]

@MartinJohns Yeah that's probably it. It's been 5 years, I wonder why it's still open.

[MartinJohns]

I wonder why it's still open.

Priorities. There are more important things to work on than some rarely reported bugs involving fairly unsound types, and the manpower is limited.

[coolCucumber-cat]

@MartinJohns It's pretty damn common and it's very obvious incorrect. There are some way more useless things that are being worked on and it's also been 5 years.

[andrewbranch]

Talked briefly about this with @ahejlsberg and decided to revisit making the resulting object type { [key: string]: string | number }—that is, if you spread in an index signature, the resulting type should have an index signature whose value type is the union of all property types and index signature types seen in the target object—and see what breaks in the extended test suites.