T extends unknown is observably-different from an unconstrained T

[mkantor]

🔎 Search Terms

"extends unknown" "type parameter" "generic" "conditional type" "keyof" "string" "assignable"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type parameter constraints

⏯ Playground Link

https://www.typescriptlang.org/play/?jsx=0#code/C4TwDgpgBAogHgQwLZgDYQDwBUB8UC8UWUEcwEAdgCYDOUA3gL5QD8UA1hCAPYBmRUAFxQawAE4BLCgHMAULIDG3CqKgBXCuwrcA7hQDCy0WIRTgBKNhJlKtdZu16cACjjD4yNJlwBKYcalpAjw4eSUVc21DCJMzC2wXN1hEFHQEvxFxQOCoOCA

💻 Code

type Example<T> = T extends {} ? keyof T : string

const unknownConstraint = <T extends unknown>(x: Example<T>): string => x

const noConstraint = <T>(x: Example<T>): string => x
//                                                 ^
// Type 'string | number | symbol' is not assignable to type 'string'.
//   Type 'number' is not assignable to type 'string'.

🙁 Actual behavior

The T extends unknown version behaves differently from the unconstrained version.

🙂 Expected behavior

The two versions should have the same behavior, probably both being rejected since code like this is unsafe:

const key = Symbol()
const bad = unknownConstraint<{ [key]: unknown }>(key)
bad.toUpperCase() // boom

Additional information about the issue

I was asked about this and eventually narrowed down the behavior to the example above.

#61203 is also possibly related?

[LukeAbby]

My theory is that this is for backwards compatibility when symbol keys were being added.

You can also get this behavior with any type whenever the apparent type of keyof T does NOT extend symbol | number.

For example:

const objectConstraint = <T extends { abc: number }>(x: Example<T>): string => x // Ok
const numberConstraint = <T extends 123>(x: Example<T>): string => x // Ok
//                                                            ^ `keyof 123` happens to be only strings so also okay.

Notably this means that T extends {} actually errors. This furthers my hypothesis that the test is extends symbol | number ? error : do nothing because never technically extends symbol | number. To further this T extends {} | 1 doesn't error.

This does still leave unknown as a second special case though.

[mkantor]

#56652 seems possibly-relevant as well.

[RyanCavanaugh]

This is indeed a back-compat allowance; for a long time the default constraint was closer to {} than unknown (admittedly really neither). Hopefully we can eventually remove it but for now, unconstrained type parameters will sometimes appear to extend {}.

[typescript-bot]

This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.