`errorType` does not propagate with non-distributive conditional types

[mizdra]

🔎 Search Terms

non-distributive, conditional type, errorType

🕗 Version & Regression Information

• This issue has been reproducible since v5.1.6. In earlier versions, it likely behaves differently due to the impact of Propagate errorType in getConditionalType #53801.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#code/C4TwDgpgBAogTnA9nAKuaBeKpKIGZQCuAdgNbGIDuxAagIYA2hEA3ALABQO0AglFvCSp0UCAA9gEYgBMAzkTIVqUAPzY4zKAC4oeRrNacA9EahmAeis7coAIX5QA2oORpIAXVESpcpyXJUxJ5qwBrQOnoMBuwcJmZQlpxAA

💻 Code

type ErrorType = typeof unknownValue;
type A = ErrorType extends unknown ? true : false;
type B = [ErrorType] extends [unknown] ? true : false;

🙁 Actual behavior

A is any type, and B is true type.

🙂 Expected behavior

Both A and B should be any type.

Additional information about the issue

As far as I know, errorType propagates as errorType regardless of the type operation. However, only in the case of non-distributive conditional types, errorType does not propagate. Is this the intended behavior?

[mizdra]

Upon reflection, this might be similar to asking whether [ErrorType] is equal to ErrorType. And I believe the answer is no.

The array containing ErrorType as an element is of type Array. Therefore, [ErrorType] extends [unknown] ? true : false should evaluate to true.

Does this mean the behavior reported in this issue should be considered not a bug, but rather the intended specification?

[Andarist]

Upon reflection, this might be similar to asking whether [ErrorType] is equal to ErrorType. And I believe the answer is no.

Yeah, the same as { prop: ErrorType } not being the same as ErrorType. Reducing those things eagerly would actually degrade in-IDE experience.

[RyanCavanaugh]

Does this mean the behavior reported in this issue should be considered not a bug, but rather the intended specification?

The intended behavior is that you should never be in this situation (this might sound like I'm joking but for real, we do not make any guarantees about what happens after line 1 in this program)

[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.