`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.