Repository navigation
A strange behavior when checking extends ReadonlyArray<any> #59430
Description
🔎 Search Terms
"extends", "ReadonlyArray"
🕗 Version & Regression Information
- This changed between versions 5.5.3
⏯ Playground Link
💻 Code
type A = ReadonlyArray<'A'>
type B = ReadonlyArray<'B'>
type AcceptA<T extends A> = T
type Test1 = AcceptA<A> //accepted
type Test2 = AcceptA<B> //error
type Test3 = AcceptA<B extends ReadonlyArray<any> ? B : B> // accepted🙁 Actual behavior
It could be accepted in Test3 but it's still the same B in Test2 which has an error.
🙂 Expected behavior
Show an error as Test2.
Additional information about the issue
No response
Activity
[-]A strange behavior when checking `extends Readonly<any>`[/-][+]A strange behavior when checking `extends ReadonlyArray<any>`[/+]jcalz commentedon Jul 26, 2024
SO question
Ryan Cavanaugh (@RyanCavanaugh) Are we sure this is a bug? The type
B extends ReadonlyArray<any> ? B : Bis a substitution type likeB & ReadonlyArray<any>(due to #37348), and that intersection is indeed allowed inAcceptAbecause of the unsoundness ofany. It's weird, but why would someone writeB extends ReadonlyArray<any> ? B : Bin the first place except to expose some substitution-type behavior?RyanCavanaugh commentedon Jul 26, 2024
Joe Calzaretta (@jcalz) yeah, good observation. I agree
ruojianll commentedon Jul 27, 2024
Joe Calzaretta (@jcalz) Ryan Cavanaugh (@RyanCavanaugh) But isn't it a bug?
Test3shouldn't acceptBin result.ruojianll commentedon Jul 27, 2024
The true value
Bis should not beB & ReadonlyArray<any>. If the condition is true, why do we need to narrow down the true value?ruojianll commentedon Jul 29, 2024
I still consider it as a bug. Anders Hejlsberg (@ahejlsberg)
typescript-bot commentedon Aug 1, 2024
This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.