Description
openedon Jul 17, 2024
🔎 Search Terms
conditional type infer unknown
conditional type mapped type inference
🕗 Version & Regression Information
This is the behavior in every version I tried (3.3.3 to 5.5.3), and I reviewed the FAQ for entries about conditional types, mapped types, and structural inference.
⏯ Playground Link
💻 Code
type Mapped<T> = { [P in keyof T]: T[P] }
type ComponentA = Mapped<{ field: number }>
type ComponentB = { field: number }
type Infer1<T> = Mapped<T extends never ? null : T>
declare function infer1<T>(c: Infer1<T>): T
infer1({} as ComponentA).field = 1 // ✅
infer1({} as ComponentB).field = 1 // ❌ Error: Object is of type 'unknown'
type Infer2<T> = T extends never ? null : Mapped<T>
declare function infer2<T>(c: Infer2<T>): T
infer2({} as ComponentA).field = 1 // ✅
infer2({} as ComponentB).field = 1 // ✅
🙁 Actual behavior
infer1(ComponentA)
passed inference but infer1(ComponentB)
failed with unknown
.
However, if it's the other way around — Conditional ? Mapped<T>
instead of Mapped<Conditional ? T>
, both infer2(ComponentA)
and infer2(ComponentB)
passed inference.
🙂 Expected behavior
I expect infer1(ComponentA)
and infer1(ComponentB)
to both work, because ComponentA
and ComponentB
are essentially the same type — I'm unable to otherwise differentiate them by any means.
Additional information about the issue
I discovered this while debugging vuejs/core#11353, and the interference failure there was eventually reduced to this report.