Microsoft/TypeScript
Case for inference failure in `T extends F<T>`
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#51 377 ouverte le 1 nov. 2022
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Description
Bug Report
🔎 Search Terms
circular type parameter constraint
Related issues: #40439 #30134
🕗 Version & Regression Information
Tested with 4.8.4
⏯ Playground Link
💻 Code
declare const f:
<T extends F<T>>(t: T) => T
type F<T> =
{ a: unknown
, b: (a: T extends { a: infer X } ? X : never) => unknown
}
f({
a: "hello",
b: x => x.toUpperCase()
})
// doesn't compile because infers `x` as `unknown` instead of `string`
f({
a: "hello",
b: (x: string) => x.toUpperCase()
})
// compiles
Note that there is a workaround but it's only a workaround for this minimal case and not for the real world case from which this minimal case was derived.
🙁 Actual behavior
x in the first f call gets inferred as unknown
🙂 Expected behavior
x in the first f call should get inferred as string