Microsoft/TypeScript
Auf GitHub ansehen`ReturnType` and `InstanceType` don't work for functions/constructors with `never` in their parameters
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#55.667 geöffnet am 7. Sept. 2023
Domain: lib.d.tsEffort: CasualExperimentation NeededHelp WantedSuggestion
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Beschreibung
🔎 Search Terms
ReturnType InstanceType any never
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about
ReturnTypeandInstanceType.
⏯ Playground Link
💻 Code
type _1 = ReturnType<(...args: never) => 'foo'>
// ^? type _1 = any
type _2 = InstanceType<new (...args: never) => 'foo'>
// ^? type _2 = any
🙁 Actual behavior
ReturnType and InstanceType always evaluate to any when given a function/constructor whose parameters are ...args: never (or ...args: never[], a: never, b: string, etc).
🙂 Expected behavior
ReturnType and InstanceType should return the actual return/instance type.
Additional information about the issue
Changing the parameter constraint in the conditions of these types from any to never seems to fix the issue:
type ReturnType<T extends (...args: any) => any> = T extends (...args: never) => infer R ? R : any
type InstanceType<T extends abstract new (...args: any) => any> = T extends abstract new (...args: never) => infer R ? R : any
I don't think this will cause any regressions, and did some spot-checking to validate that, but haven't run the entire test suite or anything like that yet.