`ReturnType` and `InstanceType` don't work for functions/constructors with `never` in their parameters

🔎 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 ReturnType and InstanceType.

⏯ Playground Link

https://www.typescriptlang.org/play#code/C4TwDgpgBA+gjFAvFAShYBXATgOwCrgQA8AFAHQUCGWA5gM4BcUOEAbhFgJRIB8UA5ADMA9sP48AUAHopUOQD0A-BImhIsAExIoASRx1glHAGMIBSERYB3KOSq1GzNh26I+Q0eOmyFioA

💻 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.

リンク先の TypeScript Playground にある 2 つの例を再現し、次に ReturnType と InstanceType の宣言を見つけて、それらの条件付きパラメーター制約を提案されている never 形式と比較してください。never パラメーターを持つ関数とコンストラクターのカバレッジを追加し、推論された型が any ではなく 'foo' になることを確認してください。