`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.

Reproduce the two examples in the linked TypeScript Playground, then locate the declarations of ReturnType and InstanceType and compare their conditional parameter constraints with the proposed never form. Add coverage for functions and constructors with never parameters, and verify that the inferred types are 'foo' rather than any.