typescript ReturnType is “any” if parameter type is “never”

[tylim88]

I found alternative solution on stack overflow but was advised to open an issue here

TypeScript Version: 3.8.3

Search Terms:
ReturnType
Parameters
Function
any
never

Code

const abc = (a:never):number=>1
    
type a = ReturnType< typeof abc> //any

const abc2 = (a:number, b:never):number=>1

type a2 = ReturnType< typeof abc2> //any

Expected behavior:
type a and a2 should be number regardless of the types of parameters

Actual behavior:
type a and a2 are any if any of the types of the parameters are never

Playground
link

the original question is posted on here with details describing the use case
link

[jcalz]

This is interesting:

type Expected = ((arg: string) => 0) extends (...args: any) => infer R ? "expected" : "unexpected"; // expected 
type Unexpected = ((arg: never) => 0) extends (...args: any) => infer R ? "expected" : "unexpected"; // unexpected
type Expected2 = ((arg: never) => 0) extends (...args: any) => any ? "expected" : "unexpected"; // expected
type Unexpected2 = ((arg: never) => 0) extends ((...args: any) => 0) ? "expected" : "unexpected" // unexpected

There's probably an existing issue for this but I didn't see one.

[MartinJohns]

Duplicate of #35432.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.