ReturnType<ReturnType<>> in type alias results in never

[ghost]

TypeScript Version: 2.8.0-dev.20180216

Search Terms: ReturnType conditional type

Code

export {};
type ReturnType<T extends (...args: any[]) => any> = T extends (...args: any[]) => infer R ? R : any;
type ReturnTypeReturnType<T extends (...args: any[]) => (...args: any[]) => any> = ReturnType<ReturnType<T>>;
type Expected = ReturnType<ReturnType<() => () => "abcdef">>; // correctly `"abcdef"`
type Actual = ReturnTypeReturnType<() => () => "abcdef">; // `never`
const x: Actual = "abcdef"; // Error

Expected behavior:

Actual same as Expected.

Actual behavior:

Actual is never.

The issue may be fixed by changing the definition of ReturnType to be T extends (...args: any[]) => infer R ? R : never;. Based on the rules in #21496 the behavior with any may be by design, but the global ReturnType definition currently uses any, so maybe the fix should be in lib.d.ts.

[falsandtru]

I think anyway ReturnTypeReturnType<() => () => "abcdef"> shouldn't reach any. So we don't have to fix the ReturnType definition.

[mhegazy]

I can not reproduce this on latest.