TypeScript Version: 3.9.0-beta

Search Terms: recursive, recursive generic

Code

type Intersect<U extends any[], R = unknown> =
    ((...u: U) => any) extends ((h: infer H, ...t: infer T) => any)
        ? Intersect<T, R & H>
        : R

const value: Intersect<[{a: number}, {b: number}]> = {
    a: 1,
    b: 2
}

Note: the usual workaround of an immediately-indexed intermediate object type still works.

Expected behavior: It to type check without warnings

Actual behavior:

• At line 1: Type alias 'Intersect' circularly references itself. (2456)
• At line 3: Type 'Intersect' is not generic. (2315)
• At line 6: Type 'Intersect' is not generic. (2315)

Playground Link: Link

Related Issues:

• Circularities Only Blocked Sometimes #37426 (root cause might be this)
• Recursive type alias in union type breaks in 3.8.3 #37344
• Probably about 25%-30% of all other recursive type issues.
• My use case is basically solving Tuple iteration and merging #26058 in userland, and doing this with SomeTuple[number] won't work because I don't want to recurse.