[Swift] Recursive opaque type fails compile https://bsky.app/profile/inamiy.bsky.social/post/3k633r4kez72o

recursive-opaque-type.swift

// Recursive opaque type fails compile because recursiveness can’t deterministically pick the right type to replace `some P`.

protocol P {}

struct Parent<Child: P>: P {
    let children: [Child]
}

func recursive() -> some P {
    // ERROR: Function opaque return type was inferred as 'Parent<some P>', which defines the opaque type in terms of itself
    Parent(children: [recursive()])
}

// ERROR: Function declares an opaque return type, but has no return statements in its body from which to infer an underlying type
func recursive2<T: P>(_ x: T) -> some P {
    return recursive2(x)
}