bug: functions dislike iteration?

[KiaraGrouwstra]

TypeScript Version: 2.4.1

Code

export type TupleHasIndex<
    Arr extends any[],
    I extends number
> = ({[K in keyof Arr]: '1' } & Array<'0'>)[I];
// ^ #15768, TS2536 `X cannot be used to index Y` on generic
export type Inc = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
export type TupleToUnion<R extends any[], I extends number = 0, Acc = never> =
    { 1: TupleToUnion<R, Inc[I], Acc | R[I]>, 0: Acc }[TupleHasIndex<R, I>];
type TupleToUnionTest1 = TupleToUnion<['a', 'b']>;
// "a" | "b"
type TupleToUnionTest2 = TupleToUnion<string[]>;
// never

declare function toUnion<R extends string[]>(arr: R): TupleToUnion<R>;
const test1 = toUnion(['a', 'b']);
const arr: ['a', 'b'] = ['a', 'b'];
const test2 = toUnion(arr);
// ^ both: expecting 'a'|'b', got never

Expected behavior:
'a'|'b'

Actual behavior:
never

I'd reported a similar failure at #12290 (comment), though the thread had already been closed. These are possibly related to #16018, of generic types similarly failing when wrapped, even without functions.

[KiaraGrouwstra]

I retested this again on master now, where test2 does in fact work. Closing as no longer relevant until I can find a new repro for master.