When all:

• Foo is an interface from extending Array or ReadonlyArray
• An object bar has a property with type Foo | undefined
• Optional chaining used to array-index Foo
• ...after a property access to bar

Then, the transpiled lua does not get +1 on the array indexing.

interface Foo extends Array<number> {}
const b: {arr?: Foo} = {arr:[1,2]}

// incorrectly prints 1
print(b.arr?.[1])

const arr: Foo | undefined = b.arr
print(arr?.[1])

// all correctly prints 2
print(b.arr![1])

const arr2 : Foo | undefined = [1,2]
print(arr2?.[1])

const c: {arr?: number[]} = {arr:[1,2]}
print(c.arr?.[1])

const d: {arr: Foo} = {arr:[1,2]}
print(c.arr?.[1])

TSTL v1.28.1

I'm amazed and annoyed how many stars had to align to encounter this bug, and that it can't be more miminal.