interface Iter[T] {
mut:
    next() ?T
}

fn (mut it Iter[T]) collect() []T {
    mut data := []T{}
    for {
        val := it.next() or { break }
        data << val
    }
    return data
}

struct ArrayIter[T] {
    data []T
mut:
    index int
}

fn (mut it ArrayIter[T]) next[T]() ?T {
    if it.index >= it.data.len {
        return none
    }
    defer {
        it.index++
    }
    return it.data[it.index]
}

fn test_generics_interface_method() {
    mut iter := Iter[int](ArrayIter[int]{
        data: [1, 2, 3]
    })
    assert iter.collect() == [1, 2, 3]
}