v/vlib/v/tests/generics_interface_with_multi_generic_structs_test.v

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

fn (it Iter[T]) skip[T](n int) Iter[T] {
	return SkipIter[T]{
		n: n
	}
}

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

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

struct SkipIter[T] {
mut:
	n    int
	iter Iter[T]
}

fn (mut it SkipIter[T]) next[T]() ?T {
	for it.n > 0 {
		it.iter.next()?
		it.n--
	}
	return it.iter.next()
}

fn test_generics_interface_with_multi_generic_structs() {
	mut x := Iter[int](ArrIter[int]{
		data: [1, 2, 3]
	})
	println(x)

	mut ret := x.next() or { 0 }
	println(ret)
	assert ret == 1

	ret = x.next() or { 0 }
	println(ret)
	assert ret == 2

	ret = x.next() or { 0 }
	println(ret)
	assert ret == 3
}