test.nim

proc next[R](input: iterator(): R): R =
    for v in input:
          return v

iterator t: int {.closure.} =
  yield 1

t.next

# test.nim(2, 13) Error: type mismatch: got (iterator (): int{.closure.})
# but expected one of: 
# system.items(E: typedesc[enum]): iter[typedesc[enum]]
# system.items(a: set[T]): iter[T]
# system.items(a: cstring): iter[char]
# system.items(a: string): iter[char]
# system.items(a: array[IX, T]): iter[T]
# system.items(a: seq[T]): iter[T]
# system.items(a: openarray[T]): iter[T]