Here's a simplified scenario.
It's a major blocker for any kind of lazy map/filter library, such as https://github.com/def-/nim-iterutils/blob/53f4a30e57f01cd8cf9d5b1dd22053eac82b25f2/src/iterutils.nim#L303 "TODO: Currently fail" where it shows map/filter etc cannot be composed.

proc myiter*(n:int): auto =
  iterator it: int {.closure.} =
    for x in 0..n:
      yield x
  return it

proc iterFun*[T](a:T): auto =
  iterator it: int {.closure.} =
    for x in a():
      yield x
  return it

proc test()=
  let it1 = myiter(5)
  let it2 = iterFun(it1)

  # nothing is printed here:
  for a in it2(): echo a

  echo "done"

test()

this prints "done"
it should print 0 1 2 3 4 5 done

EDIT this is a regression, I just verified this worked in 0.9.4 (which was nimrod)

$git_clone_D/temp/nim/Nim3/bin/nimrod c -r t26_iterator_compose2.nim
0
1
2
3
4
5
done

$git_clone_D/temp/nim/Nim3/bin/nimrod --version
Nimrod Compiler Version 0.9.4 (2018-07-02) [MacOSX: amd64]
Copyright (c) 2006-2014 by Andreas Rumpf

NOTE: I also verified https://github.com/def-/nim-iterutils used to work with 0.9.4:

git clone https://github.com/def-/nim-iterutils
cd nim-iterutils
git checkout 40b83d2de580b93c5ef96100a72df1ea4981a7a8
$git_clone_D/temp/nim/Nim3/bin/nimrod c -r src/iterutils.nim

NOTE: to build 0.9.4, unfortunately choosenim didn't work, see but I posted in dom96/choosenim#66 (I show there how I built 0.9.4 , not sure if there's a simpler way)

EDIT
I just tried with v0.13.0, that version already has the regression.

Not sure how to build nim for versions between v0.9.4 and v0.13.0 because https://github.com/nim-lang/csources.git doesn't have any git tag between these and ./bin/nimrod c koch fails for csources.git:v0.9.4 with Nim:v0.12.0

EDIT fixed in c554c2a