compiler error if combine closure iterator, normal iterator and template

[a-mr]

Compiler reports internal error. (Found when working on #15612).

Example

template declare(body: untyped) =
  if true:
    body
  else:
    body

iterator search(): int {.closure.} =
  yield 11

iterator walk(): int =
  declare():
    yield 0

template process(iter: untyped) =
  for x in iter:
    echo x

for _ in walk():
  process(search())

Current Output

Error: internal error: expr: var not init search_385876041

Expected Output

to compile

Possible Solution

Additional Information

nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2020-11-09
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: 53eca459f1d96c710238be95bd3fb048b336f2f3
active boot switches: -d:release

Interestingly, this code works with small modifications such as:

• if make search normal iterator
• if make walk closure iterator
• if change if ... body else: body to just body in declare

[a-mr]

Another weird interaction between templates and closure iterators:

iterator walk(): (int, int) {.closure.} =
  yield (0,1)

for (x0,_) in walk():
  echo x0

Error: internal error: expr(nkVarTuple); unknown node kind

With just a scalar output type (or with a normal iterator) it works.