Internal error with auto return in closure iterator

[mratsim]

I stumbled upon the following internal error with the following code:

proc flatIterator[T](s: openarray[T]): auto {.noSideEffect.}=
    result = iterator(): auto =
        when (T is not seq|array):
            for item in s:
                yield item
        else:
            yield 123456

let it = flatIterator(@[@[1,2],@[3,4]])

Error

flatten_iterator.nim(2, 14) Error: internal error: expr: var not init result_104065
No stack traceback available
To create a stacktrace, rerun compilation with ./koch temp c <file>

Changing the signature to int works

proc flatIterator[T](s: openarray[T]): auto {.noSideEffect.}=
    result = iterator(): int = #<------------ Now it compiles
        when (T is not seq|array):
            for item in s:
                yield item
        else:
            yield 123456

let it = flatIterator(@[@[1,2],@[3,4]])

Context:
Following @zah suggestion in #5708. I'm trying to create a flat iterator that iterates on arbitrary nested openarrays. Inline iterators cannot be recursive (#555) so I'm trying through a closure iterator.

[mratsim]

Another error with genTypeInfo, if it's not related I will open another case:

proc flatIterator[N,T](s: array[N, T], U: typedesc): iterator(): U {.noSideEffect.}=
    result = iterator(): U =
        when T is array:
            let it = flatIterator(s, U)
            for item in it():
                yield item
        else:
            for item in s:
                yield item

let it = flatIterator([[1,2],[3,4]], int)

Error:

Error: internal error: genTypeInfo(tyTypeDesc)
No stack traceback available
To create a stacktrace, rerun compilation with ./koch temp c <file>

[mratsim]

For reference this is the working code I use that avoids all those issues

iterator flatIter[T](s: openarray[T]): auto {.noSideEffect.}=
  ## Inline iterator on any-depth seq or array
  ## Returns values in order
  for item in s:
    when item is array|seq:
      for subitem in flatIter(item):
        yield subitem
    else:
      yield item

@mratsim for info - this compiles with devel:

proc flatIterator[N,T](s: array[N, T], U: typedesc): iterator(): U {.noSideEffect.}=
    result = iterator(): U =
        when T is array:
            let it = flatIterator(s, U)
            for item in it():
                yield item
        else:
            for item in s:
                yield item

let it = flatIterator([[1,2],[3,4]], int)

All examples here compile (except for the one that was said to work :-). The infinite recursion blows the stack though, but if modified works fine.