Dynamic dispatch broken if base method returns generic var type

[hohlerde]

Code example:

type
  Command* = ref object of RootObj
    d*: seq[cuchar]

  CmdSeparation* = ref object of Command
  BaseJob* = ref object of RootObj
  SeparationJob* = ref object of BaseJob

var
  workerThread: Thread[tuple[dummy:int]]
  

proc getJob*[T: BaseJob](t: typedesc[T]): T =
  result = new(T)
  

method serialize*(self: Command): var seq[cuchar] {.base, gcsafe.} =
  echo "Command"
  # the lines below cause warning: use {.base.} for base methods; baseless methods are deprecated [UseBase] for all other serialize methods
  self.d = newSeq[cuchar](10)
  result = self.d


method serialize*(self: CmdSeparation): var seq[cuchar] =
  echo "CmdSeparation"
  result = procCall Command(self).serialize()


proc doCmdWork(cmd: Command) =
  discard cmd.serialize()


method execute*(self: BaseJob) {.base, gcsafe.} =
  echo "BaseJob"


method execute*(self: SeparationJob) = 
  echo "SeparationJob"
  var cmd = CmdSeparation()
  doCmdWork(cmd)
  procCall BaseJob(self).execute()


proc doJobWork(job: BaseJob) =
  job.execute()


proc workerRun(params: tuple[dummy:int]) {.thread.} =
  var job = getJob(SeparationJob)
  doJobWork(job)


proc main*() =
  createThread(workerThread, workerRun, (dummy: 0))
  joinThread(workerThread)


when isMainModule:
  main()

Compiler gives a warning:

test.nim(24, 8) Warning: use {.base.} for base methods; baseless methods are deprecated [UseBase]

Expected console output:

SeparationJob
CmdSeparation
Command
BaseJob

Actual console output:

SeparationJob
Command
BaseJob

[hohlerde]

Nim version used:

Nim Compiler Version 0.17.3 (2017-11-14) [Windows: i386]
Copyright (c) 2006-2017 by Andreas Rumpf

git hash: 22ceab0fbb29241f72149fe4d53749d57861ba6f
active boot switches: -d:release

Also happens to be the same with amd64 on windows and also seen the same behaviour on a Raspberry Pi running raspbian.