Internal error: (filename: ccgstmts.nim, line: 1126) while returning result with no modification

[mratsim]

I was investigating and adding a test case to this issue #5325 and came upon this internal crash

  # Error: internal error: (filename: ccgstmts.nim, line: 1126)
  # No stack traceback available
  # To create a stacktrace, rerun compilation with ./koch temp c <file>

Test case

type
  Gate[TT] = ref object {.inheritable.}
    # Base operator or layer
    # Inherit from it and add a forward and backward method.

  Node[TT] = ref NodeObj[TT]

  NodeObj {.acyclic.} [TT] = object
    # Store an operator/layer + its parent
    gate: Gate[TT]
    parents: array[2, Node[TT]]

  Context*[TT] = ref object
    ## Tape / Wengert list. Contains the list of applied operations or layers
    ## i.e. Directed Acyclic Graph
    nodes: seq[Node[TT]]

  Variable*[TT] = object
    ## Wrapper for values
    tape: Context[TT]
    value: TT # TT should be a Tensor[T] or CudaTensor[T] or a scalar

method backward*[TT](self: Gate[TT], gradient: TT): array[2,TT] {.base.} =
  raise newException(ValueError, "backward method is not implemented")

method forward*[TT](self: Gate[TT], a, b: Variable[TT]): Variable[TT] {.base.} =
  raise newException(ValueError, "forward method is not implemented")

proc newContext*(TT: typedesc): Context[TT] {.noSideEffect.} =
  ## Initialize a context (Tape / Wengert list)
  new result
  result.nodes = newSeq[Node[TT]]()

#############################################################
type AddGate {.final.} [TT] = ref object of Gate[TT]
  arity: int
  a_shape: seq[int]
  b_shape: seq[int]


method forward*[TT](self: AddGate[TT], a, b: Variable[TT]): Variable[TT] =
  return Variable[TT](tape: a.tape, value: a.value + b.value)

method backward*[TT](self: AddGate[TT], gradient: TT): array[2,TT] =
  result #<--- crash
  # Error: internal error: (filename: ccgstmts.nim, line: 1126)
  # No stack traceback available
  # To create a stacktrace, rerun compilation with ./koch temp c <file>

  # Using the next statements instead do not crash
  # result[0] = 1.TT
  # result[1] = 1.TT

var ctx = newContext float32

[genotrance]

No longer crashes with #head.

-------- SNIPPET --------
type
  Gate[TT] = ref object {.inheritable.}
    # Base operator or layer
    # Inherit from it and add a forward and backward method.

  Node[TT] = ref NodeObj[TT]

  NodeObj {.acyclic.} [TT] = object
    # Store an operator/layer + its parent
    gate: Gate[TT]
    parents: array[2, Node[TT]]

  Context*[TT] = ref object
    ## Tape / Wengert list. Contains the list of applied operations or layers
    ## i.e. Directed Acyclic Graph
    nodes: seq[Node[TT]]

  Variable*[TT] = object
    ## Wrapper for values
    tape: Context[TT]
    value: TT # TT should be a Tensor[T] or CudaTensor[T] or a scalar

method backward*[TT](self: Gate[TT], gradient: TT): array[2,TT] {.base.} =
  raise newException(ValueError, "backward method is not implemented")

method forward*[TT](self: Gate[TT], a, b: Variable[TT]): Variable[TT] {.base.} =
  raise newException(ValueError, "forward method is not implemented")

proc newContext*(TT: typedesc): Context[TT] {.noSideEffect.} =
  ## Initialize a context (Tape / Wengert list)
  new result
  result.nodes = newSeq[Node[TT]]()

#############################################################
type AddGate {.final.} [TT] = ref object of Gate[TT]
  arity: int
  a_shape: seq[int]
  b_shape: seq[int]


method forward*[TT](self: AddGate[TT], a, b: Variable[TT]): Variable[TT] =
  return Variable[TT](tape: a.tape, value: a.value + b.value)

method backward*[TT](self: AddGate[TT], gradient: TT): array[2,TT] =
  result #<--- crash
  # Error: internal error: (filename: ccgstmts.nim, line: 1126)
  # No stack traceback available
  # To create a stacktrace, rerun compilation with ./koch temp c <file>

  # Using the next statements instead do not crash
  # result[0] = 1.TT
  # result[1] = 1.TT

var ctx = newContext float32
-------------------------

-------- OUTPUT --------
Hint: used config file 'c:\users\gt\Desktop\DL\programming\nimdevel\config\nim.cfg' [Conf]
Hint: system [Processing]
Hint: temp [Processing]
CC: temp
CC: stdlib_system
Hint:  [Link]
Hint: operation successful (12044 lines compiled; 1.047 sec total; 10.766MiB peakmem; Debug Build) [SuccessX]

Ran successfully
------------------------

-------- NIMTEMP --------
Hint: used config file 'c:\users\gt\Desktop\DL\programming\nimdevel\config\nim.cfg' [Conf]
Hint: system [Processing]
Hint: temp [Processing]
CC: temp
CC: stdlib_system
Hint:  [Link]
Hint: operation successful (12044 lines compiled; 2.219 sec total; 10.758MiB peakmem; Debug Build) [SuccessX]

Ran successfully
-------------------------

-------- VERSION --------
Nim Compiler Version 0.18.1 [Windows: i386]
Compiled at 2018-07-05
Copyright (c) 2006-2018 by Andreas Rumpf

git hash: 596de2a3b935ff21ffb9ab821d7c079003eacb24
active boot switches: -d:release
-------------------------