'invalid type ... in this context' when function, recursive ref and case types are used together

[zielmicha]

The following code produces f.nim(17, 17) Error: invalid type: 'ArgsDef' in this context: 'proc (name: string, valueType: int): ArgDef'

type
  ParserThunk[T] = (proc(): T)

  ArgDefType* = enum
    adtValue
    adtMoreArgs

  ArgDef* = ref object
    case typ*: ArgDefType
    of adtValue:
      valueType: int
    of adtMoreArgs:
      args*: ParserThunk[ArgsDef]

  ArgsDef* = ParserThunk[seq[ArgDef]]

proc valueArgDef(name: string, valueType: int): ArgDef =
  new(result)
  result.typ = adtValue
  result.valueType = valueType

However, this compiles (ArgsDef is 'inlined'):

type
  ParserThunk[T] = (proc(): T)

  ArgDefType* = enum
    adtValue
    adtMoreArgs

  ArgDef* = ref object
    case typ*: ArgDefType
    of adtValue:
      valueType: int
    of adtMoreArgs:
      args*: ParserThunk[ParserThunk[seq[ArgDef]]]

proc valueArgDef(name: string, valueType: int): ArgDef =
  new(result)
  result.typ = adtValue
  result.valueType = valueType

Other simplification, such as removing case type make this code compile. I've also managed to produce longer variations of this which crash the compiler.