'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.

[Araq]

Reordering also helps, put ArgsDef before ArgDef.

[genotrance]

Still fails with #head.

Hint: used config file 'c:\users\gt\Desktop\DL\programming\nimdevel\config\nim.cfg' [Conf]
Hint: system [Processing]
Hint: temp [Processing]
temp.nim(19, 17) Error: invalid type: 'ArgsDef' in this context: 'proc (name: string, valueType: int): ArgDef' for proc
Traceback (most recent call last)
nim.nim(121)             nim
nim.nim(77)              handleCmdLine
main.nim(173)            mainCommand
main.nim(77)             commandCompileToC
modules.nim(242)         compileProject
modules.nim(182)         compileModule
passes.nim(250)          processModule
passes.nim(139)          processTopLevelStmt
sem.nim(576)             myProcess
sem.nim(544)             semStmtAndGenerateGenerics
semstmts.nim(1975)       semStmt
semexprs.nim(797)        semExprNoType
semexprs.nim(2416)       semExpr
semstmts.nim(1724)       semProc
semstmts.nim(1640)       semProcAux
sem.nim(231)             paramsTypeCheck
sem.nim(228)             typeAllowedCheck
msgs.nim(1055)           localError
msgs.nim(1037)           liMessage
msgs.nim(891)            handleError
msgs.nim(876)            quit

[foldl]

Hi there. Any updates?

[metagn]

Now gives 'ArgsDef' is not a concrete type. Maybe we should just document that forward declared structural types do not work, or maybe this is documented already. Mutually recursive types are mentioned here without any mention of which types can be recursive (but it also says "to simplify structural type checking, recursive tuples are not valid" here which might be a bad location since recursive structural types and recursive types in general are different)

It is possible to support this as Nimony does by saving the type AST during forward declarations and inlining it and typechecking it when used but the design of the Nim compiler prefers keeping alias types etc. so it would not fit in well