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10565 :: Invalid type void in a sequence in a generic object variant fails

• Patitotective (orginal) [2023-10-22T19:32:39+02:00] view original

  Compiling the following code

  type
    Obj*[T] = object
      case a*: bool
      of false: discard
      of true:
        b*: seq[T]
  
  var obj = Obj[void](a: false)
  Run

  Fails in

  ~/test/a.nim(8, 20) template/generic instantiation of `setLen` from here
  ~/.choosenim/toolchains/nim-2.0.0/lib/system/seqs_v2.nim(126, 18) template/generic instantiation of `len` from here
  ~/.choosenim/toolchains/nim-2.0.0/lib/system.nim(711, 13) Error: invalid type: 'void' in this context: 'proc (x: seq[void]): int{.noSideEffect, gcsafe.}' for proc
  
  Run

  I think this happens because even though the true branch isn't used it tries to see if it's valid, and a sequence of void isn't valid.

  A quick fix is to change void to any other type

  var obj = Obj[bool](a: false)
  Run

  Is this a bug? Should it be reported?

---

• Araq (orginal) [2023-10-22T21:18:36+02:00] view original

  No, it's just you not understanding types. ;-)

  Obj[void] is an invalid type, the active case branch is irrelevant. Types capture the invariants of the data, types do not change depending on runtime values.

---

• choltreppe (orginal) [2023-10-23T15:41:16+02:00] view original

  You could do something like:

  type
    Obj*[T] = object
      case a*: bool
      of false: discard
      of true:
        when T is void: discard
        else:
          b*: seq[T]
  Run

  so the b field only exists when the type isnt bool. But in this case the variants seem pretty useless and you could just do:

  type
    Obj*[T] = object
      when T is void:
        discard
      else:
        b*: seq[T]
  Run

---