Unable to make a const instance of an inherited, generic object.

[linkmonitor]

Compilation fails when it includes a const instance of an object inherited from a generic object.

Example

type
  Point1[T] = object of RootObj
    x:T

  Point2[T] = object of Point1[T]
    y:T

proc main =
  var a = Point2[int](x:1, y:2)
  echo a

  let b = Point2[int](x:1, y:2)
  echo b

  const c = Point2[int](x:1, y:2)
  echo c

when isMainModule:
  main()

Current Output

main.nim(15, 24) Error: cannot create null element for: [T]

Expected Output

(y: 2, x: 1)
(y: 2, x: 1)
(y: 2, x: 1)

Additional Information

Updated my toolchain using choosenim approximately 6 hours ago.

$ nim -v
Nim Compiler Version 0.19.9 [Linux: amd64]
Compiled at 2019-05-16
Copyright (c) 2006-2019 by Andreas Rumpf

active boot switches: -d:release

[mratsim]

This shouldn't be allowed and caught during semcheck.

Point2 should be a ref object of Point1 because it adds new field and that will change the size of the type.

[krux02]

@mratsim: Well I disagree. I think is not code where you should use a ref type. I think it is just a bug in the VM.

[mratsim]

I think it brings extra complexity with little benefits.
For example you cannot store them in a sequence:

type
  Point1 = object of RootObj
    x: int

  Point2 = object of Point1
    y: int

  PointString = object of Point1
    s: string

var a: seq[Point1]

a.add Point1(x: 1)
a.add Point2(x: 2, y: 2)
a.add PointString(x: 3, s: "s")

echo a

[linkmonitor]

Whether or not inheritance is appropriate for this snippet, I think it's reasonable to assume there are cases where non-ref inheritance is useful.

I don't use the "generic inherited point" pattern in code; it's an example I use to explore what's possible given the language's features.

[krux02]

@mratsim I know what non ref inheritance means. Nobody tries to do that here.

I was thinking that the combination of generics and inheritance at compile time causes the problems here, but aparently that is not the cause of this problem. The snippet refuses to work even without generics:

type
  Point1 = object of RootObj
    x: int

  Point2 = object of Point1
    y: int

proc main =
  var a = Point2(x:1, y:2)
  echo a

  let b = Point2(x:1, y:2)
  echo b


  const c = Point2(x:1, y:2)

  echo c

static:
  let d = Point2(x:1, y:2)
  echo d

when isMainModule:
  main()

[Araq]

const objects are not allowed to inherit and are not allowed to have case in them, the compiler's error message should say so, but doesn't.