`{.inheritable.}` does not work for generic objects

[alaviss]

Example

type
  X[T] {.inheritable.} = object
  Y = object of X[int]

Actual Output

test.nim(3, 14) Error: inheritance only works with non-final objects; for X[system.int] to be inheritable it must be 'object of RootObj' instead of 'object'

Expected Output

There should be no errors

Additional Information

• This works fine in mainline Nim
• I'm using this in union.nim to create virtual object roots for generics matching.