array construction of ptr generic object with subtype relation failed

[jangko]

the first example is the working example

type
  Fruit = object of RootObj
    name: string
  Apple = object of Fruit
  Banana = object of Fruit

var
  ir = Fruit(name: "Fruit")
  ia = Apple(name: "apple")
  ib = Banana(name: "banana")

let x = [ia.addr, ib.addr, ir.addr]
for c in x:
  echo cast[int](c)

the second example, differ only in generics, failed to compile

type
  Vehicle[T] = object of RootObj
    tire: T
  Car[T] = object of Vehicle[T]
  Bike[T] = object of Vehicle[T]

var v = Vehicle[int](tire: 3)
var c = Car[int](tire: 4)
var b = Bike[int](tire: 2)

let y = [b.addr, c.addr, v.addr]

this bug was found during #4799 investigation, and needed to fixed before #4799
note: ref object and ref generic is ok, only ptr generic failed