inconsistent internal representation of generic objects array construction

[jangko]

the deeper I dig into #4799, the more I scratch my head. here is another meta-bug I think I need to isolate and fixed separately from #4799.

I know this one may looks like #7712 and #7637 because on surface it looks like object slicing. Indeed it sliced the object. But the problem highlighted here is the inconsistent internal representation of array construction of generic objects.

I will use macro to demonstrate the problem and to avoid object slicing at runtime. The problem itself not of macros module, but it originate from commonType algorithm of array construction semcheck.

import macros

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

macro peek(n: typed): untyped =
  echo getTypeImpl(n).treeRepr
  echo "---------"

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

peek([v, c, b])
peek([c, b, v]) # change elements position

output:

BracketExpr
  Sym "array"
  Infix
    Ident ".."
    IntLit 0
    IntLit 2
  BracketExpr        # expected output: Vehicle[int]
    Sym "Vehicle"    
    Sym "int"
---------
BracketExpr
  Sym "array"
  Infix
    Ident ".."
    IntLit 0
    IntLit 2
  Sym "Vehicle"   # lost it's generic identity: Vehicle[?]
---------