fixes #7818, correct internal representation of generic objects array construction (#7824)

* defer skiptypes
* defer skiptypes for tyRef & tyPtr
* remove unneeded skipTypes

tests/array/t7818.nim

@@ -0,0 +1,45 @@
+discard """
+  msg: '''BracketExpr
+  Sym "array"
+  Infix
+    Ident ".."
+    IntLit 0
+    IntLit 2
+  BracketExpr
+    Sym "Vehicle"
+    Sym "int"
+---------
+BracketExpr
+  Sym "array"
+  Infix
+    Ident ".."
+    IntLit 0
+    IntLit 2
+  BracketExpr
+    Sym "Vehicle"
+    Sym "int"
+---------'''
+"""
+
+# bug #7818
+# this is not a macro bug, but array construction bug
+# I use macro to avoid object slicing
+# see #7712 and #7637
+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([c, b, v])
+peek([v, c, b])