fixes #5264 (#5520); inheriting from generic object

tests/types/tparameterizedparent4.nim

@@ -0,0 +1,30 @@
+discard """
+  file: "tparameterizedparent4.nim"
+  line: 23
+  errormsg: "redefinition of 'grain'"
+"""
+# bug #5264
+type
+  Texture = enum
+    Smooth
+    Coarse
+
+  FruitBase = object of RootObj
+    color: int
+    grain: string
+
+  Apple[T] = object of T
+    width: int
+    tast_e: float64
+    case kind: Texture
+    of Smooth:
+      skin: float64
+    of Coarse:
+      grain: int
+
+proc setColor(self: var FruitBase, c: int) =
+  self.color = c
+
+var x = Apple[FruitBase](kind: Smooth, skin: 1.5)
+x.setColor(14)
+echo x