fixes #7637; assignments are not allowed to slice object; minor breaking change

tests/objects/tobj_asgn_dont_slice.nim

@@ -0,0 +1,24 @@
+discard """
+  outputsub: '''ObjectAssignmentError'''
+  exitcode: "1"
+"""
+
+# bug #7637
+type
+  Fruit = object of RootObj
+    name*: string
+  Apple = object of Fruit
+  Pear = object of Fruit
+
+method eat(f: Fruit) {.base.} =
+  raise newException(Exception, "PURE VIRTUAL CALL")
+
+method eat(f: Apple) =
+  echo "fruity"
+
+method eat(f: Pear) =
+  echo "juicy"
+
+let basket = [Apple(name:"a"), Pear(name:"b")]
+
+eat(basket[0])