Return valid MapSet when union-ing a legacy MapSet (#6534)

Author: howleysv

lib/elixir/lib/map_set.ex

@@ -353,7 +353,8 @@ defmodule MapSet do
     %{map_set | map: Map.merge(map1, map2)}
   end
   def union(%MapSet{map: map1}, %MapSet{map: map2}) do
-    new_from_list(Map.keys(map1) ++ Map.keys(map2), [])
+    map = new_from_list(Map.keys(map1) ++ Map.keys(map2), [])
+    %MapSet{map: map}
   end
 
   defp order_by_size(map1, map2) when map_size(map1) > map_size(map2), do: {map2, map1}

lib/elixir/test/elixir/map_set_test.exs

@@ -108,4 +108,38 @@ defmodule MapSetTest do
     list = MapSet.to_list(MapSet.new(5..120))
     assert Enum.sort(list) == Enum.to_list(5..120)
   end
+
+  test "MapSet v1 compatibility" do
+    result = 1..5 |> map_set_v1() |> MapSet.new()
+    assert MapSet.equal?(result, MapSet.new(1..5))
+
+    result = MapSet.put(map_set_v1(1..5), 6)
+    assert MapSet.equal?(result, MapSet.new(1..6))
+
+    result = MapSet.union(map_set_v1(1..5), MapSet.new(6..10))
+    assert MapSet.equal?(result, MapSet.new(1..10))
+
+    result = MapSet.intersection(map_set_v1(1..10), MapSet.new(6..15))
+    assert MapSet.equal?(result, MapSet.new(6..10))
+
+    result = MapSet.difference(map_set_v1(1..10), MapSet.new(6..50))
+    assert MapSet.equal?(result, MapSet.new(1..5))
+
+    result = MapSet.delete(map_set_v1(1..10), 1)
+    assert MapSet.equal?(result, MapSet.new(2..10))
+
+    assert MapSet.size(map_set_v1(1..5)) == 5
+    assert MapSet.to_list(map_set_v1(1..5)) == Enum.to_list(1..5)
+
+    assert MapSet.disjoint?(map_set_v1(1..5), MapSet.new(10..15))
+    refute MapSet.disjoint?(map_set_v1(1..5), MapSet.new(5..10))
+
+    assert MapSet.subset?(map_set_v1(3..7), MapSet.new(1..10))
+    refute MapSet.subset?(map_set_v1(7..12), MapSet.new(1..10))
+  end
+
+  defp map_set_v1(enumerable) do
+    map = Map.new(1..5, &{&1, true})
+    %{__struct__: MapSet, map: map}
+  end
 end