Remove redundant line empty when we eliminate negations (#15196)

Co-authored-by: Guillaume Duboc <guilduboc@gmail.com>

Author: josevalim

lib/elixir/lib/module/types/descr.ex

@@ -5126,10 +5126,8 @@ defmodule Module.Types.Descr do
          zip_empty_intersection?(elements, neg_elements) do
       [{tag, elements}]
     else
-      tuple_dnf_union(
-        tuple_elim_size(n, m, tag, elements, neg_tag),
+      tuple_elim_size(n, m, tag, elements, neg_tag) ++
         tuple_elim_content([], tag, elements, neg_elements)
-      )
     end
   end
 
@@ -5207,19 +5205,6 @@ defmodule Module.Types.Descr do
     end)
   end
 
-  # Prefer the smaller on the left
-  defp tuple_dnf_union(dnf1, dnf2) do
-    # Union of tuple DNFs is just concatenation,
-    # but we do our best to remove duplicates.
-    with [tuple1] <- dnf1,
-         [tuple2] <- dnf2,
-         optimized when optimized != nil <- maybe_optimize_tuple_union(tuple1, tuple2) do
-      [optimized]
-    else
-      _ -> dnf1 ++ (dnf2 -- dnf1)
-    end
-  end
-
   defp tuple_union(
          bdd_leaf(tag1, elements1) = tuple1,
          bdd_leaf(tag2, elements2) = tuple2
@@ -5313,23 +5298,17 @@ defmodule Module.Types.Descr do
   end
 
   # Transforms a bdd into a union of tuples with no negations.
-  # Note: it is important to compose the results with
-  # tuple_dnf_union/2 to avoid duplicates
   defp tuple_bdd_to_dnf_no_negations(bdd) do
     bdd_to_dnf(bdd)
-    |> Enum.reduce([], fn {pos, negs}, acc ->
+    |> Enum.flat_map(fn {pos, negs} ->
       case non_empty_tuple_literals_intersection(pos) do
-        :empty ->
-          acc
-
-        {tag, elements} ->
-          if tuple_line_empty?(tag, elements, negs) do
-            acc
-          else
-            tuple_eliminate_negations(tag, elements, negs) |> tuple_dnf_union(acc)
-          end
+        :empty -> []
+        {tag, elements} -> tuple_eliminate_negations(tag, elements, negs)
       end
     end)
+    # We want to avoid each_singleton? from failing,
+    # so we remove contiguous duplicates (cheaper than uniq)
+    |> Enum.dedup()
   end
 
   defp tuple_bdd_to_dnf_with_negations(bdd) do

lib/elixir/test/elixir/module/types/descr_test.exs

@@ -1430,6 +1430,16 @@ defmodule Module.Types.DescrTest do
       refute singleton?(open_tuple([]))
       refute singleton?(union(tuple([atom([:value])]), tuple([atom([:other_value])])))
       refute singleton?(union(tuple([atom([:value])]), closed_map(other: atom([:value]))))
+
+      # Both BDD lines produce the same singleton tuple, so the tuple DNF must not duplicate it.
+      a = tuple([union(integer(), atom([:ok])), atom([:x])])
+      b = tuple([integer(), atom([:x, :y])])
+      c = tuple([integer(), union(atom([:x]), binary())])
+
+      t = union(difference(a, b), difference(a, c))
+      # Semantically t ~= {:ok, :x}, confirmed by equal?
+      assert equal?(t, tuple([atom([:ok]), atom([:x])]))
+      assert singleton?(t)
     end
   end