Restore intersection precision

Author: gldubc

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

@@ -3489,17 +3489,19 @@ defmodule Module.Types.Descr do
             if neg_tag == :closed and map_empty?(map_intersection(bdd, neg_bdd)) do
               [{value, bdd} | acc]
             else
-              diff_bdd = map_difference(bdd, neg_bdd)
+              intersection_value = intersection(value, neg_value)
 
-              cond do
-                value == neg_value or subtype?(value, neg_value) ->
-                  if map_empty?(diff_bdd), do: acc, else: [{value, diff_bdd} | acc]
+              if empty?(intersection_value) do
+                [{value, bdd} | acc]
+              else
+                diff_bdd = map_difference(bdd, neg_bdd)
 
-                map_empty?(diff_bdd) ->
+                if map_empty?(diff_bdd) do
                   prepend_pair_unless_empty_diff(value, neg_value, bdd, acc)
-
-                true ->
-                  prepend_pair_unless_empty_diff(value, neg_value, bdd, [{value, diff_bdd} | acc])
+                else
+                  acc = [{intersection_value, diff_bdd} | acc]
+                  prepend_pair_unless_empty_diff(value, neg_value, bdd, acc)
+                end
               end
             end
           end)

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

@@ -2894,9 +2894,7 @@ defmodule Module.Types.DescrTest do
     end
 
     test "with projected negative maps and no popped value projection" do
-      # map_put/3 passes nil as the popped value accumulator because it only
-      # needs the map side. The final negative covers the whole popped map shape
-      # but not the key value, so this exercises that nil shortcut directly.
+      # map_put/3 passes nil as the popped value accumulator because it only needs the map side.
       map =
         projected_negative_map(100)
         |> difference(open_map(k: atom(), x: term()))