From c26f0eeb553ee75cdd5b834f8fac2c976c66668f Mon Sep 17 00:00:00 2001
From: Guillaume Duboc <27832828+gldubc@users.noreply.github.com>
Date: Sun, 3 May 2026 22:26:51 +0200
Subject: [PATCH] Fix wrong intersection with union of dynamic()

---
 lib/elixir/lib/module/types/descr.ex               |  2 --
 lib/elixir/test/elixir/module/types/descr_test.exs |  5 +++++
 lib/elixir/test/elixir/module/types/expr_test.exs  | 14 ++++++++++++++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/lib/elixir/lib/module/types/descr.ex b/lib/elixir/lib/module/types/descr.ex
index 063ed4a8fb..f074240e85 100644
--- a/lib/elixir/lib/module/types/descr.ex
+++ b/lib/elixir/lib/module/types/descr.ex
@@ -448,8 +448,6 @@ defmodule Module.Types.Descr do
   """
   def intersection(:term, other), do: remove_optional(other)
   def intersection(other, :term), do: remove_optional(other)
-  def intersection(%{dynamic: :term}, other), do: dynamic(remove_optional(other))
-  def intersection(other, %{dynamic: :term}), do: dynamic(remove_optional(other))
 
   def intersection(left, right) do
     is_gradual_left = gradual?(left)
diff --git a/lib/elixir/test/elixir/module/types/descr_test.exs b/lib/elixir/test/elixir/module/types/descr_test.exs
index 1278f2b821..5eb59c512b 100644
--- a/lib/elixir/test/elixir/module/types/descr_test.exs
+++ b/lib/elixir/test/elixir/module/types/descr_test.exs
@@ -1500,6 +1500,11 @@ defmodule Module.Types.DescrTest do
         assert truthiness(dynamic(type)) == :always_false
       end
 
+      assert equal?(
+               intersection(union(atom(), dynamic()), union(atom(), dynamic())),
+               union(atom(), dynamic())
+             )
+
       for type <-
             [negation(atom()), atom([true]), negation(atom([false, nil])), atom([:ok]), integer()] do
         assert truthiness(type) == :always_true
diff --git a/lib/elixir/test/elixir/module/types/expr_test.exs b/lib/elixir/test/elixir/module/types/expr_test.exs
index c0a5483a26..087d901a11 100644
--- a/lib/elixir/test/elixir/module/types/expr_test.exs
+++ b/lib/elixir/test/elixir/module/types/expr_test.exs
@@ -1425,6 +1425,20 @@ defmodule Module.Types.ExprTest do
       assert typecheck!([x = 123, y = 456.0], x == y) == boolean()
     end
 
+    test "preserves static components across gradual self-intersections" do
+      assert typecheck!(
+               [x],
+               (
+                 # y :: :foo or dynamic()
+                 y = if :rand.uniform() > 0.5, do: :foo, else: x
+                 # y's type intersected with itself
+                 y = y
+
+                 y
+               )
+             ) == union(atom([:foo]), dynamic())
+    end
+
     test "in dynamic mode" do
       assert typedyn!([x = 123, y = 456.0], x < y) == dynamic(boolean())
       assert typedyn!([x = 123, y = 456.0], x == y) == dynamic(boolean())