From 9ef280820a8d204d5eeb9fa66fb00c6b9f9ed462 Mon Sep 17 00:00:00 2001
From: Lukasz Samson <lukaszsamson@gmail.com>
Date: Sat, 29 Aug 2026 10:33:14 +0200
Subject: [PATCH 1/2] Preserve left element types in list subtraction

Teach the type system that list subtraction returns a possibly-empty sublist of its left operand while preserving proper-list validation for both operands.

Assisted-by: Codex:GPT-5
---
 lib/elixir/lib/module/types/apply.ex              | 11 +++++++++++
 lib/elixir/test/elixir/module/types/expr_test.exs |  8 ++++++++
 2 files changed, 19 insertions(+)

diff --git a/lib/elixir/lib/module/types/apply.ex b/lib/elixir/lib/module/types/apply.ex
index 0eba019ad87..88f16aff955 100644
--- a/lib/elixir/lib/module/types/apply.ex
+++ b/lib/elixir/lib/module/types/apply.ex
@@ -1152,6 +1152,17 @@ defmodule Module.Types.Apply do
     end
   end
 
+  defp remote_apply(:erlang, :--, _info, [left, right], stack) do
+    case {list_of(left), list_of(right)} do
+      {{_, list_of}, {_, _}} ->
+        result = if list_of, do: list(list_of), else: empty_list()
+        {:ok, return(result, [left, right], stack)}
+
+      _ ->
+        {:error, badremote(:erlang, :--, [left, right])}
+    end
+  end
+
   @struct_key atom([:__struct__])
   @nil_atom atom([nil])
 
diff --git a/lib/elixir/test/elixir/module/types/expr_test.exs b/lib/elixir/test/elixir/module/types/expr_test.exs
index 6d26ff8db48..e3d51340938 100644
--- a/lib/elixir/test/elixir/module/types/expr_test.exs
+++ b/lib/elixir/test/elixir/module/types/expr_test.exs
@@ -173,6 +173,14 @@ defmodule Module.Types.ExprTest do
                    non_empty_list(term()), term()
                """
     end
+
+    test "--" do
+      assert typecheck!([x], [1, 2, 3] -- x) == list(integer())
+      assert typecheck!([x], [] -- x) == empty_list()
+
+      assert typeerror!([x], [1, 2, 3] -- String.to_integer(x)) |> strip_ansi() =~
+               "incompatible types given to Kernel.--/2"
+    end
   end
 
   describe "funs" do

From 3a7af7a2f3699836991a487b18507281837168e6 Mon Sep 17 00:00:00 2001
From: Lukasz Samson <lukaszsamson@gmail.com>
Date: Sun, 30 Aug 2026 08:46:49 +0200
Subject: [PATCH 2/2] Add :erlang.--/2 parametric TODO and cover static
 non-empty subtraction

---
 lib/elixir/lib/module/types/apply.ex              | 2 ++
 lib/elixir/test/elixir/module/types/expr_test.exs | 1 +
 2 files changed, 3 insertions(+)

diff --git a/lib/elixir/lib/module/types/apply.ex b/lib/elixir/lib/module/types/apply.ex
index 88f16aff955..c0ac9a0f1e9 100644
--- a/lib/elixir/lib/module/types/apply.ex
+++ b/lib/elixir/lib/module/types/apply.ex
@@ -1153,6 +1153,8 @@ defmodule Module.Types.Apply do
   end
 
   defp remote_apply(:erlang, :--, _info, [left, right], stack) do
+    # TODO: remove once we add parametric types, this will just be:
+    # list(a), list(term()) -> list(a)
     case {list_of(left), list_of(right)} do
       {{_, list_of}, {_, _}} ->
         result = if list_of, do: list(list_of), else: empty_list()
diff --git a/lib/elixir/test/elixir/module/types/expr_test.exs b/lib/elixir/test/elixir/module/types/expr_test.exs
index e3d51340938..6d1275c2f3f 100644
--- a/lib/elixir/test/elixir/module/types/expr_test.exs
+++ b/lib/elixir/test/elixir/module/types/expr_test.exs
@@ -176,6 +176,7 @@ defmodule Module.Types.ExprTest do
 
     test "--" do
       assert typecheck!([x], [1, 2, 3] -- x) == list(integer())
+      assert typecheck!([1] -- [1]) == list(integer())
       assert typecheck!([x], [] -- x) == empty_list()
 
       assert typeerror!([x], [1, 2, 3] -- String.to_integer(x)) |> strip_ansi() =~