From c9821ac688907b32df76b99e9486b9fa207ca894 Mon Sep 17 00:00:00 2001
From: ksss <co000ri@gmail.com>
Date: Tue, 22 Sep 2026 23:19:46 +0900
Subject: [PATCH] Support `nil` in return value.

---
 core/thread.rbs            | 2 +-
 test/stdlib/Thread_test.rb | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/core/thread.rbs b/core/thread.rbs
index c5da4e6ffd..7c18fa65a9 100644
--- a/core/thread.rbs
+++ b/core/thread.rbs
@@ -495,7 +495,7 @@ class Thread < Object
   # native thread, it returns *nil*. If the Ruby implementation uses M:N thread
   # model, the ID may change depending on the timing.
   #
-  def native_thread_id: () -> Integer
+  def native_thread_id: () -> Integer?
 
   # <!--
   #   rdoc-file=thread.c
diff --git a/test/stdlib/Thread_test.rb b/test/stdlib/Thread_test.rb
index 96df8d8e5f..d0d2b4ad50 100644
--- a/test/stdlib/Thread_test.rb
+++ b/test/stdlib/Thread_test.rb
@@ -42,6 +42,8 @@ def test_native_thread_id
       "() -> Integer",
       Thread.current, :native_thread_id
     )
+
+    assert_send_type "() -> nil", Thread.new{}.join, :native_thread_id
   end
 
   def test_raise