From 3b4aff13b545def35195e1ffac108568cffb83a3 Mon Sep 17 00:00:00 2001
From: Yusuke Endoh <mame@ruby-lang.org>
Date: Tue, 1 Sep 2026 17:45:03 +0900
Subject: [PATCH] Guard against an include/extend cycle in on_ancestors_updated

`module Foo; module Util; include Foo; end; extend Util; end` is valid Ruby but
made `on_ancestors_updated` recurse until SystemStackError. `include`/`prepend`
cycles are rejected by Ruby itself, so `extend` was the first way to reach it.
Add the same guard `each_descendant` has, plus scenarios for extend resolution.
---
 lib/typeprof/core/env/module_entity.rb |  2 +
 scenario/class/extend-cycle.rb         | 20 +++++++
 scenario/class/extend.rb               | 77 ++++++++++++++++++++++++++
 3 files changed, 99 insertions(+)
 create mode 100644 scenario/class/extend-cycle.rb
 create mode 100644 scenario/class/extend.rb

diff --git a/lib/typeprof/core/env/module_entity.rb b/lib/typeprof/core/env/module_entity.rb
index 06dc7e89..c961266b 100644
--- a/lib/typeprof/core/env/module_entity.rb
+++ b/lib/typeprof/core/env/module_entity.rb
@@ -482,6 +482,8 @@ def on_parent_modules_changed(genv)
     end
 
     def on_ancestors_updated(genv, base_mod)
+      # `include` and `extend` can form a cycle in valid Ruby
+      return if base_mod == self
       @child_modules.each_key {|child_mod| child_mod.on_ancestors_updated(genv, base_mod || self) }
       @static_reads.each_value do |static_reads|
         static_reads.each do |static_read|
diff --git a/scenario/class/extend-cycle.rb b/scenario/class/extend-cycle.rb
new file mode 100644
index 00000000..da3d0b87
--- /dev/null
+++ b/scenario/class/extend-cycle.rb
@@ -0,0 +1,20 @@
+## update
+module Foo
+  module Util
+    include Foo
+    def helper = "h"
+  end
+  extend Util
+end
+
+def f = Foo.helper
+
+## assert
+module Foo
+  module Util
+    def helper: -> String
+  end
+end
+class Object
+  def f: -> String
+end
diff --git a/scenario/class/extend.rb b/scenario/class/extend.rb
new file mode 100644
index 00000000..b74bbc1f
--- /dev/null
+++ b/scenario/class/extend.rb
@@ -0,0 +1,77 @@
+## update
+module A
+  def a = 1
+end
+module B
+  def b = "s"
+end
+class C
+  extend A
+  extend B
+end
+
+def f = [C.a, C.b]
+
+## assert
+module A
+  def a: -> Integer
+end
+module B
+  def b: -> String
+end
+class C
+  extend A
+  extend B
+end
+class Object
+  def f: -> [Integer, String]
+end
+
+## update
+module M
+  def hi = "from M"
+end
+class C
+  extend M
+  def self.hi = 42
+end
+
+def f = C.hi
+
+## assert
+module M
+  def hi: -> String
+end
+class C
+  extend M
+  def self.hi: -> Integer
+end
+class Object
+  def f: -> Integer
+end
+
+## update
+module Inner
+  def deep = 1.0
+end
+module Outer
+  include Inner
+end
+class C
+  extend Outer
+end
+
+def f = C.deep
+
+## assert
+module Inner
+  def deep: -> Float
+end
+module Outer
+end
+class C
+  extend Outer
+end
+class Object
+  def f: -> Float
+end