From a9dfb05649875d7edd069fb329021b4dea3397cc Mon Sep 17 00:00:00 2001
From: dak2 <32436625+dak2@users.noreply.github.com>
Date: Sun, 25 Oct 2026 18:47:18 +0900
Subject: [PATCH] Fix recursive superclass check in singleton_ancestors

The superclass recursion reset building_ancestors, so a cycle raised
SystemStackError instead of RecursiveAncestorError.
---
 .../definition_builder/ancestor_builder.rb    |  2 +-
 test/rbs/ancestor_builder_test.rb             | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/lib/rbs/definition_builder/ancestor_builder.rb b/lib/rbs/definition_builder/ancestor_builder.rb
index 7cc2c7543..3e5d0a137 100644
--- a/lib/rbs/definition_builder/ancestor_builder.rb
+++ b/lib/rbs/definition_builder/ancestor_builder.rb
@@ -602,7 +602,7 @@ def singleton_ancestors(type_name, building_ancestors: [])
         when Definition::Ancestor::Singleton
           super_name = super_class.name
 
-          super_ancestors = singleton_ancestors(super_name, building_ancestors: [])
+          super_ancestors = singleton_ancestors(super_name, building_ancestors: building_ancestors)
           ancestors.unshift(*super_ancestors.ancestors)
         end
 
diff --git a/test/rbs/ancestor_builder_test.rb b/test/rbs/ancestor_builder_test.rb
index bbd4d2d61..634f862d0 100644
--- a/test/rbs/ancestor_builder_test.rb
+++ b/test/rbs/ancestor_builder_test.rb
@@ -729,6 +729,25 @@ module Y[A]
     end
   end
 
+  def test_singleton_ancestors_recursive_superclass
+    SignatureManager.new do |manager|
+      manager.files[Pathname("foo.rbs")] = <<EOF
+class A < B
+end
+
+class B < A
+end
+EOF
+      manager.build do |env|
+        builder = DefinitionBuilder::AncestorBuilder.new(env: env)
+
+        assert_raises(RecursiveAncestorError) do
+          builder.singleton_ancestors(type_name("::A"))
+        end
+      end
+    end
+  end
+
   def test_build_invalid_type_application
     SignatureManager.new do |manager|
       manager.files[Pathname("foo.rbs")] = <<EOF