Fix recursive superclass check in singleton_ancestors

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
 

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