merge revision(s) ebb96fa8808317ad53a4977bff26cf755d68077e: [Backport #17321]

	Fix singleton class cloning

	Before this commit, `clone` gave different results depending on whether the original object
	had an attached singleton class or not.

	Consider the following setup:
	```
	class Foo; end
	Foo.singleton_class.define_method(:foo) {}

	obj = Foo.new

	obj.singleton_class if $call_singleton

	clone = obj.clone
	```

	When `$call_singleton = false`, neither `obj.singleton_class.singleton_class` nor
	`clone.singleton_class.singleton_class` own any methods.

	However, when `$call_singleton = true`, `clone.singleton_class.singleton_class` would own a copy of
	`foo` from `Foo.singleton_class`, even though `obj.singleton_class.singleton_class` does not.

	The latter case is unexpected and results in a visibly different clone, depending on if the original object
	had an attached class or not.

	Co-authored-by: Ufuk Kayserilioglu <ufuk.kayserilioglu@shopify.com>
	---
	 class.c                 | 31 ++++++++++++++++++++++---------
	 test/ruby/test_class.rb | 47 +++++++++++++++++++++++++++++++++++++++++++++++
	 2 files changed, 69 insertions(+), 9 deletions(-)

1 files changed, 47 insertions, 0 deletions

diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index ca78473026..46485c4fd2 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -483,6 +483,53 @@ class TestClass < Test::Unit::TestCase
     assert_equal(:foo, d.foo)
   end
 
+  def test_clone_singleton_class_exists
+    klass = Class.new do
+      def self.bar; :bar; end
+    end
+
+    o = klass.new
+    o.singleton_class
+    clone = o.clone
+
+    assert_empty(o.singleton_class.instance_methods(false))
+    assert_empty(clone.singleton_class.instance_methods(false))
+    assert_empty(o.singleton_class.singleton_class.instance_methods(false))
+    assert_empty(clone.singleton_class.singleton_class.instance_methods(false))
+  end
+
+  def test_clone_when_singleton_class_of_singleton_class_exists
+    klass = Class.new do
+      def self.bar; :bar; end
+    end
+
+    o = klass.new
+    o.singleton_class.singleton_class
+    clone = o.clone
+
+    assert_empty(o.singleton_class.instance_methods(false))
+    assert_empty(clone.singleton_class.instance_methods(false))
+    assert_empty(o.singleton_class.singleton_class.instance_methods(false))
+    assert_empty(clone.singleton_class.singleton_class.instance_methods(false))
+  end
+
+  def test_clone_when_method_exists_on_singleton_class_of_singleton_class
+    klass = Class.new do
+      def self.bar; :bar; end
+    end
+
+    o = klass.new
+    o.singleton_class.singleton_class.define_method(:s2_method) { :s2 }
+    clone = o.clone
+
+    assert_empty(o.singleton_class.instance_methods(false))
+    assert_empty(clone.singleton_class.instance_methods(false))
+    assert_equal(:s2, o.singleton_class.s2_method)
+    assert_equal(:s2, clone.singleton_class.s2_method)
+    assert_equal([:s2_method], o.singleton_class.singleton_class.instance_methods(false))
+    assert_equal([:s2_method], clone.singleton_class.singleton_class.instance_methods(false))
+  end
+
   def test_singleton_class_p
     feature7609 = '[ruby-core:51087] [Feature #7609]'
     assert_predicate(self.singleton_class, :singleton_class?, feature7609)