diff --git a/lib/robe/core_ext.rb b/lib/robe/core_ext.rb
index c80fbcb..26ff00b 100644
--- a/lib/robe/core_ext.rb
+++ b/lib/robe/core_ext.rb
@@ -3,14 +3,6 @@ class Module
     alias_method :__name__, :name
   end
 
-  if method_defined?(:singleton_class?)
-    alias_method :__singleton_class__?, :singleton_class?
-  else
-    def __singleton_class__?
-      self != Class && ancestors.first != self
-    end
-  end
-
   unless method_defined?(:__singleton_class__)
     alias_method :__singleton_class__, :singleton_class
   end
diff --git a/lib/robe/sash.rb b/lib/robe/sash.rb
index 41bdc2e..2d2df0f 100644
--- a/lib/robe/sash.rb
+++ b/lib/robe/sash.rb
@@ -50,7 +50,7 @@ def find_method_owner(mod, inst, sym)
 
     def method_spec(method)
       owner, inst = method.owner, nil
-      if owner.__singleton_class__?
+      if owner.singleton_class?
         # FIXME: Use https://docs.ruby-lang.org/en/3.2/Class.html#method-i-attached_object
         name = owner.to_s[/Class:([A-Z][^\(> ]*)/, 1] # defined in an eigenclass
       elsif name = name_cache[owner]
diff --git a/lib/robe/sash/includes_tracker.rb b/lib/robe/sash/includes_tracker.rb
index 1cfac97..2dbb891 100644
--- a/lib/robe/sash/includes_tracker.rb
+++ b/lib/robe/sash/includes_tracker.rb
@@ -27,7 +27,7 @@ def self.maybe_scan
 
           ObjectSpace.each_object(Module) do |cl|
             next unless cl.respond_to?(:included_modules)
-            next if cl.__singleton_class__?
+            next if cl.singleton_class?
             cl.included_modules.each { |mod| includers[mod] << [cl, true] }
             sc = cl.__singleton_class__
             sc.included_modules.each { |mod| includers[mod] << [cl, nil] }
diff --git a/lib/robe/type_space.rb b/lib/robe/type_space.rb
index 27c70de..a097fec 100644
--- a/lib/robe/type_space.rb
+++ b/lib/robe/type_space.rb
@@ -37,10 +37,8 @@ def scan_with(scanner)
       unless instance
         singleton_ancestors = obj.singleton_class.ancestors
 
-        if RUBY_VERSION >= "2.1.0"
-          # Ruby 2.1 includes all singletons in the ancestors chain
-          singleton_ancestors.reject!(&:__singleton_class__?)
-        end
+        # Ruby 2.1+ includes all singletons in the ancestors chain
+        singleton_ancestors.reject!(&:singleton_class?)
 
         scanner.scan(singleton_ancestors, true, false)
       end
diff --git a/lib/robe/visor.rb b/lib/robe/visor.rb
index 17a1f73..144902e 100644
--- a/lib/robe/visor.rb
+++ b/lib/robe/visor.rb
@@ -8,7 +8,7 @@ class Visor
     SearchError = Class.new(StandardError)
 
     def each_object(*args)
-      ObjectSpace.each_object(*args).reject { |m| m.__singleton_class__? }
+      ObjectSpace.each_object(*args).reject { |m| m.singleton_class? }
     end
 
     # Returns descendants and (modules only) also includers.