Merge pull request rails#41095 from Shopify/singleton-duplicable

Define Singleton#duplicable? and return false

Author: rafaelfranca

activesupport/lib/active_support/core_ext/object/duplicable.rb

@@ -47,3 +47,14 @@ def duplicable?
     false
   end
 end
+
+require "singleton"
+
+module Singleton
+  # Singleton instances are not duplicable:
+  #
+  # Class.new.include(Singleton).instance.dup # TypeError (can't dup instance of singleton
+  def duplicable?
+    false
+  end
+end

activesupport/test/core_ext/object/duplicable_test.rb

@@ -6,7 +6,7 @@
 require "active_support/core_ext/numeric/time"
 
 class DuplicableTest < ActiveSupport::TestCase
-  RAISE_DUP = [method(:puts), method(:puts).unbind]
+  RAISE_DUP = [method(:puts), method(:puts).unbind, Class.new.include(Singleton).instance]
   ALLOW_DUP = ["1", "symbol_from_string".to_sym, Object.new, /foo/, [], {}, Time.now, Class.new, Module.new, BigDecimal("4.56"), nil, false, true, 1, 2.3, Complex(1), Rational(1)]
 
   def test_duplicable