2 files changed, 9 insertions, 3 deletions
diff --git a/activesupport/lib/active_support/core_ext/enumerable.rb b/activesupport/lib/active_support/core_ext/enumerable.rb
index 8ebe758078..ae08fd90c1 100644
--- a/activesupport/lib/active_support/core_ext/enumerable.rb
+++ b/activesupport/lib/active_support/core_ext/enumerable.rb
@@ -115,9 +115,14 @@ end
 # and fall back to the compatible implementation, but that's much slower than
 # just calling the compat method in the first place.
 if Array.instance_methods(false).include?(:sum) && !(%w[a].sum rescue false)
-  class Array
-    alias :orig_sum :sum
+  # Using Refinements here in order not to expose our internal method
+  using Module.new {
+    refine Array do
+      alias :orig_sum :sum
+    end
+  }
 
+  class Array
     def sum(init = nil, &block) #:nodoc:
       if init.is_a?(Numeric) || first.is_a?(Numeric)
         init ||= 0
diff --git a/activesupport/lib/active_support/core_ext/object/duplicable.rb b/activesupport/lib/active_support/core_ext/object/duplicable.rb
index c671d34673..9485e74816 100644
--- a/activesupport/lib/active_support/core_ext/object/duplicable.rb
+++ b/activesupport/lib/active_support/core_ext/object/duplicable.rb
@@ -73,7 +73,8 @@ end
 
 class Symbol
   begin
-    :symbol.dup
+    :symbol.dup # Ruby 2.4.x.
+    'symbol_from_string'.to_sym.dup # Some symbols can't `dup` in Ruby 2.4.0.
   rescue TypeError
 
     # Symbols are not duplicable: