3 files changed, 61 insertions, 0 deletions
diff --git a/activesupport/CHANGELOG b/activesupport/CHANGELOG
index fd0bd573c1..8d15d6622f 100644
--- a/activesupport/CHANGELOG
+++ b/activesupport/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* object.duplicable? returns true if object.dup is safe. False for nil, true, false, symbols, and numbers; true otherwise.  #9333 [sur]
+
 * Time, Date and DateTime #advance accept :weeks option.  #9866 [Geoff Buesing]
 
 * Fix Time#years_ago and #years_since from leap days.  #9865 [Geoff Buesing]
diff --git a/activesupport/lib/active_support/core_ext/duplicable.rb b/activesupport/lib/active_support/core_ext/duplicable.rb
new file mode 100644
index 0000000000..adbbfd8c60
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/duplicable.rb
@@ -0,0 +1,37 @@
+class Object
+  # Can you safely .dup this object?
+  # False for nil, false, true, symbols, and numbers; true otherwise.
+  def duplicable?
+    true
+  end
+end
+
+class NilClass #:nodoc:
+  def duplicable?
+    false
+  end
+end
+
+class FalseClass #:nodoc:
+  def duplicable?
+    false
+  end
+end
+
+class TrueClass #:nodoc:
+  def duplicable?
+    false
+  end
+end
+
+class Symbol #:nodoc:
+  def duplicable?
+    false
+  end
+end
+
+class Numeric #:nodoc:
+  def duplicable?
+    false
+  end
+end
diff --git a/activesupport/test/core_ext/duplicable_test.rb b/activesupport/test/core_ext/duplicable_test.rb
new file mode 100644
index 0000000000..7f8c0f7d3b
--- /dev/null
+++ b/activesupport/test/core_ext/duplicable_test.rb
@@ -0,0 +1,22 @@
+require File.dirname(__FILE__) + '/../abstract_unit'
+
+class DuplicableTest < Test::Unit::TestCase
+  NO  = [nil, false, true, :symbol, 1, 2.3, BigDecimal.new('4.56')]
+  YES = ['1', Object.new, /foo/, [], {}]
+
+  def test_duplicable
+    NO.each do |v|
+      assert !v.duplicable?
+      begin
+        v.dup
+        fail
+      rescue Exception
+      end
+    end
+
+    YES.each do |v|
+      assert v.duplicable?
+      assert_nothing_raised { v.dup }
+    end
+  end
+end