Date#freeze bug doesn't affect Ruby 1.9

activesupport/lib/active_support/core_ext/date/behavior.rb

@@ -12,16 +12,26 @@ def acts_like_date?
         # the methods with one that caches the result in an instance variable.
         # If a Date is frozen but the memoized method hasn't been called, the
         # first call will result in a frozen object error since the memo
-        # instance variable is uninitialized. Work around by eagerly memoizing
-        # before freezing.
-        def freeze #:nodoc:
-          self.class.private_instance_methods(false).each do |m|
-            if m.to_s =~ /\A__\d+__\Z/
-              instance_variable_set(:"@#{m}", [send(m)])
+        # instance variable is uninitialized.
+        #
+        # Work around by eagerly memoizing before freezing.
+        #
+        # Ruby 1.9 uses a preinitialized instance variable so it's unaffected.
+        # This hack is as close as we can get to feature detection:
+        begin
+          ::Date.today.freeze.jd
+        rescue => frozen_object_error
+          if frozen_object_error.message =~ /frozen/
+            def freeze #:nodoc:
+              self.class.private_instance_methods(false).each do |m|
+                if m.to_s =~ /\A__\d+__\Z/
+                  instance_variable_set(:"@#{m}", [send(m)])
+                end
+              end
+
+              super
             end
           end
-
-          super
         end
       end
     end