Date#freeze bug doesn't affect Ruby 1.9

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

@@ -1,3 +1,5 @@
+require 'date'
+
 module ActiveSupport #:nodoc:
   module CoreExtensions #:nodoc:
     module Date #:nodoc:
@@ -10,18 +12,25 @@ module ActiveSupport #:nodoc:
 
         # Date memoizes some instance methods using metaprogramming to wrap
         # 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:
+        if (Date.today.freeze.inspect rescue false)
+          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
-          end
 
-          super
+            super
+          end
         end
       end
     end