diff --git a/activesupport/lib/active_support/inflector.rb b/activesupport/lib/active_support/inflector.rb
index 4921b99677cef4adbfbaf4e3f8018e48e1d0dd56..9fb7540ea2c0954aeee96e1b1b21b1c458dc9a06 100644
--- a/activesupport/lib/active_support/inflector.rb
+++ b/activesupport/lib/active_support/inflector.rb
@@ -330,47 +330,30 @@ def foreign_key(class_name, separate_class_name_and_id_with_underscore = true)
       underscore(demodulize(class_name)) + (separate_class_name_and_id_with_underscore ? "_id" : "id")
     end
 
-    # Ruby 1.9 introduces an inherit argument for Module#const_get and
-    # #const_defined? and changes their default behavior.
-    if Module.method(:const_get).arity == 1
-      # Tries to find a constant with the name specified in the argument string:
-      #
-      #   "Module".constantize     # => Module
-      #   "Test::Unit".constantize # => Test::Unit
-      #
-      # The name is assumed to be the one of a top-level constant, no matter whether
-      # it starts with "::" or not. No lexical context is taken into account:
-      #
-      #   C = 'outside'
-      #   module M
-      #     C = 'inside'
-      #     C               # => 'inside'
-      #     "C".constantize # => 'outside', same as ::C
-      #   end
-      #
-      # NameError is raised when the name is not in CamelCase or the constant is
-      # unknown.
-      def constantize(camel_cased_word)
-        names = camel_cased_word.split('::')
-        names.shift if names.empty? || names.first.empty?
-
-        constant = Object
-        names.each do |name|
-          constant = constant.const_defined?(name) ? constant.const_get(name) : constant.const_missing(name)
-        end
-        constant
-      end
-    else
-      def constantize(camel_cased_word) #:nodoc:
-        names = camel_cased_word.split('::')
-        names.shift if names.empty? || names.first.empty?
-
-        constant = Object
-        names.each do |name|
-          constant = constant.const_get(name, false) || constant.const_missing(name)
-        end
-        constant
-      end
+    # Tries to find a constant with the name specified in the argument string:
+    #
+    #   "Module".constantize     # => Module
+    #   "Test::Unit".constantize # => Test::Unit
+    #
+    # The name is assumed to be the one of a top-level constant, no matter whether
+    # it starts with "::" or not. No lexical context is taken into account:
+    #
+    #   C = 'outside'
+    #   module M
+    #     C = 'inside'
+    #     C               # => 'inside'
+    #     "C".constantize # => 'outside', same as ::C
+    #   end
+    #
+    # NameError is raised when the name is not in CamelCase or the constant is
+    # unknown.
+    def constantize(camel_cased_word)
+      names = camel_cased_word.split('::')
+      names.shift if names.empty? || names.first.empty?
+
+      constant = Object
+      names.each { |name| constant = constant.const_get(name) }
+      constant
     end
 
     # Turns a number into an ordinal string used to denote the position in an
diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index d8c93dc9ae265c530607c651f38bf29ae2de5b9c..2c422ebe72759a657983d10fcaea2db9cd493c88 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -2,8 +2,21 @@
 require 'inflector_test_cases'
 
 module Ace
+  module Extension
+    def self.included(base)
+      base.extend(ClassMethods)
+    end
+
+    module ClassMethods
+      def mission_accomplished?
+        false
+      end
+    end
+  end
+
   module Base
     class Case
+      include Extension
     end
   end
 end
@@ -167,7 +180,9 @@ def test_constantize
   end
 
   def test_constantize_does_lexical_lookup
-    assert_raises(NameError) { ActiveSupport::Inflector.constantize("Ace::Base::InflectorTest") }
+    assert_equal InflectorTest, ActiveSupport::Inflector.constantize("Ace::Base::InflectorTest")
+    assert_nothing_raised { Ace::Base::Case::ClassMethods }
+    assert_nothing_raised { assert_equal Ace::Base::Case::ClassMethods, ActiveSupport::Inflector.constantize("Ace::Base::Case::ClassMethods") }
   end
 
   def test_ordinal