1 files changed, 38 insertions, 20 deletions

diff --git a/activesupport/lib/active_support/core_ext/class/attribute.rb b/activesupport/lib/active_support/core_ext/class/attribute.rb
index 802d988af2..7928efb871 100644
--- a/activesupport/lib/active_support/core_ext/class/attribute.rb
+++ b/activesupport/lib/active_support/core_ext/class/attribute.rb
@@ -1,11 +1,23 @@
-require 'active_support/core_ext/kernel/singleton_class'
-require 'active_support/core_ext/module/remove_method'
-require 'active_support/core_ext/array/extract_options'
+# frozen_string_literal: true
+
+require "active_support/core_ext/kernel/singleton_class"
+require "active_support/core_ext/module/redefine_method"
+require "active_support/core_ext/array/extract_options"
 
 class Class
   # Declare a class-level attribute whose value is inheritable by subclasses.
   # Subclasses can change their own value and it will not impact parent class.
   #
+  # ==== Options
+  #
+  # * <tt>:instance_reader</tt> - Sets the instance reader method (defaults to true).
+  # * <tt>:instance_writer</tt> - Sets the instance writer method (defaults to true).
+  # * <tt>:instance_accessor</tt> - Sets both instance methods (defaults to true).
+  # * <tt>:instance_predicate</tt> - Sets a predicate method (defaults to true).
+  # * <tt>:default</tt> - Sets a default value for the attribute (defaults to nil).
+  #
+  # ==== Examples
+  #
   #   class Base
   #     class_attribute :setting
   #   end
@@ -20,14 +32,14 @@ class Class
   #   Base.setting                # => true
   #
   # In the above case as long as Subclass does not assign a value to setting
-  # by performing <tt>Subclass.setting = _something_ </tt>, <tt>Subclass.setting</tt>
+  # by performing <tt>Subclass.setting = _something_</tt>, <tt>Subclass.setting</tt>
   # would read value assigned to parent class. Once Subclass assigns a value then
   # the value assigned by Subclass would be returned.
   #
   # This matches normal Ruby method inheritance: think of writing an attribute
   # on a subclass as overriding the reader method. However, you need to be aware
   # when using +class_attribute+ with mutable structures as +Array+ or +Hash+.
-  # In such cases, you don't want to do changes in places but use setters:
+  # In such cases, you don't want to do changes in place. Instead use setters:
   #
   #   Base.setting = []
   #   Base.setting                # => []
@@ -68,32 +80,35 @@ class Class
   #   object.setting = false  # => NoMethodError
   #
   # To opt out of both instance methods, pass <tt>instance_accessor: false</tt>.
+  #
+  # To set a default value for the attribute, pass <tt>default:</tt>, like so:
+  #
+  #   class_attribute :settings, default: {}
   def class_attribute(*attrs)
     options = attrs.extract_options!
-    instance_reader = options.fetch(:instance_accessor, true) && options.fetch(:instance_reader, true)
-    instance_writer = options.fetch(:instance_accessor, true) && options.fetch(:instance_writer, true)
+    instance_reader    = options.fetch(:instance_accessor, true) && options.fetch(:instance_reader, true)
+    instance_writer    = options.fetch(:instance_accessor, true) && options.fetch(:instance_writer, true)
     instance_predicate = options.fetch(:instance_predicate, true)
+    default_value      = options.fetch(:default, nil)
 
     attrs.each do |name|
-      remove_possible_singleton_method(name)
+      singleton_class.silence_redefinition_of_method(name)
       define_singleton_method(name) { nil }
 
-      remove_possible_singleton_method("#{name}?")
+      singleton_class.silence_redefinition_of_method("#{name}?")
       define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate
 
       ivar = "@#{name}"
 
-      remove_possible_singleton_method("#{name}=")
+      singleton_class.silence_redefinition_of_method("#{name}=")
       define_singleton_method("#{name}=") do |val|
         singleton_class.class_eval do
-          remove_possible_method(name)
-          define_method(name) { val }
+          redefine_method(name) { val }
         end
 
         if singleton_class?
           class_eval do
-            remove_possible_method(name)
-            define_method(name) do
+            redefine_method(name) do
               if instance_variable_defined? ivar
                 instance_variable_get ivar
               else
@@ -106,8 +121,7 @@ class Class
       end
 
       if instance_reader
-        remove_possible_method name
-        define_method(name) do
+        redefine_method(name) do
           if instance_variable_defined?(ivar)
             instance_variable_get ivar
           else
@@ -115,13 +129,17 @@ class Class
           end
         end
 
-        remove_possible_method "#{name}?"
-        define_method("#{name}?") { !!public_send(name) } if instance_predicate
+        redefine_method("#{name}?") { !!public_send(name) } if instance_predicate
       end
 
       if instance_writer
-        remove_possible_method "#{name}="
-        attr_writer name
+        redefine_method("#{name}=") do |val|
+          instance_variable_set ivar, val
+        end
+      end
+
+      unless default_value.nil?
+        self.send("#{name}=", default_value)
       end
     end
   end