Allow default to be configured for Enum

```ruby
class Book < ActiveRecord::Base
  enum status: [:proposed, :written, :published], _default: :published
end

Book.new.status # => "published"
```

activerecord/CHANGELOG.md

+*   Allow default to be configured for Enum.
+
+    ```ruby
+    class Book < ActiveRecord::Base
+      enum status: [:proposed, :written, :published], _default: :published
+    end
+
+    Book.new.status # => "published"
+    ```
+
+    *Ryuta Kamizono*
+
 *   Support `where` with comparison operators (`>`, `>=`, `<`, and `<=`).
 
     ```ruby

activerecord/lib/active_record/attributes.rb

@@ -205,13 +205,13 @@ module ClassMethods
       # tracking is performed. The methods +changed?+ and +changed_in_place?+
       # will be called from ActiveModel::Dirty. See the documentation for those
       # methods in ActiveModel::Type::Value for more details.
-      def attribute(name, cast_type = Type::Value.new, **options)
+      def attribute(name, cast_type = nil, **options, &block)
         name = name.to_s
         reload_schema_from_cache
 
         self.attributes_to_define_after_schema_loads =
           attributes_to_define_after_schema_loads.merge(
-            name => [cast_type, options]
+            name => [cast_type || block, options]
           )
       end
 
@@ -246,9 +246,14 @@ def define_attribute(
       def load_schema! # :nodoc:
         super
         attributes_to_define_after_schema_loads.each do |name, (type, options)|
-          if type.is_a?(Symbol)
+          case type
+          when Symbol
             adapter_name = ActiveRecord::Type.adapter_name_from(self)
             type = ActiveRecord::Type.lookup(type, **options.except(:default), adapter: adapter_name)
+          when Proc
+            type = type[type_for_attribute(name)]
+          else
+            type ||= Type::Value.new
           end
 
           define_attribute(name, type, **options.slice(:default))

activerecord/lib/active_record/enum.rb

@@ -159,9 +159,14 @@ def assert_valid_value(value)
 
     def enum(definitions)
       klass = self
+
       enum_prefix = definitions.delete(:_prefix)
       enum_suffix = definitions.delete(:_suffix)
       enum_scopes = definitions.delete(:_scopes)
+
+      default = {}
+      default[:default] = definitions.delete(:_default) if definitions.key?(:_default)
+
       definitions.each do |name, values|
         assert_valid_enum_definition_values(values)
         # statuses = { }
@@ -177,7 +182,7 @@ def enum(definitions)
         detect_enum_conflict!(name, "#{name}=")
 
         attr = attribute_alias?(name) ? attribute_alias(name) : name
-        decorate_attribute_type(attr, :enum) do |subtype|
+        attribute(attr, **default) do |subtype|
           EnumType.new(attr, enum_values, subtype)
         end
 

activerecord/test/cases/enum_test.rb

@@ -583,6 +583,15 @@ def self.name; "Book"; end
     assert_equal :integer, Book.type_for_attribute("status").type
   end
 
+  test "overloaded default" do
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = "books"
+      enum status: [:proposed, :written, :published], _default: :published
+    end
+
+    assert_equal "published", klass.new.status
+  end
+
   test "scopes can be disabled" do
     klass = Class.new(ActiveRecord::Base) do
       self.table_name = "books"