Unify decorate_attribute_type and attribute

Follow-up to 75c309c7ad.

As a result of these changes, attributes can have their type and default
value configured separately.  Similar behavior was implemented in #39380,
but only for attributes that derive (and do not override) their type
from the database.

activerecord/lib/active_record/attribute_methods/serialization.rb

@@ -129,7 +129,7 @@ module ActiveRecord
             Coders::YAMLColumn.new(attr_name, class_name_or_coder)
           end
 
-          decorate_attribute_type(attr_name.to_s, **options) do |cast_type|
+          attribute(attr_name, **options) do |cast_type|
             if type_incompatible_with_serialize?(cast_type, class_name_or_coder)
               raise ColumnNotSerializableError.new(attr_name, cast_type)
             end

activerecord/lib/active_record/attributes.rb

@@ -208,14 +208,21 @@ module ActiveRecord
       # 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 = nil, **options, &block)
+      def attribute(name, cast_type = nil, **options, &decorator)
         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 || block, options]
-          )
+        prev_cast_type, prev_options, prev_decorator = attributes_to_define_after_schema_loads[name]
+
+        unless cast_type && prev_cast_type
+          cast_type ||= prev_cast_type
+          options = prev_options || options if options.empty?
+          decorator ||= prev_decorator
+        end
+
+        self.attributes_to_define_after_schema_loads = attributes_to_define_after_schema_loads.merge(
+          name => [cast_type, options, decorator]
+        )
       end
 
       # This is the low level API which sits beneath +attribute+. It only
@@ -248,8 +255,16 @@ module ActiveRecord
 
       def load_schema! # :nodoc:
         super
-        attributes_to_define_after_schema_loads.each do |name, (type, options)|
-          define_attribute(name, _lookup_cast_type(name, type, options), **options.slice(:default))
+        attributes_to_define_after_schema_loads.each do |name, (type, options, decorator)|
+          if type.is_a?(Symbol)
+            type = ActiveRecord::Type.lookup(type, **options.except(:default), adapter: ActiveRecord::Type.adapter_name_from(self))
+          elsif type.nil?
+            type = type_for_attribute(name)
+          end
+
+          type = decorator[type] if decorator
+
+          define_attribute(name, type, **options.slice(:default))
         end
       end
 
@@ -272,32 +287,6 @@ module ActiveRecord
           end
           _default_attributes[name] = default_attribute
         end
-
-        def decorate_attribute_type(attr_name, **default)
-          type, options = attributes_to_define_after_schema_loads[attr_name]
-
-          default.with_defaults!(default: options[:default]) if options&.key?(:default)
-
-          attribute(attr_name, **default) do |cast_type|
-            if type && !type.is_a?(Proc)
-              cast_type = _lookup_cast_type(attr_name, type, options)
-            end
-
-            yield cast_type
-          end
-        end
-
-        def _lookup_cast_type(name, type, options)
-          case type
-          when Symbol
-            adapter_name = ActiveRecord::Type.adapter_name_from(self)
-            ActiveRecord::Type.lookup(type, **options.except(:default), adapter: adapter_name)
-          when Proc
-            type[type_for_attribute(name)]
-          else
-            type || type_for_attribute(name)
-          end
-        end
     end
   end
 end

activerecord/lib/active_record/enum.rb

@@ -183,7 +183,7 @@ module ActiveRecord
 
         attr = attribute_alias?(name) ? attribute_alias(name) : name
 
-        decorate_attribute_type(attr, **default) do |subtype|
+        attribute(attr, **default) do |subtype|
           EnumType.new(attr, enum_values, subtype)
         end
 

activerecord/test/cases/attributes_test.rb

@@ -68,6 +68,15 @@ module ActiveRecord
       assert_equal "the overloaded default", klass.new.overloaded_string_with_limit
     end
 
+    test "attributes with overridden types keep their type when a default value is configured separately" do
+      child = Class.new(OverloadedType) do
+        attribute :overloaded_float, default: "123"
+      end
+
+      assert_equal OverloadedType.type_for_attribute("overloaded_float"), child.type_for_attribute("overloaded_float")
+      assert_equal 123, child.new.overloaded_float
+    end
+
     test "extra options are forwarded to the type caster constructor" do
       klass = Class.new(OverloadedType) do
         attribute :starts_at, :datetime, precision: 3, limit: 2, scale: 1, default: -> { Time.now.utc }
@@ -295,6 +304,15 @@ module ActiveRecord
       assert_equal 123, model.non_existent_decimal
     end
 
+    test "attributes not backed by database columns keep their type when a default value is configured separately" do
+      child = Class.new(OverloadedType) do
+        attribute :non_existent_decimal, default: "123"
+      end
+
+      assert_equal OverloadedType.type_for_attribute("non_existent_decimal"), child.type_for_attribute("non_existent_decimal")
+      assert_equal 123, child.new.non_existent_decimal
+    end
+
     test "attributes not backed by database columns properly interact with mutation and dirty" do
       child = Class.new(ActiveRecord::Base) do
         self.table_name = "topics"