Allow proc defaults with the Attributes API

This is a variant implementation of the changes proposed in #19914.
Unlike that PR, the change in behavior is isolated in its own class.
This is to prevent wonky behavior if a Proc is assigned outside of the
default, and it is a natural place to place the behavior required by #19921
as well.

Close #19914.

[Sean Griffin & Kir Shatrov]

activerecord/lib/active_record/attribute/user_provided_default.rb

@@ -0,0 +1,19 @@
+require 'active_record/attribute'
+
+module ActiveRecord
+  class Attribute # :nodoc:
+    class UserProvidedDefault < FromUser
+      def initialize(name, value, type)
+        super(name, value, type)
+      end
+
+      def type_cast(value)
+        if value.is_a?(Proc)
+          super(value.call)
+        else
+          super
+        end
+      end
+    end
+  end
+end

activerecord/lib/active_record/attributes.rb

@@ -1,3 +1,5 @@
+require 'active_record/attribute/user_provided_default'
+
 module ActiveRecord
   # See ActiveRecord::Attributes::ClassMethods for documentation
   module Attributes
@@ -236,7 +238,11 @@ def define_default_attribute(name, value, type, from_user:)
         if value == NO_DEFAULT_PROVIDED
           default_attribute = _default_attributes[name].with_type(type)
         elsif from_user
-          default_attribute = Attribute.from_user(name, value, type)
+          default_attribute = Attribute::UserProvidedDefault.new(
+            name,
+            value,
+            type,
+          )
         else
           default_attribute = Attribute.from_database(name, value, type)
         end

activerecord/test/cases/attributes_test.rb

@@ -125,6 +125,16 @@ def deserialize(*)
       assert_equal "from user", model.wibble
     end
 
+    test "procs for default values" do
+      klass = Class.new(OverloadedType) do
+        @@counter = 0
+        attribute :counter, :integer, default: -> { @@counter += 1 }
+      end
+
+      assert_equal 1, klass.new.counter
+      assert_equal 2, klass.new.counter
+    end
+
     if current_adapter?(:PostgreSQLAdapter)
       test "arrays types can be specified" do
         klass = Class.new(OverloadedType) do