Document and add spec for delegating to constants in Forwardable

Fixes [Bug #13142]

lib/forwardable.rb

@@ -122,7 +122,8 @@ module Forwardable
   end
 
   # Takes a hash as its argument.  The key is a symbol or an array of
-  # symbols.  These symbols correspond to method names.  The value is
+  # symbols.  These symbols correspond to method names, instance variable
+  # names, or constant names (see def_delegator).  The value is
   # the accessor to which the methods will be delegated.
   #
   # :call-seq:
@@ -160,9 +161,12 @@ module Forwardable
 
   # Define +method+ as delegator instance method with an optional
   # alias name +ali+. Method calls to +ali+ will be delegated to
-  # +accessor.method+.
+  # +accessor.method+.  +accessor+ should be a method name, instance
+  # variable name, or constant name.  Use the full path to the
+  # constant if providing the constant name.
   #
   #   class MyQueue
+  #     CONST = 1
   #     extend Forwardable
   #     attr_reader :queue
   #     def initialize
@@ -170,12 +174,14 @@ module Forwardable
   #     end
   #
   #     def_delegator :@queue, :push, :mypush
+  #     def_delegator 'MyQueue::CONST', :to_i
   #   end
   #
   #   q = MyQueue.new
   #   q.mypush 42
   #   q.queue    #=> [42]
   #   q.push 23  #=> NoMethodError
+  #   q.to_i     #=> 1
   #
   def def_instance_delegator(accessor, method, ali = method)
     gen = Forwardable._delegator_method(self, accessor, method, ali)

test/test_forwardable.rb

@@ -3,6 +3,7 @@ require 'test/unit'
 require 'forwardable'
 
 class TestForwardable < Test::Unit::TestCase
+  INTEGER = 42
   RECEIVER = BasicObject.new
   RETURNED1 = BasicObject.new
   RETURNED2 = BasicObject.new
@@ -27,6 +28,16 @@ class TestForwardable < Test::Unit::TestCase
     end
   end
 
+  def test_def_instance_delegator_constant
+    %i[def_delegator def_instance_delegator].each do |m|
+      cls = forwardable_class do
+        __send__ m, 'TestForwardable::INTEGER', :to_i
+      end
+
+      assert_equal 42, cls.new.to_i
+    end
+  end
+
   def test_def_instance_delegator_using_args_method_as_receiver
     %i[def_delegator def_instance_delegator].each do |m|
       cls = forwardable_class(