Allow DelegateClass() to module_eval given block

Methods that return classes often module_eval the given block
(e.g. Class.new and Struct.new).  This allows DelegateClass to
work similarly.  This makes it easier to use DelegateClass
directly without subclassing, so as not to create an unnecessary
subclass.

Implements [Feature #15842]

NEWS

@@ -118,6 +118,11 @@ Date::
     new Japanese era as an informal extension, until the new JIS X 0301 is
     issued.  [Feature #15742]
 
+Delegate::
+
+  * Object#DelegateClass accepts a block and module_evals it in the context
+    of the returned class, similar to Class.new and Struct.new.
+
 ERB::
 
   * Prohibit marshaling ERB instance.

lib/delegate.rb

@@ -360,6 +360,14 @@ def Delegator.delegating_block(mid) # :nodoc:
 #     end
 #   end
 #
+# or:
+#
+#   MyClass = DelegateClass(ClassToDelegateTo) do    # Step 1
+#     def initialize
+#       super(obj_of_ClassToDelegateTo)              # Step 2
+#     end
+#   end
+#
 # Here's a sample of use from Tempfile which is really a File object with a
 # few special rules about storage location and when the File should be
 # deleted.  That makes for an almost textbook perfect example of how to use
@@ -383,7 +391,7 @@ def Delegator.delegating_block(mid) # :nodoc:
 #     # ...
 #   end
 #
-def DelegateClass(superclass)
+def DelegateClass(superclass, &block)
   klass = Class.new(Delegator)
   methods = superclass.instance_methods
   methods -= ::Delegator.public_api
@@ -410,5 +418,6 @@ def __setobj__(obj)  # :nodoc:
   klass.define_singleton_method :protected_instance_methods do |all=true|
     super(all) | superclass.protected_instance_methods
   end
+  klass.module_eval(&block) if block
   return klass
 end

test/test_delegate.rb

@@ -22,6 +22,13 @@ def test_extend
     assert_equal(:m, obj.m, "[ruby-dev:33116]")
   end
 
+  def test_delegate_class_block
+    klass = DelegateClass(Array) do
+      alias foo first
+    end
+    assert_equal(1, klass.new([1]).foo)
+  end
+
   def test_systemcallerror_eq
     e = SystemCallError.new(0)
     assert((SimpleDelegator.new(e) == e) == (e == SimpleDelegator.new(e)), "[ruby-dev:34808]")