Forwardable: Fix delegating to 'args' and 'block'

* lib/forwardable.rb (def_instance_delegator) fix delegating to
  'args' and 'block', clashing with local variables in generated
  methods.  [ruby-core:72579] [Bug #11916]

* lib/forwardable.rb (def_single_delegator): ditto.

If you have a class that uses Forwardable to delegate a method to
another object, and the method that returns the delegate object is
called `args` or `block`, then Forwardable will fail to work.

Here's a simple example:

    class ModelCreator
      extend Forwardable

      attr_reader :args

      def_delegator :args, :model_name

      def initialize(args)
        @args = args
      end
    end

    ModelCreator.new.model_name

If you run the last line above, then you'll get:

    NoMethodError: undefined method `model_name' for []:Array

This error occurs because `def_delegator` -- as it is written in Ruby --
uses metaprogramming to add methods to the class that will then delegate
to the delegate object. So it's as if we had written:

    class ModelCreator
      extend Forwardable

      attr_reader :args

      def model_name(*args, &block)
        args.model_name(*args, &block)
      end

      def initialize(args)
        @args = args
      end
    end

As you can see, `def_delegator` will not only forward the method call
onto the delegate object, it will also forward any arguments provided as
well. It is here that the bug arises: it splats all of the arguments
into a variable which is called `args`, and because of how variable
scope works in Ruby, it then attempts to call `model_name` on *this*
variable and *not* our delegate object method.

The fix is to call the delegate object method manually using `__send__`.
(This assumes, of course, that the given receiver is, in fact, the name
of a method and not the name of an instance variable, which is also a
possibility.) We use `__send__` because the delegate object method could
be private.

So, that looks like this:

    def model_name(*args, &block)
      __send__(:args).model_name(*args, &block)
    end

Because `def_delegators` and `delegate` use `def_delegator` internally,
they also get this fix as well.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53381 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 8 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 2e2202f6e6..b535efeb4f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+Wed Dec 30 11:18:42 2015  Elliot Winkler  <elliot.winkler@gmail.com>
+
+	* lib/forwardable.rb (def_instance_delegator) fix delegating to
+	  'args' and 'block', clashing with local variables in generated
+	  methods.  [ruby-core:72579] [Bug #11916]
+
+	* lib/forwardable.rb (def_single_delegator): ditto.
+
 Wed Dec 30 09:58:56 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* object.c (rb_class_inherited_p): search the corresponding