[ruby-core:73685] [Ruby trunk Bug#11916] Fix delegating to 'args' and 'block'

Issue #11916 has been updated by Usaku NAKAMURA.

Backport changed from 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN to 2.0.0: WONTFIX, 2.1: REQUIRED, 2.2: REQUIRED, 2.3: REQUIRED

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Bug #11916: Fix delegating to 'args' and 'block'
https://bugs.ruby-lang.org/issues/11916#change-56881

* Author: Elliot Winkler
* Status: Closed
* Priority: Normal
* Assignee: 
* ruby -v: 2.4.0dev
* Backport: 2.0.0: WONTFIX, 2.1: REQUIRED, 2.2: REQUIRED, 2.3: REQUIRED
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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.

---Files--------------------------------
fix-def-delegator.patch (6.99 KB)


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