[ruby-core:95326] [Ruby master Misc#16157] What is the correct and *portable* way to do generic delegation?

[eregontp@gmail.com]

Issue #16157 has been updated by Eregon (Benoit Daloze).


@jeremyevans0 That is not future proof and will break as soon as `ruby2_keywords` is removed.

I don't think we want to keep using `ruby2_keywords` forever, i.e., does it make sense to use `ruby2_keywords` in Ruby 4?
From the name and my understanding of it, `ruby2_keywords` should only be used in 2.7 for transition, 3.0 later should explicitly pass kwargs.
I think using `ruby2_keywords` is also really not pretty, so I hope once we drop 2.x support we don't need such a workaround for delegation anymore.

So I believe this is the currently complete and future-proof way to do delegation that works on Ruby 2.x - 3.0+:

```ruby
class Proxy < BasicObject
  def initialize(target)
    @target = target
  end

  if RUBY_VERSION >= 3.0
    def method_missing(*args, **kwargs, &block)
      @target.send(*args, **kwargs, &block)
    end
  else
    def method_missing(*args, &block)
      @target.send(*args, &block)
    end
    ruby2_keywords :method_missing if respond_to?(:ruby2_keywords, true) # For 2.7
  end
end
```

We need to discuss when to remove `ruby2_keywords` before closing this issue, as it affects how we do the version check, so I reopen this.

----------------------------------------
Misc #16157: What is the correct and *portable* way to do generic delegation?
https://bugs.ruby-lang.org/issues/16157#change-82034

* Author: Dan0042 (Daniel DeLorme)
* Status: Closed
* Priority: Normal
* Assignee: 
----------------------------------------
With the keyword argument changes in 2.7 we must now specify keyword arguments explicitly when doing generic delegation. But this change is not compatible with 2.6, where it adds an empty hash to the argument list of methods that do not need/accept keyword arguments.

To illustrate the problem:

```ruby
class ProxyWithoutKW < BasicObject
  def initialize(target)
    @target = target
  end
  def method_missing(*a, &b)
    @target.send(*a, &b)
  end
end

class ProxyWithKW < BasicObject
  def initialize(target)
    @target = target
  end
  def method_missing(*a, **o, &b)
    @target.send(*a, **o, &b)
  end
end

class Test
  def args(*a)   a  end
  def arg(a)     a  end
  def opts(**o)  o  end
end
                                          # 2.6        2.7              3.0
ProxyWithoutKW.new(Test.new).args(42)     # [42]       [42]             [42]        ok
ProxyWithoutKW.new(Test.new).arg(42)      # 42         42               42          ok
ProxyWithoutKW.new(Test.new).opts(k: 42)  # {:k=>42}   {:k=>42} +warn   [{:k=>42}]  incompatible with >= 2.7
ProxyWithKW.new(Test.new).args(42)        # [42, {}]   [42]             [42]        incompatible with <= 2.6
ProxyWithKW.new(Test.new).arg(42)         # error      42               42          incompatible with <= 2.6
ProxyWithKW.new(Test.new).opts(k: 42)     # {:k=>42}   {:k=>42} +warn   {:k=>42}    must ignore warning? cannot use pass_positional_hash in 2.6
```

I don't know how to solve this, so I'm asking for the **official** correct way to write portable delegation code. And by **portable** I mean code that can be used in gems that target ruby 2.6 and above.




-- 
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