[ruby-core:95317] [Ruby master Misc#16157] What is the correct and *portable* way to do generic delegation?

[merch-redmine@jeremyevans.net]

Issue #16157 has been updated by jeremyevans0 (Jeremy Evans).

Status changed from Open to Closed

`pass_keywords` was renamed to `ruby2_keywords`. The correct and portable way to handle generic delegation is now:

```ruby
class Proxy < BasicObject
  def initialize(target)
    @target = target
  end
  def method_missing(*a, &b)
    @target.send(*a, &b)
  end
  ruby2_keywords :method_missing if respond_to?(:ruby2_keywords, true)
end
```

Basically, keep your existing method definition, and after it add `ruby2_keywords :method_name if respond_to?(:ruby2_keywords, true)`.

That will work in all ruby versions until `ruby2_keywords` is removed. That is currently planned for after EOL of Ruby 2.6, but potentially it could stay longer.

I fixed `ruby2_keywords` so it works with `define_method`.  Note that generic delegation through procs is not currently implemented, but it is not difficult to add `Proc#ruby2_keywords` if it is needed.  Please request that as a feature if you want it.

----------------------------------------
Misc #16157: What is the correct and *portable* way to do generic delegation?
https://bugs.ruby-lang.org/issues/16157#change-82023

* 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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