[ruby-core:94894] [Ruby master Misc#16157] What is the correct and *portable* way to do generic delegation?

Issue #16157 has been updated by Dan0042 (Daniel DeLorme).


> Note that the `RUBY_VERSION` check is only needed in a subset of the cases.  In cases where the target method does not accept keyword arguments, no changes are needed

Yes, I know, that's exactly what I was saying.

But my point was that "Even if it doesn't have to be *changed*, it has to be *checked*, which is possibly even more work". Although now I realize that checking is easy if the tests are comprehensive enough. And by that I mean the tests have to cover not only `foo(42)` and `foo(42, flag:true)` but also (less obviously) `wrapfoo(42)` and `wrapfoo(42, flag:true)`. But even without tests you can get the warnings from production logs. The lexical analysis was just intended to get a rough idea but I think I got a bit too caught up.

> The only time you really need the `RUBY_VERSION` check is for complete argument delegation to arbitrary methods with arbitrary arguments.

Ah yes, I made a mistake there. So in my example if `to_json(*args)` outputs a warning, it's ok to change it to `to_json(*args,**kw)` even in 2.6 since it's very unlikely you'd be delegating to two different `to_json` methods, one with kwargs and one without.

So the migration procedure looks like this I think?
```
if you get a warning
   if you are delegating to a specific method
      use (*args, **kw)
   else
      check RUBY_VERSION to delegate via (*args) or (*args, **kw)
else
   don't change anything, otherwise it will break on 2.6
```

> Most methods written in C do not care if they are called with keyword arguments or a positional hash argument and will work with either.

Wow, really? This is a bit off-topic but can you explain why C methods have no trouble with the hash/keyword ambiguity? I would have assumed it was the same as with ruby methods.

---

Well, I hope everyone has comprehensive test suites.

I hope everyone will understand that just adding `**kw` can result in bugs on 2.6.

I hope this migration will go as smoothly as you think it will.

Disclaimer: I may be a worrywart.

----------------------------------------
Misc #16157: What is the correct and *portable* way to do generic delegation?
https://bugs.ruby-lang.org/issues/16157#change-81509

* Author: Dan0042 (Daniel DeLorme)
* Status: Open
* 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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