From: daniel@...42.com
Date: 2019-09-11T15:36:19+00:00
Subject: [ruby-core:94904] [Ruby master Misc#16157] What is the correct and *portable* way to do generic delegation?

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


> Prior to the keyword argument separation branch merge, it was not even possible for a C method to tell if it was called with keywords or called with a positional hash. 

That's the same situation as ruby methods used to be in. So *theoretically* C methods are susceptible to the same problem. But very very few C methods have a signature that would allow to trigger the bug. However, feast your eyes on the behavior of `warn()` (on ruby master)

```
>> warn({x:1})
ArgumentError (unknown keyword: :x)

>> warn({x:1}, uplevel:0)
(irb):4: warning: {:x=>1}

>> warn({x:1}, **{})

>> warn({x:1}, {})

>> warn({x:1}, {}, {})
{:x=>1}

>> warn({x:1}, {}, {}, {})
{:x=>1}
{}

>> warn({x:1}, {y:2}, {})
ArgumentError (unknown keyword: :y)

>> warn({x:1}, {y:2}, {}, {})
{:x=>1}
{:y=>2}
```

Of course this is all *extremely* unconventional usage and doesn't really deserve a fix. But I thought it was weird/interesting.

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

For reference, searching for only arbitrary delegation (`send`/`__send__` with `*args`) gives 179 matches in 158 files of 86 gems (not all of which will require kwargs) https://pastebin.com/HX3w5ngK
 
> The only behavior difference is for **{}, which was ignored by the parser in 2.6, and is not likely to appear in production code. 

Confirmed, not a single appearance in the 679 gems I installed.

> However, the changes are necessary to fix the problems with keyword arguments.

I still disagree with that, but that's another argument, another story ;-)




----------------------------------------
Misc #16157: What is the correct and *portable* way to do generic delegation?
https://bugs.ruby-lang.org/issues/16157#change-81518

* 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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