[ruby-core:94865] [Ruby master Misc#16157] What is the correct and *portable* way to do generic delegation?

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


I've compiled your delegate-keyword-argument-separation branch to test this. BTW in your example it should be `respond_to?(:pass_positional_hash, true)`

The portability result is... Almost there, but not quite. In 2.6 and 2.7, `o.empty?` cannot distinguish between `foo()` and `foo({})`. This results in the discrepancies found below via this test script.

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

class ProxyNew < BasicObject
  def initialize(target)
    @target = target
  end
  def method_missing(*a, **o, &b)
    if o.empty?
      @target.send(*a, &b)
    else
      @target.send(*a, **o, &b)
    end rescue $!
  end
  pass_positional_hash(:method_missing) if respond_to?(:pass_positional_hash, true)
end

class Test
  def args(*a)   a  end
  def arg(a)     a  end
  def opts(**o)  o  end
  def arg0o(a=nil, **o) [a,o] end
  def arg1o(a, **o)     [a,o] end
end

e = Object.new
def e.write(*args) $stdout.print("warn! ") end
$stderr = e

opt = {}
hash = {k:42}
proxy = (ARGV.first=="old" ? ProxyOld : ProxyNew).new(Test.new)
p proxy.args(42)          
p proxy.arg(42)           
p proxy.arg({k:42})       
p proxy.arg({})           
p proxy.opts(k: 42)       
p proxy.arg0o(hash)       
p proxy.arg0o(hash, **{}) 
p proxy.arg0o(hash, **opt)
```

I've highlighted the two cases where we have the correct behavior in 3.0 and 2.6 ProxyOld, but not the two others.

| testcase                  | 2.6 ProxyOld    | 2.6 ProxyNew        | 2.7 ProxyNew                    | 3.0 ProxyNew   |
|---------------------------|-----------------|---------------------|---------------------------------|----------------|
| proxy.args(42)            | [42]            | [42]                | [42]                            | [42]           |
| proxy.arg(42)             | 42              | 42                  | 42                              | 42             |
| proxy.arg({k:42})         | {:k=>42}        | {:k=>42}            | {:k=>42}                        | {:k=>42}       |
| proxy.arg({})             | {}              | **error**           | **error**                       | {}             |
| proxy.opts(k: 42)         | {:k=>42}        | {:k=>42}            | {:k=>42}                        | {:k=>42}       |
| proxy.arg0o(hash)         | [nil, {:k=>42}] | [nil, {:k=>42}]     | warn! warn! [nil, {:k=>42}]     | [{:k=>42}, {}] |
| proxy.arg0o(hash, **{})   | [nil, {:k=>42}] | [nil, {:k=>42}]     | warn! warn! [nil, {:k=>42}]     | [{:k=>42}, {}] |
| proxy.arg0o(hash, **opt)  | [{:k=>42}, {}]  | **[nil, {:k=>42}]** | **warn! warn! [nil, {:k=>42}]** | [{:k=>42}, {}] |


----------------------------------------
Misc #16157: What is the correct and *portable* way to do generic delegation?
https://bugs.ruby-lang.org/issues/16157#change-81483

* 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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