[ruby-core:94923] [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).


Dan0042 (Daniel DeLorme) wrote:
> After some more thinking I believe the `pass_keywords` behavior would be appropriate for 100% of methods with an argument splat but no keywords.

We do not want this as the default behavior.  Consider:

```ruby
def foo(*args)
  bar(*quux)
  baz(*args)
end
```

Here in the call to `bar`, the purpose is not to pass arguments through, but to call the method `quux` and splat the arguments received from that method as positional arguments.  If `bar` accepts a positional argument splat and keyword arguments, and `quux` returns an array where the last element is a hash, then the last element will be treated as keywords to `bar`, even though that is not what we want (these are the types of bugs keyword argument separation is designed to fix).

`pass_keywords` is designed only for arbitrary delegation methods, and only where backwards compatibility with Ruby <2.7 is desired.  It should eventually be removed after Ruby <2.7 is EOL.  As the implementation is based on VM frame flags, it requires extra internal work in the VM for every method call.

----------------------------------------
Misc #16157: What is the correct and *portable* way to do generic delegation?
https://bugs.ruby-lang.org/issues/16157#change-81538

* 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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