Issue #16154 has been updated by jeremyevans0 (Jeremy Evans).


I worked on an alternative approach of using `ruby2_keywords` with a hash flag approach instead of a VM frame flag approach: https://github.com/ruby/ruby/pull/2477 .  This approach is significantly less invasive to implement, and I think handles more common argument delegation cases than the VM frame flag approach (both approaches handle the simple case).  I think we should use the hash flag approach.

There have been discussions about whether to make the `ruby2_keywords` behavior the default behavior.  I do not think it is a good idea.  Consider this example:

```ruby
  def foo(*args)
    args.each do |arg|
      bar(arg, **{})
    end
  end
  def bar(*args, **kw)
    baz(*args) unless kw[:skip]
  end
  def baz(a=1, **kw)
    [h, kw]
  end
```

Here `foo` is designed to take only positional arguments, not to pass through keywords.  Let's say you call `foo(a: 1)`.  Because the last argument to foo is flagged as keywords, when `baz` is called, the `{a: 1}` hash intended to be a positional argument is treated as a keyword argument.  Note that `baz` could be at a completely different place in the program, far from the definition of `foo`, which will make cases like this difficult to debug.

If you would like to experiment with the approach of passing keywords through argument splats by default, I have a branch that implements it: https://github.com/jeremyevans/ruby/tree/ruby2_keywords-by-default .  The only `make check` failures are in the keyword arguments tests, due to empty hash splats no longer being dropped in the case where a method accepts an argument splat but no keywords.

----------------------------------------
Bug #16154: lib/delegate.rb issues keyword argument warnings
https://bugs.ruby-lang.org/issues/16154#change-81650

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.5: DONTNEED, 2.6: DONTNEED
----------------------------------------
In the current master branch, `lib/delegate.rb` (e.g. `Delegator`/`SimpleDelegator`/`DelegateClass`) does not delegate keyword arguments, leading to keyword argument warnings when using it.  It is simple to fix this by changing it to delegate keyword arguments, and I have a commit that does this (https://github.com/ruby/ruby/pull/2439/commits/30a70491b11d5235877a13d1a1d3fe0157960a64).

However, one issue with this approach, that you can notice at the bottom of the commit, is that it warns in a case where the behavior will not change between 2.6 and 3.0:

```ruby
foo = Object.new
def foo.bar(*args)
  args
end
d = SimpleDelegator.new(foo)
d.bar({a: 1})
# warns in 2.7, even though 2.6, 2.7, and 3.0 will return [{a: 1}]
```

The reason it warns even though the behavior will not change is that there is a behavior change, but the behavior change is inside the delegator.  When you call `d.bar`, the positional hash is converted to a keyword argument, which generates a warning.  When `d.bar` calls `foo.bar`, the keyword splat is converted back to a positional hash.  Because of this hash->keyword->hash conversion, the net result is behavior does not change outside the delegator, and therefore it would be great if we can somehow flag this particular case as not warning, while still warning in the case where `foo.bar` would accept keyword arguments (as in that case, behavior will change).

I came up with an implementation that makes delegate not emit the warning in the case where behavior does not change, but still emit the warning in the case where behavior will change (https://github.com/ruby/ruby/pull/2439/commits/6b7bd4f89143d692932dc89fadc341980816ee14).  This works by flagging a method that accepts keyword arguments.  If the flag is set for the method, and the method is passed a positional hash that is converted to a keyword argument, no warning is emitted, and instead a VM frame flag is set in the next frame so that keyword splats inside that method are converted to positional arguments.  The method flag is enabled by calling a `Module#pass_positional_hash` method, and this method would only exist in 2.7, since it is only for skipping this 2.7-specific warning.

I'm not sure this implementation approach is best, and would be open to any approach that allows delegate to not emit a warning for the code above, as long as it doesn't change the default keyword behavior in 2.7.

Attached is a patch that implements this, and there is also a pull request that has passed CI for it (https://github.com/ruby/ruby/pull/2439).

---Files--------------------------------
delegate-keyword-argument-separation.patch (19.1 KB)


-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>