[ruby-core:104432] [Ruby master Bug#18011] `Method#parameters` is incorrect for forwarded arguments

Issue #18011 has been updated by josh.cheek (Josh Cheek).


jeremyevans0 (Jeremy Evans) wrote in #note-1:
> Forwarded arguments is implemented as `(*args, &block)` with `ruby2_keywords`, not as `(*args, **kwargs, &block)`. If you add `def wrapper4(*r, &b) = wrapped(*r, &b),` to `methods` and then `ruby2_keywords :wrapper4` after `methods`, you can see it gives the same output as `wrapper1` and `wrapper3`.
> 
> If we want to indicate which methods have `ruby2_keywords`, one way would be to include `ruby2_keywords` into `Method#parameters` output if it is enabled for the method.  Possibly this could be done by adding `[:ruby2_keywords]` as the last value in the returned array.

Thanks for the reply, I didn't know about the `ruby2_keywords` method. I guess if the people here disagree with me, then they can close this issue.

I guess I can detect this by the argument names of `:*` and `:&`, there doesn't seem to be any other way to get that.

---

For reference to any readers, he's saying to add another example like this:

```ruby
def wrapped(ord, kw:) = [ord, {kw: kw}]

methods = [
  # ...
  def wrapper4(*r, &b)      = wrapped(*r, &b),
]

ruby2_keywords :wrapper4

methods.each do |name|
  # ...
end
```

Output:

```
# ...
wrapper4(*r, &b)
  params: [[:rest, :r], [:block, :b]]
  result: [123, {:kw=>456}]
```

---

Maybe worth noting that Ripper also parses it into the rest arg:

```
ruby -vr ripper -e '
  def args_for(sig) = %i(req opt rest req key keyrest block).zip(Ripper.sexp(sig)[1][0][2][1][1..])
  pp args_for "def m(...) end"
  pp args_for "def m(*, **) end"
'
ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [arm64-darwin20]
[[:req, nil],
 [:opt, nil],
 [:rest, [:args_forward]],
 [:req, nil],
 [:key, nil],
 [:keyrest, nil],
 [:block, nil]]
[[:req, nil],
 [:opt, nil],
 [:rest, [:rest_param, nil]],
 [:req, nil],
 [:key, nil],
 [:keyrest, [:kwrest_param, nil]],
 [:block, nil]]
```

----------------------------------------
Bug #18011: `Method#parameters` is incorrect for forwarded arguments
https://bugs.ruby-lang.org/issues/18011#change-92686

* Author: josh.cheek (Josh Cheek)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.1p64 (2021-04-05 revision 0fb782ee38) [arm64-darwin20]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
When asking a method about its parameters, forwarded arguments say they are a rest and a block (`wrapper1` in the example below).
However, when we use that signature, it raises an ArgumentError (`wrapper2` in the example below).
When I add a keyrest to the signature, it works as expected (`wrapper3` in the example below).
So I think forwarded arguments should include a keyrest in the output of `Method#parameters`

```ruby
def wrapped(ord, kw:) = [ord, {kw: kw}]

methods = [
 def wrapper1(...)         = wrapped(...),
 def wrapper2(*r, &b)      = wrapped(*r, &b),
 def wrapper3(*r, **k, &b) = wrapped(*r, **k, &b),
]

methods.each do |name|
  puts File.read(__FILE__)[/#{name}\(.*?\)/]
  puts "  params: #{method(name).parameters.inspect}"
  puts "  result: #{(method(name).call 123, kw: 456 rescue $!).inspect}"
  puts
end
```

Output:

```
wrapper1(...)
  params: [[:rest, :*], [:block, :&]]
  result: [123, {:kw=>456}]

wrapper2(*r, &b)
  params: [[:rest, :r], [:block, :b]]
  result: #<ArgumentError: wrong number of arguments (given 2, expected 1; required keyword: kw)>

wrapper3(*r, **k, &b)
  params: [[:rest, :r], [:keyrest, :k], [:block, :b]]
  result: [123, {:kw=>456}]
```



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