[ruby-core:98865] [Ruby master Feature#16456] Ruby 2.7 argument delegation (...) should be its own kind of parameter in Method#parameters

Issue #16456 has been updated by mame (Yusuke Endoh).


Adding a new type of parameters will break existing code.  In fact, if we add a new type `:delegate`, the Sorbet code written in #note-8 will `raise "Unknown parameter type: #{type}"`.  So, we must be careful.

Currently, `(...)` parameter generates a unique indicator `[:rest, :*]`.  Note that it uses an invalid variable name `:*`, so it is not produced by other parameters than `(...)`.  Thus, without ambiguity, it is already able to determine if `(...)` parameter is used or not by checking if the result of `Method#parameters` includes `[:rest, :*]`.  So, currently, I don't see a strong reason to add a new type.

That being said, feeling is important for Ruby.  Changing `:*` to `:"..."` may be acceptable, though it brings incompatibility.  I have no idea if it is worth or not.

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Feature #16456: Ruby 2.7 argument delegation (...) should be its own kind of parameter in Method#parameters
https://bugs.ruby-lang.org/issues/16456#change-86230

* Author: aaronc81 (Aaron Christiansen)
* Status: Open
* Priority: Normal
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A method defined with `...` as its parameter list is equivalent to one defined with `*args, &blk`, according to `Method#parameters`.

```ruby
def foo(...); end
p method(:foo).parameters
# => [[:rest, :*], [:block, :&]]
```

Even in Ruby 2.7, `...` and `*args, &blk` are not *quite* equivalent as the latter may produce a warning where the former does not. In Ruby 3.0 and beyond, `...` and `*args, &blk` will have a substantial semantic difference. Due to this, I don't consider the current behaviour of `Method#parameters` particularly ideal when dealing with methods using this new syntax.

If the goal of `...` is to be a "delegate everything" operator, even when parameter passing is changed like in Ruby 3.0, I would propose that `Method#parameters` considers it a unique type of parameter. For example:

```ruby
def foo(...); end
p method(:foo).parameters
# => [[:delegate, :"..."]]
```



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