[ruby-core:103368] [Ruby master Feature#17785] Allow named parameters to be keywords

Issue #17785 has been updated by byroot (Jean Boussier).


> the number of keywords is very low, which means that the cases where using a keyword as an argument name makes sense is also very low.

Variable and keyword names are not purely random though, so I don't think this statistical reasoning makes that much sense. Especially since keywords reserved names that are short and popular: `end`, `class`, etc. If you define method that generate some HTML, a `class:` named parameters is common, if you define a method that deal with period of times, `end:` is common, `if:` is common for methods taking callbacks, etc.

I'm not for adding extra syntax, but I agree with @marcandree that `binding.local_variable_get(:class)` is too slow to be used in many cases.

It would be great if the parser or VM would just optimize it away, but I understand that it's currently very tricky because both `#binding` and `Binding#local_variable_get` could have been redefined. 

----------------------------------------
Feature #17785: Allow named parameters to be keywords
https://bugs.ruby-lang.org/issues/17785#change-91461

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
We should allow named parameters to be keywords and use add a trailing `_` to the corresponding variable:

```ruby
def check(arg, class:)
  arg.is_a?(class_)
end

check(42, class: Integer) # => true
```

Currently, if we want such an API we have to use `**rest`:

```ruby
def check(arg, **rest)
  class_ = rest.fetch(:class) { raise ArgumentError('missing keyword: :class')}
  if rest.size > 1
    unknown = rest.keys - [:class]
    raise ArgumentError("unknown keyword(s): :#{unknown.join(', :')})
  end

  arg.is_a?(class_)
end
```

This is very verbose, much less convenient, much less readable, prevents `steep` from generating the proper signature, etc.

We should do the same for pattern match.



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