[ruby-core:98735] [Ruby master Feature#6869] Do not treat `_` parameter exceptionally

Issue #6869 has been updated by docx (Lukas Dolezal).


Re 

> In these descriptions, lowercase letter means the characters ''a'' though ''z'', as well as ''_'', the underscore.

I think what they mean here is that you can use it inside of a variable. Probably just did not realized the single underscore case :) But that is my interpretation.

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Feature #6869: Do not treat `_` parameter exceptionally
https://bugs.ruby-lang.org/issues/6869#change-86090

* Author: alexeymuranov (Alexey Muranov)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 3.0
----------------------------------------
I started by commenting on #6693, but i have realized that  this is a slightly different request.

I propose to not treat the variable name "`_`" exceptionally. Current behavior:

~~~ruby
 {0=>1}.each_with_index { |_,_| p _ } # [0, 1]
~~~

prints "[0, 1]", but

~~~ruby
 {1=>2}.each_with_index { |x,x| p x } # SyntaxError: (eval):2: duplicated argument name
~~~

raises  "SyntaxError: (eval):2: duplicated argument name".

Similarly for methods:

~~~ruby
 def f(_, _)
   _
 end
 f(0, 1) # => 0

 def f(x, x)
   x
 end # => SyntaxError: (eval):2: duplicated argument name
~~~

Observe also that the use of repeated `_` parameter is not consistent between methods and blocks: for methods the value is the first assigned value, and for blocks it is the array of all the assigned values.

1. I propose to use the same rule for all variables, without distinguishing `_` specially.

In particular i propose to allow to repeat any variable, not only `_`, in block or method arguments without raising an error.

There may be several solutions what the repeated argument will hold: it may hold the array of all assigned values, the first assigned value, the last assigned value, the first non-nil assigned value, or the last non-nil assigned value.

2. I propose to treat repeated arguments in methods and in blocks the same way (do not know which one).

3. For unused variables i propose to introduce a special placeholder, for example "`-`" not followed by anything other than a delimiter (comma or bracket):

~~~ruby
 each_with_index { |-, value| puts value }

 -, -, suffix = parse(name)
~~~




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