Issue #6869 has been updated by drbrain (Eric Hodel).

Category set to core
Assignee set to matz (Yukihiro Matsumoto)

Seems to be part of variable shadowing checks.  The check was added before r8857 (which was a refactor of the feature) and checking for '_' was removed in r14186.

Since it was committed by matz I think your chances at acceptance are low.
----------------------------------------
Feature #6869: Do not treat `_` parameter exceptionally
https://bugs.ruby-lang.org/issues/6869#change-28876

Author: alexeymuranov (Alexey Muranov)
Status: Open
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: 3.0


=begin
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:

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

prints "[0, 1]", but

 {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:

 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):

 each_with_index { |-, value| puts value }

 -, -, suffix = parse(name)

=end



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