[ruby-core:90690] [Ruby trunk Bug#15457] Weird behaviour when having multiple arguments of same name and splat operator

Issue #15457 has been reported by graywolf (Gray Wolf).

----------------------------------------
Bug #15457: Weird behaviour when having multiple arguments of same name and splat operator
https://bugs.ruby-lang.org/issues/15457

* Author: graywolf (Gray Wolf)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.5.3
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
When multiple arguments of the same name are provided and last one of
them has splat operator, everything gets weird.

```
+   $ cat bug.rb
def test(_a, b, *_a)
  pp _a
  pp b
end
test(*%W{a b c d e})

+   $ RUBYOPT=-rpry rvm 2.5.3 do ruby bug.rb
["c", "d", "e"]
nil

+   $ RUBYOPT=-rpry rvm 2.4.5 do ruby bug.rb
["c", "d", "e"]
nil

+   $ RUBYOPT=-rpry rvm 2.3.8 do ruby bug.rb
["c", "d", "e"]
nil
```

At least it's consistent across supported ruby versions, but I have no
idea what is going on.

I get why `_a` is `%w{c d e}`, that makes sense. But why is `b` `nil`?
That makes no sense at all.



It looks to me like an edge case when having multiple arguments of the same
name and one of them (last one?) having splat operator.

-- 
There are only two hard things in Computer Science:
cache invalidation, naming things and off-by-one errors.




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