[ruby-core:93308] [Ruby trunk Feature#15950] Allow negative length in `Array#[]`, `Array#[]=`, `String#[]`, `String[]=`

Issue #15950 has been reported by sawa (Tsuyoshi Sawada).

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Feature #15950: Allow negative length in `Array#[]`, `Array#[]=`, `String#[]`, `String[]=`
https://bugs.ruby-lang.org/issues/15950

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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To take the first n characters of a string, using `[]` is straightforward:

```ruby
"abcdefgh"[0, 3] # => "abc"
```

But to take the last n characters, we need to use n in two arguments: in the index (in negative form) in addition to the length:

```ruby
"abcdefgh"[-3, 3] # => "fgh"
```

This is cumbersome.

I wish negative length to be allowed, and be interpreted as measuring leftward (while cycling the receiver if necessary).

```ruby
"abcdefgh"[0, -3] # => "fgh"
"abcdefgh"[5, -3] # => "cde"
```




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