Issue #17279 has been reported by mrkn (Kenta Murata).

----------------------------------------
Feature #17279: Allow a negative step in Range#step with a block
https://bugs.ruby-lang.org/issues/17279

* Author: mrkn (Kenta Murata)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
`Range#step` prohibits a negative step when a block is given.

```
>> (6..3).step(-1) {|i| p i }
Traceback (most recent call last):
        5: from /home/mrkn/.rbenv/versions/2.7/bin/irb:23:in `<main>'
        4: from /home/mrkn/.rbenv/versions/2.7/bin/irb:23:in `load'
        3: from /home/mrkn/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/irb-1.2.4/exe/irb:11:in `<top (required)>'
        2: from (irb):1
        1: from (irb):1:in `step'
ArgumentError (step can't be negative)
```

But `Range#step` allows a negative step when it is called without a block.  In this case, `Range#step` creates an ArithmeticSequence, and `ArithmeticSequence#each` can iterate with a negative step.

```
>> (6..3).step(-1).each {|i| p i }
6
5
4
3
=> ((6..3).step(-1))
```

I think the prohibition of a negative step in `Range#step` has already been meaningless, so it may be better to permit it for consistency.



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