[ruby-core:108127] [Ruby master Bug#18247] weird results for `Array#slice` or `Array#[]` with argument of type `Enumerator::ArithmeticSequence`

Issue #18247 has been updated by jeremyevans0 (Jeremy Evans).


This is still an issue in the master branch.  I've submitted a pull request to fix it: https://github.com/ruby/ruby/pull/5739

----------------------------------------
Bug #18247: weird results for `Array#slice` or `Array#[]` with argument of type `Enumerator::ArithmeticSequence`
https://bugs.ruby-lang.org/issues/18247#change-97101

* Author: lxxxvi (Mario Sch端ttel)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
There are weird results for `Array#slice` or `Array#[]` with argument of type `Enumerator::ArithmeticSequence`.

Particularly most results with negative steps are weird. For example:

```ruby
@array = [0, 1, 2, 3, 4, 5]

@array.slice((2..).step(-1))          # =>  [2, 1, 0]
@array.slice((2..).step(-2))          # =>  [2, 0]
@array.slice((2..).step(-3))          # =>  [2]
@array.slice((2..).step(-4))          # =>  [0]           # ???  I'd have expected [2]

@array.slice((-3..).step(-1))         # =>  [3, 2, 1, 0]
@array.slice((-3..).step(-2))         # =>  [3, 1]
@array.slice((-3..).step(-3))         # =>  [3, 0]
@array.slice((-3..).step(-4))         # =>  [3]
@array.slice((-3..).step(-5))         # =>  [0]           # ???  I'd have expected [3]
```

Have a look at these examples with "beginless" ranges and negative steps.
Particularly the ones exluding the end value (`...`) seem wrong:


```ruby
@array = [0, 1, 2, 3, 4, 5]

## end with zero index
@array.slice((..0).step(-1))        # =>  [5, 4, 3, 2, 1, 0]
@array.slice((...0).step(-1))       # =>  [5, 4, 3, 2, 1, 0]

@array.slice((..0).step(-2))        # =>  [5, 3, 1]
@array.slice((...0).step(-2))       # =>  [5, 3, 1]

@array.slice((..0).step(-10))       # =>  [0]
@array.slice((...0).step(-10))      # =>  [0]

## end with positive index
@array.slice((..3).step(-1))        # =>  [5, 4, 3]
@array.slice((...3).step(-1))       # =>  [5, 4, 3]

@array.slice((..3).step(-2))        # =>  [5, 3]
@array.slice((...3).step(-2))       # =>  [5, 3]

@array.slice((..3).step(-10))       # =>  [3]
@array.slice((...3).step(-10))      # =>  [3]

## end with negative index
@array.slice((..-2).step(-1))       # =>  [5, 4]
@array.slice((...-2).step(-1))      # =>  [5, 4]

@array.slice((..-2).step(-2))       # =>  [5]
@array.slice((...-2).step(-2))      # =>  [5]

@array.slice((..-2).step(-10))      # =>  [4]
@array.slice((...-2).step(-10))     # =>  [4]
```

Finally, some examples where the range is "inverted".
Particularly, the results for the ones excluding the end value (`...`) look strange.
But all examples seem weird, but I might not understand the concept of arithmetic sequence in depth.

```ruby
@array = [0, 1, 2, 3, 4, 5]

# start and end with positive index
@array.slice((3..1).step(-1))       # =>  [3, 2, 1]
@array.slice((3...1).step(-1))      # =>  [2, 1]

@array.slice((3..1).step(-2))       # =>  [3, 1]
@array.slice((3...1).step(-2))      # =>  [2]

@array.slice((3..1).step(-10))      # =>  [1]
@array.slice((3...1).step(-10))     # =>  [1]

# start with negative index, end with positive index
@array.slice((-2..1).step(-1))      # =>  [4, 3, 2, 1]
@array.slice((-2...1).step(-1))     # =>  [3, 2, 1]

@array.slice((-2..1).step(-2))      # =>  [4, 2]
@array.slice((-2...1).step(-2))     # =>  [3, 1]

@array.slice((-2..1).step(-10))     # =>  [1]
@array.slice((-2...1).step(-10))    # =>  [1]

# start with positive index, end with negative index
@array.slice((4..-4).step(-1))      # =>  [4, 3, 2]
@array.slice((4...-4).step(-1))     # =>  [3, 2]

@array.slice((4..-4).step(-2))      # =>  [4, 2]
@array.slice((4...-4).step(-2))     # =>  [3]

@array.slice((4..-4).step(-10))     # =>  [2]
@array.slice((4...-4).step(-10))    # =>  [2]

# start with negative index, end with negative index
@array.slice((-2..-4).step(-1))     # =>  [4, 3, 2]
@array.slice((-2...-4).step(-1))    # =>  [3, 2]

@array.slice((-2..-4).step(-2))     # =>  [4, 2]
@array.slice((-2...-4).step(-2))    # =>  [3]

@array.slice((-2..-4).step(-10))    # =>  [2]
@array.slice((-2...-4).step(-10))   # =>  [2]
```

Found while writing specs for this method in https://github.com/ruby/spec/pull/857



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