[ruby-core:100187] [Ruby master Feature#17195] Freeze Enumerator::ArithmeticSequence objects

Issue #17195 has been updated by Eregon (Benoit Daloze).


I just realized, this is problematic because `Enumerator::ArithmeticSequence` is a subclass of `Enumerator` and not of `Range`.
If it was a subclass of Range then I think it would be fine to freeze it.

But since it subclasses Enumerator, it has mutably methods like `Enumerator#next`:
```ruby
[7] pry(main)> enum = (1..5).step(2)
=> ((1..5).step(2))
[8] pry(main)> enum.next
=> 1
[9] pry(main)> enum.next
=> 3
[10] pry(main)> enum.next
=> 5
[11] pry(main)> enum.next
StopIteration: iteration reached an end
from (pry):11:in `next'
```

@mrkn do you remember why ArithmeticSequence is an Enumerator subclass and not a Range subclass?

----------------------------------------
Feature #17195: Freeze Enumerator::ArithmeticSequence objects
https://bugs.ruby-lang.org/issues/17195#change-87758

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
----------------------------------------
Now, all Ranges are frozen (#15504).

Enumerator::ArithmeticSequence is very similar to Range, just with an extra `step`.
They're essentially already immutable, except that one could use set instance variables, but it seems of little use.

So, should we make Enumerator::ArithmeticSequence frozen too?



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