Issue #13700 has been reported by sos4nt (Stefan Schler).

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Bug #13700: Enumerable#sum may not work for Ranges subclasses due to optimization
https://bugs.ruby-lang.org/issues/13700

* Author: sos4nt (Stefan Schler)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-darwin15]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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`Enumerable#sum` is optimized for integer ranges. Unfortunately, this can break subclasses:

```ruby
class StepTwoRange < Range
  def each(&block)
    step(2, &block)
  end
end

r = StepTwoRange.new(0, 10)
r.to_a     #=> [0, 2, 4, 6, 8, 10]

r.to_a.sum #=> 30
r.sum      #=> 55
```

The optimization should therefore only be triggered for instances of `Range` and not for instances of subclasses.



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