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


Side note: in my opinion, `Enumerable` should not check the receiver's class to provide specific optimizations. Instead, `Range` should override `sum` and handle the optimization itself. `Range#sum` would also be a better place to document this behavior. (it still doesn't solve the subclassing issue, though)

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Bug #13700: Enumerable#sum may not work for Ranges subclasses due to optimization
https://bugs.ruby-lang.org/issues/13700#change-65585

* 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. (or more specifically, not for subclasses overriding `each`)

If this behavior is intentional, it should at least be documented.



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