Bug #13700: Enumerable#sum may not work for Ranges subclasses due to optimization

• Status: Closed
• 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

[sos4nt (Stefan Schüßler)]

Enumerable#sum is optimized for integer ranges. Unfortunately, this can break subclasses:

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.

[sos4nt (Stefan Schüßler)]

• Description updated (diff)

[sos4nt (Stefan Schüßler)]

• Description updated (diff)

[shevegen (Robert A. Heiler)]

Reminds me a bit of what hanmac wrote elsewhere; I can't find it right now and forgot it mostly already but I think he also mentioned some unexpected behaviour when ... subclassing I think? Or some custom class that he wrote...

[Hanmac (Hans Mackowiak)]

shevegen (Robert A. Heiler) wrote:

Reminds me a bit of what hanmac wrote elsewhere; I can't find it right now and forgot it mostly already but I think he also mentioned some unexpected behaviour when ... subclassing I think? Or some custom class that he wrote...

my comment was for https://bugs.ruby-lang.org/issues/13663
with String#upto you can't overwrite the internal String#<=>

[sos4nt (Stefan Schüßler)]

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)

• Status changed from Open to Closed

Applied in changeset git|e496e96547b64c3a2fa6f285c3bc9bd21a245ac6.

---

Document that Enumerable#sum may not respect redefinition of Range#each

It already documented that it may not respect redefinition
of Integer#+.

Fixes [Bug #13700]