Fix Enumerable#sum redefined warning

[mtsmfm]

The purpose of this PR is to remove redefining warning, not speed-up Enumerable#sum.

@amatsuda asked me to fix this warning.

We (I and @amatsuda) know #25202 already addressed this warning but there is no action for a while.

We think Rails 5.1 should be released with no warning.

---

If we require 'active_support/core_ext/enumerable' on Ruby 2.4,
we'll see following warn because Enumerable#sum and Array#sum
are added in Ruby 2.4.

rails/rails/activesupport/lib/active_support/core_ext/enumerable.rb:20: warning: method redefined; discarding old sum

The minimal way to fix the warning is alias sum sum.

$ ruby -v
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]

$ ruby -w -e "def a; end; def a; end"
-e:1: warning: method redefined; discarding old a
-e:1: warning: previous definition of a was here

$ ruby -w -e "def a; end; alias a a; def a; end"

But this behavior is not intended. (@amatsuda was told by @ko1)
So we should use alias as a meaningful way.

Ruby 2.4's sum (orig_sum) assumes an identity is 0 when we omit identity
so we can delegate to orig_sum with explicit identity only.
In a strict sense, we can detect identity by check instance's class
but we don't care at this time about that because calling Enumerable#sum is rare.
In many cases, we will call Array#sum.

[rails-bot]

r? @schneems

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[pixeltrix]

Do we even need to define it for Ruby 2.4? I assume there must be, but I can't spot any difference between our implementation and Ruby 2.4's implementation in terms of outcomes for a given set of arguments.

[mtsmfm]

I can't spot any difference between our implementation and Ruby 2.4's implementation in terms of outcomes for a given set of arguments.

Sorry, I noticed a mistake about feature detection 🙇‍♂️
I'll fix that 💨

---

The difference is following case:

$ ruby -e '(?a..?b).sum'
begin
  (?a..?b).sum
rescue => e
  p e # => #<TypeError: String can't be coerced into Integer>
end

require 'active_support/core_ext/enumerable'

p (?a..?b).sum # => "ab"

[mtsmfm]

If we run only the test, it will pass.

$ bundle exec ruby -I test test/core_ext/enumerable_test.rb
Run options: --seed 41825

# Running:

...........

Finished in 0.001698s, 6477.8854 runs/s, 70667.8405 assertions/s.

11 runs, 120 assertions, 0 failures, 0 errors, 0 skips

If we apply following patch, the test fill fail 😨

diff --git a/activesupport/test/core_ext/enumerable_test.rb b/activesupport/test/core_ext/enumerable_test.rb
index 4f1ab993b8..2a786fc874 100644
--- a/activesupport/test/core_ext/enumerable_test.rb
+++ b/activesupport/test/core_ext/enumerable_test.rb
@@ -1,6 +1,7 @@
 require "abstract_unit"
 require "active_support/core_ext/array"
 require "active_support/core_ext/enumerable"
+require "active_support/core_ext/object/json"

 Payment = Struct.new(:price)
 ExpandedPayment = Struct.new(:dollars, :cents)

  1) Error:
EnumerableTests#test_sums:
NoMethodError: undefined method `orig_sum' for #<EnumerableTests::GenericEnumerable:0x00000002898f58>
    /app/activesupport/lib/active_support/core_ext/enumerable.rb:35:in `sum'
    test/core_ext/enumerable_test.rb:51:in `test_sums'


  2) Error:
EnumerableTests#test_empty_sums:
NoMethodError: undefined method `orig_sum' for #<EnumerableTests::GenericEnumerable:0x00000002893238 @values=[]>
    /app/activesupport/lib/active_support/core_ext/enumerable.rb:35:in `sum'
    test/core_ext/enumerable_test.rb:97:in `test_empty_sums'


  3) Error:
EnumerableTests#test_range_sums:
NoMethodError: undefined method `orig_sum' for 1..4:Range
    /app/activesupport/lib/active_support/core_ext/enumerable.rb:35:in `sum'
    /app/activesupport/lib/active_support/core_ext/enumerable.rb:127:in `sum'
    test/core_ext/enumerable_test.rb:112:in `test_range_sums'

11 runs, 83 assertions, 0 failures, 3 errors, 0 skips

The cause is using refinements with prepend.

using Module.new {
  refine Enumerable do
    alias :orig_sum :sum
  end
}

module Enumerable
  def sum(*args)
    orig_sum(*args)
  end
end

class GenericEnumerable
  include Enumerable

  def each
  end
end

# GenericEnumerable.new.sum # if we uncomment this line, `GenericEnumerable#sum` will work
Enumerable.prepend(Module.new) # if we comment out this line, `GenericEnumerable#sum` will work

p GenericEnumerable.new.sum # undefined method `orig_sum' for #<GenericEnumerable:0x0000000127c120 @values=[1, 2, 3]> (NoMethodError)

It may be not intended behavior (I created the tickect https://bugs.ruby-lang.org/issues/13446).

Anyway, we can't use refinements here 😢

[matthewd]

Let's consider a longer name, then: if we have to pollute Enumerable, we want something that couldn't possibly conflict with a user's own monkeypatching strategy, for example.

original_sum_with_required_identity? Or maybe that's too long.

Either way, we should also make it private.

[mtsmfm]

We call internal method for Hash _deep_transform_keys_in_object.

rails/activesupport/lib/active_support/core_ext/hash/keys.rb

Line 143 in 8776a71

def _deep_transform_keys_in_object(object, &block)

So let's prepend _.

[kamipo]

Array#sum is also using refinements (571f0f3). If anyone use prepend to Array, is it affected by https://bugs.ruby-lang.org/issues/13446?

[mtsmfm]

(Strangely but) no it's not 🙃

using Module.new {
  refine Array do
    alias :orig_sum :sum
  end
}

class Array
  def sum(*args)
    orig_sum(*args)
  end
end

Array.prepend(Module.new)

p Array.new.sum #=> 0

[kamipo]

weird... thanks anyway.

[pixeltrix]

@kamipo I think this is because Array is a class but Enumerable is a module

[kamipo]

Ah, that's a good point!

[mtsmfm]

Test is failed but it seems not related to this PR.

[rafaelfranca]

Backported in a7f6200 and 52ff5de