Fix Enumerable#sum redefined warning

If we require 'active_support/core_ext/enumerable' on Ruby 2.4,
we'll see following warning 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`.

1 files changed, 45 insertions, 23 deletions

diff --git a/activesupport/lib/active_support/core_ext/enumerable.rb b/activesupport/lib/active_support/core_ext/enumerable.rb
index 90d7d2947f..4f120d4b45 100644
--- a/activesupport/lib/active_support/core_ext/enumerable.rb
+++ b/activesupport/lib/active_support/core_ext/enumerable.rb
@@ -1,28 +1,50 @@
 module Enumerable
-  # Calculates a sum from the elements.
+  # Enumerable#sum was added in Ruby 2.4 but it only works with Numeric elements
+  # when we omit an identity.
   #
-  #  payments.sum { |p| p.price * p.tax_rate }
-  #  payments.sum(&:price)
-  #
-  # The latter is a shortcut for:
-  #
-  #  payments.inject(0) { |sum, p| sum + p.price }
-  #
-  # It can also calculate the sum without the use of a block.
-  #
-  #  [5, 15, 10].sum # => 30
-  #  ['foo', 'bar'].sum # => "foobar"
-  #  [[1, 2], [3, 1, 5]].sum # => [1, 2, 3, 1, 5]
-  #
-  # The default sum of an empty list is zero. You can override this default:
-  #
-  #  [].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)
-  def sum(identity = nil, &block)
-    if block_given?
-      map(&block).sum(identity)
-    else
-      sum = identity ? inject(identity, :+) : inject(:+)
-      sum || identity || 0
+  # We tried shimming it to attempt the fast native method, rescue TypeError,
+  # and fall back to the compatible implementation, but that's much slower than
+  # just calling the compat method in the first place.
+  if Enumerable.instance_methods(false).include?(:sum) && !((?a..?b).sum rescue false)
+    # We can't use Refinements here because Refinements with Module which will be prepended
+    # doesn't work well https://bugs.ruby-lang.org/issues/13446
+    alias :_original_sum_with_required_identity :sum
+    private :_original_sum_with_required_identity
+    # Calculates a sum from the elements.
+    #
+    #  payments.sum { |p| p.price * p.tax_rate }
+    #  payments.sum(&:price)
+    #
+    # The latter is a shortcut for:
+    #
+    #  payments.inject(0) { |sum, p| sum + p.price }
+    #
+    # It can also calculate the sum without the use of a block.
+    #
+    #  [5, 15, 10].sum # => 30
+    #  ['foo', 'bar'].sum # => "foobar"
+    #  [[1, 2], [3, 1, 5]].sum # => [1, 2, 3, 1, 5]
+    #
+    # The default sum of an empty list is zero. You can override this default:
+    #
+    #  [].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)
+    def sum(identity = nil, &block)
+      if identity
+        _original_sum_with_required_identity(identity, &block)
+      elsif block_given?
+        map(&block).sum(identity)
+      else
+        inject(:+) || 0
+      end
+    end
+  else
+    def sum(identity = nil, &block)
+      if block_given?
+        map(&block).sum(identity)
+      else
+        sum = identity ? inject(identity, :+) : inject(:+)
+        sum || identity || 0
+      end
     end
   end