Ruby 2.4: compat with new Array#sum

Ruby 2.4 introduces `Array#sum`, but it only supports numeric elements,
breaking our `Enumerable#sum` which supports arbitrary `Object#+`.
To fix, override `Array#sum` with our compatible implementation.

Native Ruby 2.4:

    %w[ a b ].sum
    # => TypeError: String can't be coerced into Fixnum

With `Enumerable#sum` shim:

    %w[ a b ].sum
    # => 'ab'

We tried shimming the fast path and falling back to the compatible path
if it fails, but that ends up slower even in simple causes due to the cost
of exception handling. Our only choice is to override the native `Array#sum`
with our `Enumerable#sum`.

activesupport/CHANGELOG.md

@@ -1,3 +1,26 @@
+*   `Array#sum` compat with Ruby 2.4's native method.
+
+    Ruby 2.4 introduces `Array#sum`, but it only supports numeric elements,
+    breaking our `Enumerable#sum` which supports arbitrary `Object#+`.
+    To fix, override `Array#sum` with our compatible implementation.
+
+    Native Ruby 2.4:
+
+        %w[ a b ].sum
+        # => TypeError: String can't be coerced into Fixnum
+
+    With `Enumerable#sum` shim:
+
+        %w[ a b ].sum
+        # => 'ab'
+
+    We tried shimming the fast path and falling back to the compatible path
+    if it fails, but that ends up slower even in simple causes due to the cost
+    of exception handling. Our only choice is to override the native `Array#sum`
+    with our `Enumerable#sum`.
+
+    *Jeremy Daer*
+
 *   `ActiveSupport::Duration` supports ISO8601 formatting and parsing.
 
         ActiveSupport::Duration.parse('P3Y6M4DT12H30M5S')

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -104,3 +104,17 @@ def sum(identity = 0)
     end
   end
 end
+
+# Array#sum was added in Ruby 2.4 but it only works with Numeric elements.
+#
+# 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 Array.instance_methods(false).include?(:sum) && (%w[a].sum rescue true)
+  class Array
+    def sum(*args) #:nodoc:
+      # Use Enumerable#sum instead.
+      super
+    end
+  end
+end

activesupport/test/core_ext/enumerable_test.rb

@@ -10,22 +10,22 @@ def +(p) self.class.new(price + p.price) end
 end
 
 class EnumerableTests < ActiveSupport::TestCase
-
   class GenericEnumerable
     include Enumerable
+
     def initialize(values = [1, 2, 3])
       @values = values
     end
 
     def each
-      @values.each{|v| yield v}
+      @values.each { |v| yield v }
     end
   end
 
   def test_sums
     enum = GenericEnumerable.new([5, 15, 10])
     assert_equal 30, enum.sum
-    assert_equal 60, enum.sum { |i| i * 2}
+    assert_equal 60, enum.sum { |i| i * 2 }
 
     enum = GenericEnumerable.new(%w(a b c))
     assert_equal 'abc', enum.sum
@@ -70,6 +70,24 @@ def test_range_sums
     assert_equal 42, (10...10).sum(42)
   end
 
+  def test_array_sums
+    enum = [5, 15, 10]
+    assert_equal 30, enum.sum
+    assert_equal 60, enum.sum { |i| i * 2 }
+
+    enum = %w(a b c)
+    assert_equal 'abc', enum.sum
+    assert_equal 'aabbcc', enum.sum { |i| i * 2 }
+
+    payments = [ Payment.new(5), Payment.new(15), Payment.new(10) ]
+    assert_equal 30, payments.sum(&:price)
+    assert_equal 60, payments.sum { |p| p.price * 2 }
+
+    payments = [ SummablePayment.new(5), SummablePayment.new(15) ]
+    assert_equal SummablePayment.new(20), payments.sum
+    assert_equal SummablePayment.new(20), payments.sum { |p| p }
+  end
+
   def test_index_by
     payments = GenericEnumerable.new([ Payment.new(5), Payment.new(15), Payment.new(10) ])
     assert_equal({ 5 => Payment.new(5), 15 => Payment.new(15), 10 => Payment.new(10) },