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`.

1 files changed, 21 insertions, 3 deletions

diff --git a/activesupport/test/core_ext/enumerable_test.rb b/activesupport/test/core_ext/enumerable_test.rb
index f09b7d8850..976c8b2b81 100644
--- a/activesupport/test/core_ext/enumerable_test.rb
+++ b/activesupport/test/core_ext/enumerable_test.rb
@@ -10,22 +10,22 @@ class SummablePayment < Payment
 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 @@ class EnumerableTests < ActiveSupport::TestCase
     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) },