Optimize Range#sum only for integers [#2489]

activesupport/lib/active_support/core_ext/enumerable.rb

@@ -113,9 +113,10 @@ module Enumerable
 end
 
 class Range #:nodoc:
-  # Optimize range sum to use arithmetic progression if a block is not given.
+  # Optimize range sum to use arithmetic progression if a block is not given and
+  # we have a range of numeric values.
   def sum(identity=0, &block)
-    return super if block_given?
+    return super if block_given? || !(first.instance_of?(Integer) && last.instance_of?(Integer))
     actual_last = exclude_end? ? (last - 1) : last
     (actual_last - first + 1) * (actual_last + first) / 2
   end

activesupport/test/core_ext/enumerable_test.rb

@@ -64,6 +64,7 @@ class EnumerableTests < Test::Unit::TestCase
     assert_equal 20, (1..4).sum { |i| i * 2 }
     assert_equal 10, (1..4).sum
     assert_equal 6, (1...4).sum
+    assert_equal 'abc', ('a'..'c').sum
   end
 
   def test_each_with_object