Fix Range#sum optimized version

At 1bd4d1c67459a91415ee73a8f55d2309c0d62a87 was added Range#sum
optimized version for arithmetic progressions. This improvment injected
a defect with not integer range boundaries. The defect was fixed by
e0adfa82c05f9c975005f102b4bcaebfcd17d241. The second commit really
disabled optimization at all because in Ruby integer-valued numbers are
instances of Fixnum and Bignum classes. We should #use is_a?
(#kind_of?) method instead #instance_of? to check if value is numerical:

    1.class                 # => Fixnum
    1.instance_of?(Integer) # => false
    1.is_a?(Integer)        # => true

    -100_000_000_000.class                 # => Bignum
    -100_000_000_000.instance_of?(Integer) # => false
    -100_000_000_000.is_a?(Integer)        # => true

Moreover original implementation of Range#sum has a defect with reverse
range boundaries. If the first boundary is less than the second range is
empty. Current commit fixes and tests this case too.

1 files changed, 6 insertions, 2 deletions

diff --git a/activesupport/lib/active_support/core_ext/enumerable.rb b/activesupport/lib/active_support/core_ext/enumerable.rb
index 02d5a7080f..03efe6a19a 100644
--- a/activesupport/lib/active_support/core_ext/enumerable.rb
+++ b/activesupport/lib/active_support/core_ext/enumerable.rb
@@ -65,11 +65,15 @@ class Range #:nodoc:
   # 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)
-    if block_given? || !(first.instance_of?(Integer) && last.instance_of?(Integer))
+    if block_given? || !(first.is_a?(Integer) && last.is_a?(Integer))
       super
     else
       actual_last = exclude_end? ? (last - 1) : last
-      (actual_last - first + 1) * (actual_last + first) / 2
+      if actual_last >= first
+        (actual_last - first + 1) * (actual_last + first) / 2
+      else
+        identity
+      end
     end
   end
 end