Treat combined durations as a single unit

Prior to this commit, `3.months - 3.months` would result in a duration
that has the "parts" of `[[:months, 3], [:months, -3]]`. This would mean
that it was subtly different than `2.months - 2.months`. When applied to
a time, the date might actually change if the resulting day doesn't
exist however many months in the future, even though in both cases we
just wanted to add `0`, which should always be an identity operation.

With this change, we now store the parts as a hash, so `3.months -
3.months` is simply stored as `{ months: 0 }`.

1 files changed, 11 insertions, 0 deletions

diff --git a/activesupport/test/core_ext/duration_test.rb b/activesupport/test/core_ext/duration_test.rb
index b7b4a9dd00..26a9ea4aee 100644
--- a/activesupport/test/core_ext/duration_test.rb
+++ b/activesupport/test/core_ext/duration_test.rb
@@ -345,6 +345,17 @@ class DurationTest < ActiveSupport::TestCase
     end
   end
 
+  def test_adding_durations_do_not_hold_prior_states
+    time = Time.parse("Nov 29, 2016")
+    # If the implementation adds and subtracts 3 months, the
+    # resulting date would have been in February so the day will
+    # change to the 29th.
+    d1 = 3.months - 3.months
+    d2 = 2.months - 2.months
+
+    assert_equal time + d1, time + d2
+  end
+
   private
     def eastern_time_zone
       if Gem.win_platform?