Deprecate adding two Time instances with ActiveSupport::TimeWithZone

Co-authored-by: Eileen M. Uchitelle <[email protected]>
Co-authored-by: John Hawthorn <[email protected]>

Ref: rails#52084

Subtracting two time instances results in a Float representing the
time between the instances.

Adding two time instances should not work, but it does.

```ruby
10.days.ago + 10.days.ago

```

 [This change](https://github.com/rails/rails/pull/52084/files#diff-aa0ae5ccf92f812f874b632afe70375c52772636f927fe6e34ffeaebf54af9d1L303) removed the rescue statement that had made this possible.
 Now, it is a breaking change to have made that easy mistake. This commit
 instead deprecates this change until Rails 8.0 and raises a deprecation
 warning when adding two Time instances.

Author: Schwad

activesupport/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Deprecate addition for `Time` instances with `ActiveSupport::TimeWithZone`.
+
+    Previously adding time instances together such as `10.days.ago + 10.days.ago` produced a nonsensical future date. This behavior is deprecated and will be removed in Rails 8.0.
+
+    *Nick Schwaderer*
+
 *   Support rfc2822 format for Time#to_fs & Date#to_fs.
 
     *Akshay Birajdar*

activesupport/lib/active_support/time_with_zone.rb

@@ -300,7 +300,16 @@ def +(other)
       if duration_of_variable_length?(other)
         method_missing(:+, other)
       else
-        result = utc + other
+        begin
+          result = utc + other
+        rescue TypeError
+          ActiveSupport.deprecator.warn(
+            "Adding an instance of #{other.class} to an instance of #{self.class} is deprecated. This behavior will raise " \
+            "a `TypeError` in Rails 8.1."
+          )
+          result = utc.since(other)
+          result.in_time_zone(time_zone)
+        end
         result.in_time_zone(time_zone)
       end
     end

activesupport/test/core_ext/time_with_zone_test.rb

@@ -400,6 +400,13 @@ def test_no_limit_on_times
     assert_equal [0, 0, 19, 31, 12, -8001], (twz - 10_000.years).to_a[0, 6]
   end
 
+  def test_plus_two_time_instances_raises_deprecation_warning
+    twz = ActiveSupport::TimeWithZone.new(Time.utc(2000, 1, 1), @time_zone)
+    assert_deprecated(ActiveSupport.deprecator) do
+      twz + 10.days.ago
+    end
+  end
+
   def test_plus_with_duration
     assert_equal Time.utc(2000, 1, 5, 19, 0, 0), (@twz + 5.days).time
   end