Fix implicit calculations with scalars and durations

Previously calculations where the scalar is first would be converted
to a duration of seconds but this causes issues with dates being
converted to times, e.g:

    Time.zone = "Beijing"           # => Asia/Shanghai
    date = Date.civil(2017, 5, 20)  # => Mon, 20 May 2017
    2 * 1.day                       # => 172800 seconds
    date + 2 * 1.day                # => Mon, 22 May 2017 00:00:00 CST +08:00

Now the `ActiveSupport::Duration::Scalar` calculation methods will try
to maintain the part structure of the duration where possible, e.g:

    Time.zone = "Beijing"           # => Asia/Shanghai
    date = Date.civil(2017, 5, 20)  # => Mon, 20 May 2017
    2 * 1.day                       # => 2 days
    date + 2 * 1.day                # => Mon, 22 May 2017

Fixes #29160, #28970.

1 files changed, 35 insertions, 6 deletions

diff --git a/activesupport/lib/active_support/duration.rb b/activesupport/lib/active_support/duration.rb
index d4424ed792..39deb2313f 100644
--- a/activesupport/lib/active_support/duration.rb
+++ b/activesupport/lib/active_support/duration.rb
@@ -37,27 +37,56 @@ module ActiveSupport
       end
 
       def +(other)
-        calculate(:+, other)
+        if Duration === other
+          seconds   = value + other.parts[:seconds]
+          new_parts = other.parts.merge(seconds: seconds)
+          new_value = value + other.value
+
+          Duration.new(new_value, new_parts)
+        else
+          calculate(:+, other)
+        end
       end
 
       def -(other)
-        calculate(:-, other)
+        if Duration === other
+          seconds   = value - other.parts[:seconds]
+          new_parts = other.parts.map { |part, other_value| [part, -other_value] }.to_h
+          new_parts = new_parts.merge(seconds: seconds)
+          new_value = value - other.value
+
+          Duration.new(new_value, new_parts)
+        else
+          calculate(:-, other)
+        end
       end
 
       def *(other)
-        calculate(:*, other)
+        if Duration === other
+          new_parts = other.parts.map { |part, other_value| [part, value * other_value] }.to_h
+          new_value = value * other.value
+
+          Duration.new(new_value, new_parts)
+        else
+          calculate(:*, other)
+        end
       end
 
       def /(other)
-        calculate(:/, other)
+        if Duration === other
+          new_parts = other.parts.map { |part, other_value| [part, value / other_value] }.to_h
+          new_value = new_parts.inject(0) { |total, (part, value)| total + value * Duration::PARTS_IN_SECONDS[part] }
+
+          Duration.new(new_value, new_parts)
+        else
+          calculate(:/, other)
+        end
       end
 
       private
         def calculate(op, other)
           if Scalar === other
             Scalar.new(value.public_send(op, other.value))
-          elsif Duration === other
-            Duration.seconds(value).public_send(op, other)
           elsif Numeric === other
             Scalar.new(value.public_send(op, other))
           else