[Fix rubocop#776] Fix an incorrect autocorrect for Rails/Presence

Fixes rubocop#776.

This PR fixes an incorrect autocorrect for `Rails/Presence`
when using arithmetic operation in `else` branch.

changelog/fix_an_incorrect_autocorrect_for_rails_presence.md

@@ -0,0 +1 @@
+* [#776](https://github.com/rubocop/rubocop-rails/pull/776): Fix an incorrect autocorrect for `Rails/Presence` when using arithmetic operation in `else` branch. ([@koic][])

lib/rubocop/cop/rails/presence.rb

@@ -122,19 +122,19 @@ def replacement(receiver, other)
         end
 
         def build_source_for_or_method(other)
-          if other.parenthesized? || other.method?('[]') || !other.arguments?
+          if other.parenthesized? || other.method?('[]') || other.arithmetic_operation? || !other.arguments?
             " || #{other.source}"
           else
-            method = range_between(
-              other.source_range.begin_pos,
-              other.first_argument.source_range.begin_pos - 1
-            ).source
-
+            method = method_range(other).source
             arguments = other.arguments.map(&:source).join(', ')
 
             " || #{method}(#{arguments})"
           end
         end
+
+        def method_range(node)
+          range_between(node.source_range.begin_pos, node.first_argument.source_range.begin_pos - 1)
+        end
       end
     end
   end

spec/rubocop/cop/rails/presence_spec.rb

@@ -49,6 +49,22 @@
     end
   RUBY
 
+  it_behaves_like 'offense', <<~RUBY.chomp, 'a.presence || b.to_f + 12.0', 1, 5
+    if a.present?
+      a
+    else
+      b.to_f + 12.0
+    end
+  RUBY
+
+  it_behaves_like 'offense', <<~RUBY.chomp, 'a.presence || b.to_f * 12.0', 1, 5
+    if a.present?
+      a
+    else
+      b.to_f * 12.0
+    end
+  RUBY
+
   it_behaves_like 'offense', 'a if a.present?', 'a.presence', 1, 1
   it_behaves_like 'offense', 'a unless a.blank?', 'a.presence', 1, 1
   it_behaves_like 'offense', <<~RUBY.chomp, <<~FIXED.chomp, 1, 7