[Fix rubocop#309] Fix an error for Performance/MapCompact

Fixes rubocop#309.

This PR fixes an error for `Performance/MapCompact`
when using `map(&:do_something).compact` and there is a line break after
`map.compact` and assigning with `||=`.

changelog/fix_an_error_for_performance_map_compact.md

@@ -0,0 +1 @@
+* [#309](https://github.com/rubocop/rubocop-performance/issues/309): Fix an error for `Performance/MapCompact` when using `map(&:do_something).compact` and there is a line break after `map.compact` and assigning with `||=`. ([@koic][])

lib/rubocop/cop/performance/map_compact.rb

@@ -67,7 +67,7 @@ def on_send(node)
         def remove_compact_method(corrector, map_node, compact_node, chained_method)
           compact_method_range = compact_node.loc.selector
 
-          if compact_node.multiline? && chained_method&.loc.respond_to?(:selector) && chained_method.dot? &&
+          if compact_node.multiline? && chained_method&.loc.respond_to?(:selector) && use_dot?(chained_method) &&
              !map_method_and_compact_method_on_same_line?(map_node, compact_node) &&
              !invoke_method_after_map_compact_on_same_line?(compact_node, chained_method)
             compact_method_range = compact_method_with_final_newline_range(compact_method_range)
@@ -78,6 +78,10 @@ def remove_compact_method(corrector, map_node, compact_node, chained_method)
           corrector.remove(compact_method_range)
         end
 
+        def use_dot?(node)
+          node.respond_to?(:dot?) && node.dot?
+        end
+
         def map_method_and_compact_method_on_same_line?(map_node, compact_node)
           compact_node.loc.selector.line == map_node.loc.selector.line
         end

spec/rubocop/cop/performance/map_compact_spec.rb

@@ -119,6 +119,20 @@
       RUBY
     end
 
+    it 'registers an offense when using `map(&:do_something).compact` and there is a line break after' \
+       '`map.compact` and assigning with `||=`' do
+      expect_offense(<<~RUBY)
+        foo[1] ||= collection
+                   .map(&:do_something).compact
+                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `filter_map` instead.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        foo[1] ||= collection
+                   .filter_map(&:do_something)
+      RUBY
+    end
+
     it 'registers an offense when using `map { ... }.compact.first` with single-line method calls' do
       expect_offense(<<~RUBY)
         collection.map { |item| item.do_something }.compact.first