Merge pull request #375 from koic/fix_an_error_for_performance_map_me…

…thod_chain

[Fix #374] Fix an error for `Performance/MapMethodChain`

changelog/fix_an_error_for_performance_map_method_chain.md

@@ -0,0 +1 @@
+* [#374](https://github.com/rubocop/rubocop-performance/issues/374): Fix an error for `Performance/MapMethodChain` when using `map` method chain without receiver. ([@koic][])

lib/rubocop/cop/performance/map_method_chain.rb

@@ -68,6 +68,7 @@ def on_send(node)
 
         private
 
+        # rubocop:disable Metrics/CyclomaticComplexity
         def find_begin_of_chained_map_method(node, map_args)
           return unless (chained_map_method = node.receiver)
           return if !chained_map_method.call_type? || !RESTRICT_ON_SEND.include?(chained_map_method.method_name)
@@ -77,10 +78,11 @@ def find_begin_of_chained_map_method(node, map_args)
 
           receiver = chained_map_method.receiver
 
-          return chained_map_method unless receiver.call_type? && block_pass_with_symbol_arg?(receiver.first_argument)
+          return chained_map_method unless receiver&.call_type? && block_pass_with_symbol_arg?(receiver.first_argument)
 
           find_begin_of_chained_map_method(chained_map_method, map_args)
         end
+        # rubocop:enable Metrics/CyclomaticComplexity
       end
     end
   end

spec/rubocop/cop/performance/map_method_chain_spec.rb

@@ -44,6 +44,13 @@
     RUBY
   end
 
+  it 'registers an offense when using `map` method chain without receiver' do
+    expect_offense(<<~RUBY)
+      map(&:foo).map(&:bar)
+      ^^^^^^^^^^^^^^^^^^^^^ Use `map { |x| x.foo.bar }` instead of `map` method chain.
+    RUBY
+  end
+
   it 'does not register an offense when using `do_something` method chain and receiver is a method call' do
     expect_no_offenses(<<~RUBY)
       array.do_something(&:foo).do_something(&:bar)