[Fix rubocop#207] Fix an error for Performance/Sum

Fixes rubocop#207.

This PR fixes an error for `Performance/Sum` when using
`map(&do_something).sum` without receiver.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#207](https://github.com/rubocop-hq/rubocop-performance/issues/207): Fix an error for `Performance/Sum` when using `map(&do_something).sum` without receiver. ([@koic][])
+
 ## 1.9.2 (2021-01-01)
 
 ### Bug fixes

lib/rubocop/cop/performance/sum.rb

@@ -150,7 +150,9 @@ def autocorrect_sum_map(corrector, sum, map, init)
           replacement = build_good_method(init, block_pass)
 
           corrector.remove(sum_range)
-          corrector.replace(map_range, ".#{replacement}")
+
+          dot = '.' if map.receiver
+          corrector.replace(map_range, "#{dot}#{replacement}")
         end
 
         def sum_method_range(node)
@@ -228,7 +230,11 @@ def build_block_bad_method(method, init, var_acc, var_elem, body)
         end
 
         def method_call_with_args_range(node)
-          node.receiver.source_range.end.join(node.source_range.end)
+          if (receiver = node.receiver)
+            receiver.source_range.end.join(node.source_range.end)
+          else
+            node.source_range
+          end
         end
       end
     end

spec/rubocop/cop/performance/sum_spec.rb

@@ -190,6 +190,17 @@
       RUBY
     end
 
+    it "registers an offense and corrects when using `#{method}(&:count).sum`" do
+      expect_offense(<<~RUBY, method: method)
+        %{method}(&:count).sum
+        ^{method}^^^^^^^^^^^^^ Use `sum { ... }` instead of `%{method} { ... }.sum`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        sum(&:count)
+      RUBY
+    end
+
     it "registers an offense and corrects when using `array.#{method}(&:count).sum(10)`" do
       expect_offense(<<~RUBY, method: method)
         array.%{method}(&:count).sum(10)