[Fix rubocop#147] Fix an error for Performance/AncestorsInclude

Fixes rubocop#147.

This PR fixes an error for `Performance/AncestorsInclude`
when using `ancestors.include? without receiver.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#147](https://github.com/rubocop-hq/rubocop-performance/issues/147): Fix an error for `Performance/AncestorsInclude` when using `ancestors.include? without receiver. ([@koic][])
+
 ## 1.7.0 (2020-07-07)
 
 ### New features

lib/rubocop/cop/performance/ancestors_include.rb

@@ -35,7 +35,9 @@ def on_send(node)
         def autocorrect(node)
           ancestors_include_candidate?(node) do |subclass, superclass|
             lambda do |corrector|
-              corrector.replace(node, "#{subclass.source} <= #{superclass.source}")
+              subclass_source = subclass ? subclass.source : 'self.class'
+
+              corrector.replace(node, "#{subclass_source} <= #{superclass.source}")
             end
           end
         end

spec/rubocop/cop/performance/ancestors_include_spec.rb

@@ -14,6 +14,17 @@
     RUBY
   end
 
+  it 'registers an offense and corrects when using `ancestors.include? without receiver`' do
+    expect_offense(<<~RUBY)
+      ancestors.include?(Klass)
+      ^^^^^^^^^^^^^^^^^^ Use `<=` instead of `ancestors.include?`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      self.class <= Klass
+    RUBY
+  end
+
   it 'does not register an offense when using `<=`' do
     expect_no_offenses(<<~RUBY)
       Class <= Kernel