[Fix rubocop#8720] Fix an error for Lint/IdentityComparison

Fixes rubocop#8720.

This PR fixes an error for `Lint/IdentityComparison` when
calling `object_id` method without receiver in LHS or RHS.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#8720](https://github.com/rubocop-hq/rubocop/issues/8720): Fix an error for `Lint/IdentityComparison` when calling `object_id` method without receiver in LHS or RHS. ([@koic][])
+
 ## 0.91.0 (2020-09-15)
 
 ### New features

lib/rubocop/cop/lint/identity_comparison.rb

@@ -26,9 +26,11 @@ def on_send(node)
           return unless compare_between_object_id_by_double_equal?(node)
 
           add_offense(node) do |corrector|
-            receiver = node.receiver.receiver.source
-            argument = node.first_argument.receiver.source
-            replacement = "#{receiver}.equal?(#{argument})"
+            receiver = node.receiver.receiver
+            argument = node.first_argument.receiver
+            return unless receiver && argument
+
+            replacement = "#{receiver.source}.equal?(#{argument.source})"
 
             corrector.replace(node, replacement)
           end

spec/rubocop/cop/lint/identity_comparison_spec.rb

@@ -31,4 +31,16 @@
       foo.equal(bar)
     RUBY
   end
+
+  it 'does not register an offense when lhs is `object_id` without receiver' do
+    expect_no_offenses(<<~RUBY)
+      object_id == bar.object_id
+    RUBY
+  end
+
+  it 'does not register an offense when rhs is `object_id` without receiver' do
+    expect_no_offenses(<<~RUBY)
+      foo.object_id == object_id
+    RUBY
+  end
 end