[Fix rubocop#626] Fix a false positive for Rails/CompactBlank

Fixes rubocop#626.

Fix a false positive for `Rails/CompactBlank` when using the receiver
of `blank?` is not a block variable.

changelog/fix_false_positive_for_rails_compact_blank.md

@@ -0,0 +1 @@
+* [#626](https://github.com/rubocop/rubocop-rails/issues/626: Fix a false positive for `Rails/CompactBlank` when using the receiver of `blank?` is not a block variable. ([@koic][])

lib/rubocop/cop/rails/compact_blank.rb

@@ -73,12 +73,17 @@ def bad_method?(node)
           return true if reject_with_block_pass?(node)
 
           if (arguments, receiver_in_block = reject_with_block?(node.parent))
-            return arguments.length == 1 || use_hash_value_block_argument?(arguments, receiver_in_block)
+            return use_single_value_block_argument?(arguments, receiver_in_block) ||
+                   use_hash_value_block_argument?(arguments, receiver_in_block)
           end
 
           false
         end
 
+        def use_single_value_block_argument?(arguments, receiver_in_block)
+          arguments.length == 1 && arguments[0].source == receiver_in_block.source
+        end
+
         def use_hash_value_block_argument?(arguments, receiver_in_block)
           arguments.length == 2 && arguments[1].source == receiver_in_block.source
         end

spec/rubocop/cop/rails/compact_blank_spec.rb

@@ -118,6 +118,14 @@
         collection.reject { |k, v| k.empty? }
       RUBY
     end
+
+    it 'does not register an offense when using the receiver of `blank?` is not a block variable' do
+      expect_no_offenses(<<~RUBY)
+        def foo(arg)
+          collection.reject { |_| arg.blank? }
+        end
+      RUBY
+    end
   end
 
   context 'Rails <= 6.0', :rails60 do