[Fix #529] Fix a false positive for Rails/LexicallyScopedActionFilter

Fixes #529.

This PR fixes a false positive for `Rails/LexicallyScopedActionFilter`
when action method is aliased by `alias_method`.

Author: koic

CHANGELOG.md

@@ -11,6 +11,7 @@
 ### Bug fixes
 
 * [#528](https://github.com/rubocop/rubocop-rails/issues/528): Fix a false positive for `Rails/HasManyOrHasOneDependent` when specifying `:dependent` strategy with double splat. ([@koic][])
+* [#529](https://github.com/rubocop/rubocop-rails/issues/529): Fix a false positive for `Rails/LexicallyScopedActionFilter` when action method is aliased by `alias_method`. ([@koic][])
 
 ## Changes
 

lib/rubocop/cop/rails/lexically_scoped_action_filter.rb

@@ -124,9 +124,10 @@ def on_send(node)
           block = parent.each_child_node(:begin).first
           return unless block
 
-          defined_methods = block.each_child_node(:def).map(&:method_name)
+          defined_action_methods = defined_action_methods(block)
+
           methods = array_values(methods_node).reject do |method|
-            defined_methods.include?(method)
+            defined_action_methods.include?(method)
           end
 
           message = message(methods, parent)
@@ -135,6 +136,26 @@ def on_send(node)
 
         private
 
+        def defined_action_methods(block)
+          defined_methods = block.each_child_node(:def).map(&:method_name)
+
+          defined_methods + aliased_action_methods(block, defined_methods)
+        end
+
+        def aliased_action_methods(node, defined_methods)
+          alias_methods = node.each_child_node(:send).select { |send_node| send_node.method?(:alias_method) }
+
+          hash_of_alias_methods = alias_methods.each_with_object({}) do |alias_method, result|
+            result[alias_method.last_argument.value] = alias_method.first_argument.value
+          end
+
+          defined_methods.each_with_object([]) do |defined_method, aliased_method|
+            if (new_method_name = hash_of_alias_methods[defined_method])
+              aliased_method << new_method_name
+            end
+          end
+        end
+
         # @param node [RuboCop::AST::Node]
         # @return [Array<Symbol>]
         def array_values(node) # rubocop:disable Metrics/MethodLength

spec/rubocop/cop/rails/lexically_scoped_action_filter_spec.rb

@@ -115,6 +115,41 @@ def logout
     RUBY
   end
 
+  it 'does not register an offense when action method is aliased by `alias_method`' do
+    expect_no_offenses(<<~RUBY)
+      class FooController < ApplicationController
+        before_action :authorize!, only: %i[index show]
+
+        def index
+        end
+        alias_method :show, :index
+
+        private
+
+        def authorize!
+        end
+      end
+    RUBY
+  end
+
+  it 'registers an offense when action method is not aliased by `alias_method`' do
+    expect_offense(<<~RUBY)
+      class FooController < ApplicationController
+        before_action :authorize!, only: %i[foo show]
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `foo` is not explicitly defined on the class.
+
+        def index
+        end
+        alias_method :show, :index
+
+        private
+
+        def authorize!
+        end
+      end
+    RUBY
+  end
+
   it "doesn't register an offense when using conditional statements" do
     expect_no_offenses <<~RUBY
       class Test < ActionController