scope now raises on "dangerous" name conflicts

Similar to dangerous attribute methods, a scope name conflict is
dangerous if it conflicts with an existing class method defined within
`ActiveRecord::Base` but not its ancestors.

See also #13389.

*Godfrey Chan*, *Philippe Creux*

activerecord/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   `scope` now raises on "dangerous" name conflicts
+
+    Similar to dangerous attribute methods, a scope name conflict is
+    dangerous if it conflicts with an existing class method defined within
+    `ActiveRecord::Base` but not its ancestors.
+
+    See also #13389.
+
+    *Godfrey Chan*, *Philippe Creux*
+
 *   Correctly send an user provided statement to a `lock!()` call.
 
         person.lock! 'FOR SHARE NOWAIT'

activerecord/lib/active_record/attribute_methods.rb

@@ -110,16 +110,34 @@ def instance_method_already_implemented?(method_name)
         end
       end
 
-      # A method name is 'dangerous' if it is already defined by Active Record, but
+      # A method name is 'dangerous' if it is already (re)defined by Active Record, but
       # not by any ancestors. (So 'puts' is not dangerous but 'save' is.)
       def dangerous_attribute_method?(name) # :nodoc:
         method_defined_within?(name, Base)
       end
 
-      def method_defined_within?(name, klass, sup = klass.superclass) # :nodoc:
+      def method_defined_within?(name, klass, superklass = klass.superclass) # :nodoc:
         if klass.method_defined?(name) || klass.private_method_defined?(name)
-          if sup.method_defined?(name) || sup.private_method_defined?(name)
-            klass.instance_method(name).owner != sup.instance_method(name).owner
+          if superklass.method_defined?(name) || superklass.private_method_defined?(name)
+            klass.instance_method(name).owner != superklass.instance_method(name).owner
+          else
+            true
+          end
+        else
+          false
+        end
+      end
+
+      # A class method is 'dangerous' if it is already (re)defined by Active Record, but
+      # not by any ancestors. (So 'puts' is not dangerous but 'new' is.)
+      def dangerous_class_method?(method_name)
+        class_method_defined_within?(method_name, Base)
+      end
+
+      def class_method_defined_within?(name, klass, superklass = klass.superclass) # :nodoc
+        if klass.respond_to?(name, true)
+          if superklass.respond_to?(name, true)
+            klass.method(name).owner != superklass.method(name).owner
           else
             true
           end

activerecord/lib/active_record/scoping/named.rb

@@ -139,6 +139,12 @@ def scope_attributes? # :nodoc:
         #   Article.published.featured.latest_article
         #   Article.featured.titles
         def scope(name, body, &block)
+          if dangerous_class_method?(name)
+            raise ArgumentError, "You tried to define a scope named \"#{name}\" " \
+              "on the model \"#{self.name}\", but Active Record already defined " \
+              "a class method with the same name."
+          end
+
           extension = Module.new(&block) if block
 
           singleton_class.send(:define_method, name) do |*args|

activerecord/test/cases/scoping/named_scoping_test.rb

@@ -266,6 +266,63 @@ def test_should_build_on_top_of_chained_scopes
     assert_equal 'lifo', topic.author_name
   end
 
+  def test_reserved_scope_names
+    klass = Class.new(ActiveRecord::Base) do
+      self.table_name = "topics"
+
+      scope :approved, -> { where(approved: true) }
+
+      class << self
+        public
+          def pub; end
+
+        private
+          def pri; end
+
+        protected
+          def pro; end
+      end
+    end
+
+    subklass = Class.new(klass)
+
+    conflicts = [
+      :create,        # public class method on AR::Base
+      :relation,      # private class method on AR::Base
+      :new,           # redefined class method on AR::Base
+      :all,           # a default scope
+    ]
+
+    non_conflicts = [
+      :find_by_title, # dynamic finder method
+      :approved,      # existing scope
+      :pub,           # existing public class method
+      :pri,           # existing private class method
+      :pro,           # existing protected class method
+      :open,          # a ::Kernel method
+    ]
+
+    conflicts.each do |name|
+      assert_raises(ArgumentError, "scope `#{name}` should not be allowed") do
+        klass.class_eval { scope name, ->{ where(approved: true) } }
+      end
+
+      assert_raises(ArgumentError, "scope `#{name}` should not be allowed") do
+        subklass.class_eval { scope name, ->{ where(approved: true) } }
+      end
+    end
+
+    non_conflicts.each do |name|
+      assert_nothing_raised do
+        klass.class_eval { scope name, ->{ where(approved: true) } }
+      end
+
+      assert_nothing_raised do
+        subklass.class_eval { scope name, ->{ where(approved: true) } }
+      end
+    end
+  end
+
   # Method delegation for scope names which look like /\A[a-zA-Z_]\w*[!?]?\z/
   # has been done by evaluating a string with a plain def statement. For scope
   # names which contain spaces this approach doesn't work.