Trust Object#dup in ActionController::Parameters, using #initialize_copy to manually duplicate the underlying parameters hash

It looks like `ActionController::Parameters#dup` is leftover from when the class inherited from `Hash`. We can just trust `#dup`, which already copies the `@permitted` instance variable (confirmed by tests). We still define a `#initialize_copy` to make `@parameters` a copy that can be mutated without affecting the original instance.

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -572,20 +572,6 @@ def values_at(*keys)
       convert_value_to_parameters(@parameters.values_at(*keys))
     end
 
-    # Returns an exact copy of the <tt>ActionController::Parameters</tt>
-    # instance. +permitted+ state is kept on the duped object.
-    #
-    #   params = ActionController::Parameters.new(a: 1)
-    #   params.permit!
-    #   params.permitted?        # => true
-    #   copy_params = params.dup # => <ActionController::Parameters {"a"=>1} permitted: true>
-    #   copy_params.permitted?   # => true
-    def dup
-      super.tap do |duplicate|
-        duplicate.permitted = @permitted
-      end
-    end
-
     # Returns a new <tt>ActionController::Parameters</tt> with all keys from
     # +other_hash+ merges into current hash.
     def merge(other_hash)
@@ -786,7 +772,7 @@ def hash_filter(params, filter)
 
       def initialize_copy(source)
         super
-        @parameters = source.instance_variable_get(:@parameters).dup
+        @parameters = @parameters.dup
       end
   end
 

actionpack/test/controller/parameters/dup_test.rb

@@ -17,9 +17,27 @@ class ParametersDupTest < ActiveSupport::TestCase
     )
   end
 
-  test "changes on a duplicate do not affect the original" do
+  test "a duplicate maintains the original's permitted status" do
+    @params.permit!
+    dupped_params = @params.dup
+    assert dupped_params.permitted?
+  end
+
+  test "a duplicate maintains the original's parameters" do
+    @params.permit!
+    dupped_params = @params.dup
+    assert_equal @params.to_h, dupped_params.to_h
+  end
+
+  test "changes to a duplicate's parameters do not affect the original" do
     dupped_params = @params.dup
     dupped_params.delete(:person)
     assert_not_equal @params, dupped_params
   end
+
+  test "changes tp a duplicate's permitted status do not affect the original" do
+    dupped_params = @params.dup
+    dupped_params.permit!
+    assert_not_equal @params, dupped_params
+  end
 end