Make mutating params#dig mutate underlying params

When #dig was called on a params object and return either a Hash or an
Array, and that value was subsquently mutated, it would not modify the
containing params object. That means that the behavior of
`params.dig(:a, :b)[:c] = 1` did not match either `params[:a][:b][:c] =
1` nor `hash.dig(:a, :b)[:c] = 1`. Similarly to
`ActionController::Parameters#[]`, use `#convert_hashes_to_parameters`
to pre-convert values and insert them in the receiving params object
prior to returning them.

2 files changed, 10 insertions, 1 deletions

diff --git a/actionpack/lib/action_controller/metal/strong_parameters.rb b/actionpack/lib/action_controller/metal/strong_parameters.rb
index 75ca282804..4789252550 100644
--- a/actionpack/lib/action_controller/metal/strong_parameters.rb
+++ b/actionpack/lib/action_controller/metal/strong_parameters.rb
@@ -590,7 +590,8 @@ module ActionController
     #   params2 = ActionController::Parameters.new(foo: [10, 11, 12])
     #   params2.dig(:foo, 1) # => 11
     def dig(*keys)
-      convert_value_to_parameters(@parameters.dig(*keys))
+      convert_hashes_to_parameters(keys.first, @parameters[keys.first])
+      @parameters.dig(*keys)
     end
 
     # Returns a new <tt>ActionController::Parameters</tt> instance that
diff --git a/actionpack/test/controller/parameters/accessors_test.rb b/actionpack/test/controller/parameters/accessors_test.rb
index 07a897a103..674b2c6266 100644
--- a/actionpack/test/controller/parameters/accessors_test.rb
+++ b/actionpack/test/controller/parameters/accessors_test.rb
@@ -284,4 +284,12 @@ class ParametersAccessorsTest < ActiveSupport::TestCase
       value.is_a?(ActionController::Parameters)
     end
   end
+
+  test "mutating #dig return value mutates underlying parameters" do
+    @params.dig(:person, :name)[:first] = "Bill"
+    assert_equal "Bill", @params.dig(:person, :name, :first)
+
+    @params.dig(:person, :addresses)[0] = { city: "Boston", state: "Massachusetts" }
+    assert_equal "Boston", @params.dig(:person, :addresses, 0, :city)
+  end
 end