Fix the config_for to always return a NonSymbolAccessDeprecatedHash:

- If you have hashes inside array, the hashes were getting initialized
  as regular HWIA wereas we want them to be
  NonSymbolAccessDeprecatedHash in order to trigger a deprecation
  warning when keys are accessed with string.

  This patch fixes that by overwriting the `[]=` to to the same
  as what HWIA does (with the difference that we don't call
  `convert_key` to not trigger a deprecation when setting value).

  I also took the liberty to extract `hash.nested_under_indifferent_access`,
  into a separate method to allow subclasses to return whatever
  they want.
  Inheriting HWIA is not common, but I think it's useful for cases
  like this one where we want to preprocess reading and writing values
  in the hash (for deprecation purposes or other reasons).

railties/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Fix `Rails.application.config_for` returning a HWIA when Hash were
+    inside Array.
+
+    *Edouard Chin*
+
 *   Fix non-symbol access to nested hashes returned from `Rails::Application.config_for`
     being broken by allowing non-symbol access with a deprecation notice.
 

railties/lib/rails/application.rb

@@ -602,10 +602,7 @@ module Rails
         end
 
         def []=(key, value)
-          if value.is_a?(Hash)
-            value = self.class.new(value)
-          end
-          super(key.to_sym, value)
+          regular_writer(key.to_sym, convert_value(value, for: :assignment))
         end
 
         private
@@ -623,6 +620,23 @@ module Rails
 
             key
           end
+
+          def convert_value(value, options = {}) # :doc:
+            if value.is_a? Hash
+              if options[:for] == :to_hash
+                value.to_hash
+              else
+                self.class.new(value)
+              end
+            elsif value.is_a?(Array)
+              if options[:for] != :assignment || value.frozen?
+                value = value.dup
+              end
+              value.map! { |e| convert_value(e, options) }
+            else
+              value
+            end
+          end
       end
   end
 end

railties/test/application/configuration_test.rb

@@ -1787,6 +1787,28 @@ module ApplicationTests
       end
     end
 
+    test "config_for loads nested custom configuration inside array from yaml with deprecated non-symbol access" do
+      app_file "config/custom.yml", <<-RUBY
+      development:
+        foo:
+          bar:
+          - baz: 1
+      RUBY
+
+      add_to_config <<-RUBY
+        config.my_custom_config = config_for('custom')
+      RUBY
+
+      app "development"
+
+      config = Rails.application.config.my_custom_config
+      assert_instance_of Rails::Application::NonSymbolAccessDeprecatedHash, config[:foo][:bar].first
+
+      assert_deprecated do
+        assert_equal 1, config[:foo][:bar].first["baz"]
+      end
+    end
+
     test "config_for makes all hash methods available" do
       app_file "config/custom.yml", <<-RUBY
       development: