5 files changed, 57 insertions, 3 deletions
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 31c1e687dc..6aa414ba6b 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -50,7 +50,7 @@ module ActiveRecord
         super.tap do
           @previous_mutation_tracker = nil
           clear_mutation_trackers
-          @changed_attributes = HashWithIndifferentAccess.new
+          @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
         end
       end
 
@@ -70,13 +70,13 @@ module ActiveRecord
 
       def changes_applied
         @previous_mutation_tracker = mutation_tracker
-        @changed_attributes = HashWithIndifferentAccess.new
+        @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
         clear_mutation_trackers
       end
 
       def clear_changes_information
         @previous_mutation_tracker = nil
-        @changed_attributes = HashWithIndifferentAccess.new
+        @changed_attributes = ActiveSupport::HashWithIndifferentAccess.new
         forget_attribute_assignments
         clear_mutation_trackers
       end
diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index cdea917380..80e61f26eb 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Soft-deprecated the top-level `HashWithIndifferentAcces` constant.
+    `ActiveSupport::HashWithIndifferentAccess` should be used instead.
+
+    *Robin Dupret* (#28157)
+
 *   In Core Extensions, make `MarshalWithAutoloading#load` pass through the second, optional
     argument for `Marshal#load( source [, proc] )`. This way we don't have to do 
     `Marshal.method(:load).super_method.call(sourse, proc)` just to be able to pass a proc.
diff --git a/activesupport/lib/active_support/hash_with_indifferent_access.rb b/activesupport/lib/active_support/hash_with_indifferent_access.rb
index f9b269ad69..1927cddf34 100644
--- a/activesupport/lib/active_support/hash_with_indifferent_access.rb
+++ b/activesupport/lib/active_support/hash_with_indifferent_access.rb
@@ -316,4 +316,6 @@ module ActiveSupport
   end
 end
 
+# :stopdoc:
+
 HashWithIndifferentAccess = ActiveSupport::HashWithIndifferentAccess
diff --git a/activesupport/test/core_ext/hash_ext_test.rb b/activesupport/test/core_ext/hash_ext_test.rb
index a6e3e59433..525ea08abd 100644
--- a/activesupport/test/core_ext/hash_ext_test.rb
+++ b/activesupport/test/core_ext/hash_ext_test.rb
@@ -8,6 +8,8 @@ require "active_support/core_ext/object/deep_dup"
 require "active_support/inflections"
 
 class HashExtTest < ActiveSupport::TestCase
+  HashWithIndifferentAccess = ActiveSupport::HashWithIndifferentAccess
+
   class IndifferentHash < ActiveSupport::HashWithIndifferentAccess
   end
 
@@ -1088,6 +1090,30 @@ class HashExtTest < ActiveSupport::TestCase
     assert_equal 1, hash[:a]
     assert_equal 3, hash[:b]
   end
+
+  def test_inheriting_from_top_level_hash_with_indifferent_access_preserves_ancestors_chain
+    klass = Class.new(::HashWithIndifferentAccess)
+    assert_equal ActiveSupport::HashWithIndifferentAccess, klass.ancestors[1]
+  end
+
+  def test_inheriting_from_hash_with_indifferent_access_properly_dumps_ivars
+    klass = Class.new(::HashWithIndifferentAccess) do
+      def initialize(*)
+        @foo = "bar"
+        super
+      end
+    end
+
+    yaml_output = klass.new.to_yaml
+
+    # `hash-with-ivars` was introduced in 2.0.9 (https://git.io/vyUQW)
+    if Gem::Version.new(Psych::VERSION) >= Gem::Version.new("2.0.9")
+      assert_includes yaml_output, "hash-with-ivars"
+      assert_includes yaml_output, "@foo: bar"
+    else
+      assert_includes yaml_output, "hash"
+    end
+  end
 end
 
 class IWriteMyOwnXML
@@ -1133,6 +1159,8 @@ class HashExtToParamTests < ActiveSupport::TestCase
 end
 
 class HashToXmlTest < ActiveSupport::TestCase
+  HashWithIndifferentAccess = ActiveSupport::HashWithIndifferentAccess
+
   def setup
     @xml_options = { root: :person, skip_instruct: true, indent: 0 }
   end
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 8ba00a2b10..6005298127 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -65,6 +65,25 @@ Overwrite /myapp/config/application.rb? (enter "h" for help) [Ynaqdh]
 
 Don't forget to review the difference, to see if there were any unexpected changes.
 
+Upgrading from Rails 5.0 to Rails 5.1
+-------------------------------------
+
+For more information on changes made to Rails 5.1 please see the [release notes](5_1_release_notes.html).
+
+### Top-level `HashWithIndifferentAccess` is soft-deprecated
+
+If your application uses the the top-level `HashWithIndifferentAccess` class, you
+should slowly move your code to use the `ActiveSupport::HashWithIndifferentAccess`
+one.
+
+It is only soft-deprecated, which means that your code will not break at the
+moment and no deprecation warning will be displayed but this constant will be
+removed in the future.
+
+Also, if you have pretty old YAML documents containg dumps of such objects,
+you may need to load and dump them again to make sure that they reference
+the right constant and that loading them won't break in the future.
+
 Upgrading from Rails 4.2 to Rails 5.0
 -------------------------------------