Added failing test case for #16131

activesupport/test/core_ext/object/json_cherry_pick_test.rb

+require 'abstract_unit'
+
+# These test cases were added to test that cherry-picking the json extensions
+# works correctly, primarily for dependencies problems reported in #16131. They
+# need to be executed in isolation to reproduce the scenario correctly, because
+# other test cases might have already loaded additional dependencies.
+
+class JsonCherryPickTest < ActiveSupport::TestCase
+  include ActiveSupport::Testing::Isolation
+
+  def test_time_as_json
+    require_or_skip 'active_support/core_ext/object/json'
+
+    expected = Time.new(2004, 7, 25)
+    actual   = Time.parse(expected.as_json)
+
+    assert_equal expected, actual
+  end
+
+  def test_date_as_json
+    require_or_skip 'active_support/core_ext/object/json'
+
+    expected = Date.new(2004, 7, 25)
+    actual   = Date.parse(expected.as_json)
+
+    assert_equal expected, actual
+  end
+
+  def test_datetime_as_json
+    require_or_skip 'active_support/core_ext/object/json'
+
+    expected = DateTime.new(2004, 7, 25)
+    actual   = DateTime.parse(expected.as_json)
+
+    assert_equal expected, actual
+  end
+
+  private
+    def require_or_skip(file)
+      require(file) || skip("'#{file}' was already loaded")
+    end
+end