JEP401 introduces some changes to boxed primitives, for example not being able to synchronize on them or make java.lang.ref references to them.

There are several test cases in java.awt, javax.swing, sanity.cient.SwingSet that fail when running with the latest Valhalla build and --enable-preview enabled. These failures are likely all related and caused in an underlying common ground. They fail with the following error:
java.lang.IdentityException: java.lang.Boolean is not an identity class
at java.base/java.util.Objects.requireIdentity(Objects.java:238)
at java.base/java.lang.ref.Reference.<init>(Reference.java:543)
at java.base/java.lang.ref.Reference.<init>(Reference.java:538)
at java.base/java.lang.ref.SoftReference.<init>(SoftReference.java:98)
at java.desktop/sun.awt.AppContext.getSoftReferenceValue(AppContext.java:752)
at java.desktop/com.apple.laf.AquaUtils$RecyclableSingleton.get(AquaUtils.java:155)
at java.desktop/com.apple.laf.AquaUtils.animationsEnabled(AquaUtils.java:208)
at java.desktop/com.apple.laf.AquaUtils.blinkMenu(AquaUtils.java:213)
at java.desktop/com.apple.laf.AquaMenuItemUI.doClick(AquaMenuItemUI.java:157)
at java.desktop/javax.swing.plaf.basic.BasicPopupMenuUI$Actions.doReturn(BasicPopupMenuUI.java:478)
at java.desktop/javax.swing.plaf.basic.BasicPopupMenuUI$Actions.actionPerformed(BasicPopupMenuUI.java:452)
at java.desktop/javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1787)
at java.desktop/javax.swing.JComponent.processKeyBinding(JComponent.java:2949)
at java.desktop/javax.swing.KeyboardManager.fireBinding(KeyboardManager.java:295)
at java.desktop/javax.swing.KeyboardManager.fireKeyboardAction(KeyboardManager.java:241)
at java.desktop/javax.swing.JComponent.processKeyBindingsForAllComponents(JComponent.java:3041)
at java.desktop/javax.swing.JComponent.processKeyBindings(JComponent.java:3033)
at java.desktop/javax.swing.JComponent.processKeyEvent(JComponent.java:2911)
at java.desktop/java.awt.Component.processEvent(Component.java:6347)
at java.desktop/java.awt.Container.processEvent(Container.java:2260)
at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:4952)
at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2318)
at java.desktop/java.awt.Component.dispatchEvent(Component.java:4784)
at java.desktop/java.awt.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.java:1819)
at java.desktop/java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:874)
at java.desktop/java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:1136)
at java.desktop/java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:1011)
at java.desktop/java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:839)
at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:4833)
at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2318)
at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2667)
at java.desktop/java.awt.Component.dispatchEvent(Component.java:4784)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:716)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:693)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

A (likely non-exhaustive list) of tests failing due to this is:
sanity/client/SwingSet/src/SwingSet2DemoTest.java
sanity/client/SwingSet/src/OptionPaneDemoTest.java
sanity/client/SwingSet/src/FrameDemoTest.java
sanity/client/SwingSet/src/FileChooserDemoTest.java
sanity/client/SwingSet/src/ComboBoxDemoTest.java
javax/swing/Popup/TaskbarPositionTest.java
javax/swing/plaf/basic/BasicComboPopup/JComboBoxPopupLocation/JComboBoxPopupLocation.java
javax/swing/plaf/aqua/TestAltUpDownComboBox.java
javax/swing/KeyboardManager/bug4345798.java
javax/swing/JTree/4618767/JTreeSelectedElementTest.java
javax/swing/JTableHeader/JTableHeaderExceptionTest.java
javax/swing/JRootPane/4670486/bug4670486.java
javax/swing/JPopupMenu/JPopupMenuFocusStealTest.java
javax/swing/JPopupMenu/6544309/bug6544309.java
javax/swing/JPopupMenu/6515446/bug6515446.java
javax/swing/JPopupMenu/4458079/bug4458079.java
javax/swing/JMenuItem/JActionCommandTest.java
javax/swing/JMenuItem/8158566/CloseOnMouseClickPropertyTest.java
javax/swing/JMenuItem/8139169/ScreenMenuBarInputTwice.java
javax/swing/JList/4618767/JListSelectedElementTest.java
javax/swing/JInternalFrame/8145896/TestJInternalFrameMaximize.java
javax/swing/JInternalFrame/8145060/TestJInternalFrameMinimize.java
javax/swing/JInternalFrame/6288609/TestJInternalFrameDispose.java
javax/swing/JComboBox/EditableComboBoxPopupPos.java
javax/swing/JComboBox/bug4530953.java
javax/swing/JComboBox/8041909/ActionListenerExceptionTest.java
javax/swing/JComboBox/6559152/bug6559152.java
javax/swing/JComboBox/4523758/bug4523758.java
java/awt/TextArea/Mixing/TextAreaMixing.java
java/awt/Mixing/LWPopupMenu.java
java/awt/KeyboardFocusmanager/TypeAhead/SubMenuShowTest/SubMenuShowTest.java
java/awt/grab/MenuDragEvents/MenuDragEvents.java
java/awt/Focus/TestWindowsLFFocus.java
java/awt/Focus/UnaccessibleChoice/AccessibleChoiceTest.java
java/awt/event/MouseEvent/DragMouseEventTest.java
java/awt/event/ComponentEvent/ComponentItemEventTest.java
java/awt/Choice/SelectBetweenPressRelease.java
java/awt/Choice/NonFocusablePopupMenuTest.java
java/awt/Choice/ChoiceDragEventsInside.java
java/awt/Choice/SelectItem/SelectNewItemTest.java
java/awt/Choice/ChoicePopupLocation/ChoicePopupLocation.java
java/awt/Choice/ChoiceMouseWheelTest/ChoiceMouseWheelTest.java
sanity/client/SwingSet/src/SwingSet2DemoTest.java
sanity/client/SwingSet/src/OptionPaneDemoTest.java
sanity/client/SwingSet/src/FrameDemoTest.java
sanity/client/SwingSet/src/FileChooserDemoTest.java
sanity/client/SwingSet/src/ComboBoxDemoTest.java
java/awt/Modal/BlockedMouseInputTest.java
sanity/client/SwingSet/src/SwingSet2DemoTest.java
sanity/client/SwingSet/src/OptionPaneDemoTest.java
sanity/client/SwingSet/src/FrameDemoTest.java
sanity/client/SwingSet/src/FileChooserDemoTest.java
sanity/client/SwingSet/src/ComboBoxDemoTest.java