My colleagues Marsela Sulku and Maria Sam are working on detecting errors with new errorprone checks and have found one impossible cast in openjdk:

Method jaxws/src/java.xml.ws/share/classes/com/sun/xml/internal/ws/spi/db/TypeInfo.java#getItemType contains
        if (type instanceof Class && ((Class)type).isArray() && !byte[].class.equals(type)) {
            Type componentType = ((Class)type).getComponentType();
            Type genericComponentType = null;
            if (genericType!= null && genericType instanceof GenericArrayType) {
                GenericArrayType arrayType = (GenericArrayType) type;
                genericComponentType = arrayType.getGenericComponentType();
                componentType = arrayType.getGenericComponentType();
            }

The cast to (GenericArrayType) cannot succeed because type is already a Class. (Probably this code has never worked or been tested)

It's very likely that this is a typo for:

--- a/src/java.xml.ws/share/classes/com/sun/xml/internal/ws/spi/db/TypeInfo.java
+++ b/src/java.xml.ws/share/classes/com/sun/xml/internal/ws/spi/db/TypeInfo.java
@@ -177,7 +177,7 @@
             Type componentType = ((Class)type).getComponentType();
             Type genericComponentType = null;
             if (genericType!= null && genericType instanceof GenericArrayType) {
- GenericArrayType arrayType = (GenericArrayType) type;
+ GenericArrayType arrayType = (GenericArrayType) genericType;
                 genericComponentType = arrayType.getGenericComponentType();
                 componentType = arrayType.getGenericComponentType();
             }