Add enum serialization test and Serialization Proxy

Author: bishabosha

library/src/scala/runtime/EnumValueSerializationProxy.java

@@ -0,0 +1,40 @@
+package scala.runtime;
+
+import java.io.Serializable;
+import java.security.AccessController;
+import java.security.PrivilegedActionException;
+import java.security.PrivilegedExceptionAction;
+
+/** A serialization proxy for singleton enum values */
+public final class EnumValueSerializationProxy implements Serializable {
+    private static final long serialVersionUID = 1L;
+    private final Class<?> enumClass;
+    private final int ordinal;
+    private static final ClassValue<Object[]> enumValues = new ClassValue<Object[]>() {
+        @Override
+        protected Object[] computeValue(Class<?> type) {
+            try {
+                return AccessController.doPrivileged((PrivilegedExceptionAction<Object[]>) () ->
+                  (Object[])type.getMethod("values").invoke(null));
+            } catch (PrivilegedActionException e) {
+                return rethrowRuntime(e.getCause());
+            }
+        }
+    };
+
+    private static <T> T rethrowRuntime(Throwable e) {
+        Throwable cause = e.getCause();
+        if (cause instanceof RuntimeException) throw (RuntimeException) cause;
+        else throw new RuntimeException(cause);
+    }
+
+    public EnumValueSerializationProxy(Class<?> enumClass, int ordinal) {
+        this.enumClass = enumClass;
+        this.ordinal = ordinal;
+    }
+
+    @SuppressWarnings("unused")
+    private Object readResolve() {
+        return enumValues.get(enumClass)[ordinal];
+    }
+}

tests/run/enums-serialization-compat.scala

@@ -0,0 +1,23 @@
+import java.io._
+import scala.util.Using
+
+enum JColor extends java.lang.Enum[JColor]:
+  case Red
+
+enum SColor:
+  case Green
+
+enum SColorTagged[T]:
+  case Blue extends SColorTagged[Unit]
+
+@main def Test = Using.Manager({ use =>
+  val buf = use(ByteArrayOutputStream())
+  val out = use(ObjectOutputStream(buf))
+  Seq(JColor.Red, SColor.Green, SColorTagged.Blue).foreach(out.writeObject)
+  val read = use(ByteArrayInputStream(buf.toByteArray))
+  val in   = use(ObjectInputStream(read))
+  val Seq(Red @ _, Green @ _, Blue @ _) = (1 to 3).map(_ => in.readObject)
+  assert(Red eq JColor.Red, JColor.Red)
+  assert(Green eq SColor.Green, SColor.Green)
+  assert(Blue eq SColorTagged.Blue, SColorTagged.Blue)
+}).get