src/java.base/share/classes/java/lang/ref/WeakReference.java

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--- 28,14 ---
  
  /**
   * Weak reference objects, which do not prevent their referents from being
   * made finalizable, finalized, and then reclaimed.  Weak references are most
   * often used to implement canonicalizing mappings.
+  * <p>
+  * The referent must not be an instance of a {@linkplain Class#isValue()
+  * value class}; such a value can never have another reference to it
+  * and cannot be held in a reference type.
   *
   * <p> Suppose that the garbage collector determines at a certain point in time
   * that an object is <a href="package-summary.html#reachability">weakly
   * reachable</a>.  At that time it will atomically clear all weak references to
   * that object and all weak references to any other weakly-reachable objects

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--- 54,12 ---
      /**
       * Creates a new weak reference that refers to the given object.  The new
       * reference is not registered with any queue.
       *
       * @param referent object the new weak reference will refer to
+      * @throws IllegalArgumentException if the referent is an instance of a
+      *         {@link Class#isValue() value class}
       */
      public WeakReference(T referent) {
          super(referent);
      }
  

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--- 68,12 ---
       * registered with the given queue.
       *
       * @param referent object the new weak reference will refer to
       * @param q the queue with which the reference is to be registered,
       *          or {@code null} if registration is not required
+      * @throws IllegalArgumentException if the referent is an instance of a
+      *         {@link Class#isValue() value class}
       */
      public WeakReference(T referent, ReferenceQueue<? super T> q) {
          super(referent, q);
      }