src/java.base/share/classes/sun/invoke/util/VerifyAccess.java

*** 131,141 ****
              return ((allowedModes & PACKAGE_ALLOWED) != 0 &&
                      isSamePackage(defc, lookupClass));
          case PRIVATE:
              // Rules for privates follows access rules for nestmates.
              return ((allowedModes & PRIVATE) != 0 &&
!                     Reflection.areNestMates(defc, lookupClass));
          default:
              throw new IllegalArgumentException("bad modifiers: "+Modifier.toString(mods));
          }
      }
  
--- 131,141 ----
              return ((allowedModes & PACKAGE_ALLOWED) != 0 &&
                      isSamePackage(defc, lookupClass));
          case PRIVATE:
              // Rules for privates follows access rules for nestmates.
              return ((allowedModes & PRIVATE) != 0 &&
!                     areNestMates(defc, lookupClass));
          default:
              throw new IllegalArgumentException("bad modifiers: "+Modifier.toString(mods));
          }
      }
  
*** 334,343 ****
--- 334,370 ----
          if (class1.getClassLoader() != class2.getClassLoader())
              return false;
          return Objects.equals(class1.getPackageName(), class2.getPackageName());
      }
  
+     /**
+      * Test if two classes are defined as part of the same package member (top-level class).
+      * If this is true, they can share private access with each other.
+      * @param class1 a class
+      * @param class2 another class
+      * @return whether they are identical or nested together
+      */
+     public static boolean areNestMates(Class<?> class1, Class<?> class2) {
+         if (class1 == class2)
+             return true;
+         if (!isSamePackage(class1, class2))
+             return false;
+         if (Reflection.areNestMates(class1, class2))
+             return true;
+         // Could be pre-nestmate nested types
+         if (getOutermostEnclosingClass(class1) != getOutermostEnclosingClass(class2))
+             return false;
+         return true;
+     }
+ 
+     private static Class<?> getOutermostEnclosingClass(Class<?> c) {
+         Class<?> pkgmem = c;
+         for (Class<?> enc = c; (enc = enc.getEnclosingClass()) != null; )
+             pkgmem = enc;
+         return pkgmem;
+     }
+ 
      private static boolean loadersAreRelated(ClassLoader loader1, ClassLoader loader2,
                                               boolean loader1MustBeParent) {
          if (loader1 == loader2 || loader1 == null
                  || (loader2 == null && !loader1MustBeParent)) {
              return true;