Description:
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It seems that since PHP7 alpha it's possible to access private static properties of upper classes. This is an unwanted regression https://3v4l.org/4XheY

Currently on master, the same example has a failing assertion and a segfault:

[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
php: php/Zend/zend_object_handlers.c:279: zend_verify_property_access: Assertion `property_info->flags & 0x200' failed.

Program received signal SIGABRT, Aborted.
0x00007ffff5ea5877 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56        return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);

Expected result:
----------------
Same error as php 5.6: Fatal error: Cannot access property B::$x in /in/4XheY on line 9

Actual result:
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On RC6 it's possible to access the private static property from the upper class.
On master it causes a segfault.