デッドロックしてるぽいね。
(lldb) bt * thread #1, name = 'test', stop reason = signal SIGSTOP * frame #0: 0x00007ffff6a72a9f libpthread.so.0`__pthread_once_slow + 95 frame #1: 0x00007ffff7c7f96f libswiftCore.so`swift_once + 95 frame #2: 0x0000000000400fef test`type metadata accessor for test.B + 47 frame #3: 0x0000000000401531 test`___lldb_unnamed_symbol7$$test + 17 frame #4: 0x00007ffff6a72ae9 libpthread.so.0`__pthread_once_slow + 169 frame #5: 0x00007ffff7c7f96f libswiftCore.so`swift_once + 95 frame #6: 0x00000000004013df test`type metadata accessor for test.B.X + 47 frame #7: 0x000000000040138e test`type metadata accessor for test.A<test.B.X> + 30 frame #8: 0x0000000000401341 test`___lldb_unnamed_symbol5$$test + 17 frame #9: 0x00007ffff6a72ae9 libpthread.so.0`__pthread_once_slow + 169 frame #10: 0x00007ffff7c7f96f libswiftCore.so`swift_once + 95 frame #11: 0x0000000000400fef test`type metadata accessor for test.B + 47 frame #12: 0x0000000000400f31 test`main + 65 frame #13: 0x00007ffff6ca1830 libc.so.6`__libc_start_main + 240 frame #14: 0x0000000000400e09 test`_start + 41