Yes, it is a bug. Microsoft acknowledged it is, the feedback report is here.
Right now the bug is in "will not fix" status and it is unclear when (if ever) it will be addressed. There is a somewhat odd workaround for it, the IntelliSense parser built into Visual Studio, written by the Edison Design Group, does complain about it. You get the red squiggles and the message:
Error: member "Test.privateData" (declared at line 10) is inaccessible