[Bug c/66397] New: sanitize=undefined triggers extra -Warray-bounds warning

["sirl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66397

            Bug ID: 66397
           Summary: sanitize=undefined triggers extra -Warray-bounds
                    warning
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

Created attachment 35691
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35691&action=edit
testcase, warns with gcc-6 -c -O2 -fsanitize=undefined

The attached testcase derived from a C++ iterator implementation relies on the
fact that the undefined pCurrent-1 is immediately nullified again by the
following pCurrent++ by the optimizer. But in current trunk r224064 the
following warning is issued:

test.c: In function 'test':
test.c:19:53: warning: array subscript is below array bounds [-Warray-bounds]
     ths->pCurrent = (ths->pStart) ? ths->pStart - 1 : (stru *) 0;
                                                     ^

Though the warning is not completely wrong, -Warray-bounds usually triggers
only when the value is really accessed, or? gcc-5.1 compiles the testcase
without warning.