[Bug sanitizer/81981] New: [8 Regression] -fsanitize=undefined makes a -Wmaybe-uninitialized warning disappear

["vincent-gcc at vinc17 dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81981

            Bug ID: 81981
           Summary: [8 Regression] -fsanitize=undefined makes a
                    -Wmaybe-uninitialized warning disappear
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at gcc dot gnu.org
  Target Milestone: ---

Consider:

int foo (int i)
{
  int t[1], u[1];
  int n = 0;

  if (i)
    {
      t[n] = i;
      u[0] = i;
    }

  return t[0] + u[0];
}

With gcc (Debian 20170823-1) 8.0.0 20170823 (experimental) [trunk revision
251306], I get:

zira% gcc-snapshot -Wmaybe-uninitialized -O2 -c tst.c -fsanitize=undefined
tst.c: In function 'foo':
tst.c:12:15: warning: 'u[0]' may be used uninitialized in this function
[-Wmaybe-uninitialized]
   return t[0] + u[0];
          ~~~~~^~~~~~

zira% gcc-snapshot -Wmaybe-uninitialized -O2 -c tst.c                     
tst.c: In function 'foo':
tst.c:12:15: warning: 'u[0]' may be used uninitialized in this function
[-Wmaybe-uninitialized]
   return t[0] + u[0];
          ~~~~~^~~~~~
tst.c:12:15: warning: 't[0]' may be used uninitialized in this function
[-Wmaybe-uninitialized]

i.e. with -fsanitize=undefined, I do not get the warning on t[0].

This is a regression. No such problem with GCC 7.2.0, 6.4.0 and 5.4.1.