[Bug c/85365] New: -Wrestrict false positives with -fsanitize=undefined

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85365

            Bug ID: 85365
           Summary: -Wrestrict false positives with -fsanitize=undefined
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

This small testcase warns 3 time when compiled with 8.0.1@r259308:

extern char a[], b[], d[];
int c, e;
char *strcpy(char *, const char *);
char *strcat(char *, const char *);
__SIZE_TYPE__ strlen(char *);

void t1(char *g) {
  strcpy(g + 4, c ? b : a);
  e = strlen(g + 4);
}

void t2(char *g) {
  strcpy(g + 4, c ? b : a);
  strcat(g + 4, d);
}

void t3(char *g) {
  strcat(g + 4, c ? b : a);
  strcat(g + 4, d);
}

# gcc-8 -c -O2 -fsanitize=undefined -Wrestrict t.c
t.c: In function 't1':
t.c:8:3: warning: 'strcpy' source argument is the same as destination
[-Wrestrict]
   strcpy(g + 4, c ? b : a);
   ^~~~~~~~~~~~~~~~~~~~~~~~
t.c: In function 't2':
t.c:13:3: warning: 'strcpy' source argument is the same as destination
[-Wrestrict]
   strcpy(g + 4, c ? b : a);
   ^~~~~~~~~~~~~~~~~~~~~~~~
t.c: In function 't3':
t.c:18:3: warning: 'strcat' source argument is the same as destination
[-Wrestrict]
   strcat(g + 4, c ? b : a);
   ^~~~~~~~~~~~~~~~~~~~~~~~

gcc-7 didn't warn for this testcase.