[Bug c/101358] New: Warn when saving a pointer to an object with temporary lifetime

[josephcsible at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101358

            Bug ID: 101358
           Summary: Warn when saving a pointer to an object with temporary
                    lifetime
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: josephcsible at gmail dot com
  Target Milestone: ---

Consider this C code:

typedef struct {
    int x[1];
} foo;

foo f(void);

int g(void) {
    int *p = f().x;
    return *p;
}

The g() function is always UB, since the return value of f() has temporary
lifetime, so doing "return *p;" is dereferencing a pointer to an object whose
lifetime has ended. (This is the case both before and after C11's change to
temporary lifetime.) Since it's obvious at compile time that p can never be
used safely, we should have a warning for it, similar to how we have
-Wreturn-local-addr to catch mistakes like this function:

int *h(void) {
    int x;
    return &x;
}