[Bug tree-optimization/101831] Spurious maybe-uninitialized warning on std::array::size

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101831

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-08-16

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
-Wmaybe-uninitialized is also issued when an uninitialized object is passed by
reference to a const-qualified argument.  This includes passing the address of
an such object to the implicit this pointer in calls to member functions.  This
form of the warning runs very early on and before any function calls have been
inlined, so it can't tell that the function doesn't actually read from the
uninitialized object.  The same effect can be reproduced in C in a call to a
non-member function (see below).  It's possible to run the early uninitialized
pass later but probably not without introducing some false negatives.  I'm not
sure that the std::array use case is common enough to justify the  potential
for the false negatives (or conversely, that the potential is significant
enough not to avoid the false positives).  So confirmed.  It requires some
thought and testing.

$ cat a.c && gcc -S -Wall -Wextra a.c
inline __attribute__ ((always_inline))
int f (const int *p) { (void)&p; return 0; }

int g (void)
{
  int i;
  return f (&i);
}
a.c: In function ‘g’:
a.c:7:10: warning: ‘i’ may be used uninitialized [-Wmaybe-uninitialized]
    7 |   return f (&i);
      |          ^~~~~~
a.c:2:5: note: by argument 1 of type ‘const int *’ to ‘f’ declared here
    2 | int f (const int *p) { (void)&p; return 0; }
      |     ^
a.c:6:7: note: ‘i’ declared here
    6 |   int i;
      |       ^