Bug 78044

Created attachment 39844 [details]
test.i

There are already filed bug reports about false positive for this warning.  This might be a dup of one of those.  

Also this warning is hard not to get false positives really for conditional unitialized variables which is why the option was split into two.  One for the hard for GCC to prove that it is not a false positive (-Wmaybe-uninitialized).

"which is why the option was split into two."

Ok, it makes sense to have one reliable option that handles fewer cases, but doesn't have false positives. And another that checks more, but with false positives. The only problem I have with -Wmaybe-uninitialized is that it is included in -Wall. I thought that -Wall was meant to collect the warnings without false positives? 

Would it be possible not to turn -Wmaybe-uninitialized on when -Wall is turned on?

Created attachment 39848 [details]
Reduced preprocessed test-case

Confirmed.

Well.  someFunction () is optimized down to just

void someFunction() ()
{
  int aOptional$4;
  struct C D.2560;
  struct C D.2555;
  void * _4;

  <bb 2>:
  C::operator mpl_::B (&D.2555);
  D.2555 ={v} {CLOBBER};
  _4 = operator new (8);
  C::operator mpl_::B (&D.2560);

  <bb 3>:
  D.2560 ={v} {CLOBBER};
  MEM[(int *)_4 + 4B] = aOptional$4_2(D);
  return;

<L0>:
  operator delete (_4, 8);
  resx 3

so unless C::operator mpl_::B (&D.2560); always throws we store aOptional$4_2(D)
(uninitialized) to MEM[(int *)_4 + 4B].  The warning itself is a bit misleading
I guess.

Thus

    int __trans_tmp_1 = p1.get_impl ();
    new (m_storage.address ()) int(__trans_tmp_1);

and

  int
  get_impl ()
  {
    union
    {
      void *ap_pvoid;
      int *as_ptype;
    } caster{m_storage.address ()};
    get_impl___trans_tmp_3 = *caster.as_ptype;
    int __trans_tmp_2 = get_impl___trans_tmp_3;
    B __trans_tmp_4 = C ();
    return __trans_tmp_2;
  }

doesn't cover up the fact that m_storage is not initialized.

This might be an artifact of the testcase reduction of course.

This still reproduces in version 7.2. Here is a Wandbox link: https://wandbox.org/permlink/5uCAr5u0xpynljhQ

Here is the code, you need to compile it with -DNDEBUG -O2:

```
#include <cassert>

class O
{
    int _val;
    bool _init = false;
public:
    O() = default;
    O(const int& i) : _val(i), _init(true) {}
    explicit operator bool () const { return _init; }
    const int& operator*() const { assert (_init); return _val; }
    void operator=(int const& i) { _init = true; _val = i; }
};

bool equal_pointees ( O const& x, O const& y )
{
  return (!x) != (!y) ? false : ( !x ? true : (*x) == (*y) ) ;
}

O getitem();

int main(int argc, const char *[])
{
  O a = getitem();
  O b;

  if (argc > 0)
    b = argc;

  if (equal_pointees(a, b))
    return 1;

  return 0;
}
```

Is there a way to give a hint to the compiler (like with an attribute) so that the warning is silenced?

The warning for the test case in comment 0 (attachment 39843 [details]) disappeared between r275012 and r275022.  The one for the test case in comment 7 disappeared after r254495.