[Bug c++/52988] std::async not executed on function returning nullptr_t

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52988

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
                 CC|                            |jason at gcc dot gnu.org
          Component|libstdc++                   |c++
      Known to fail|                            |4.6.3, 4.8.0

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-04-14 
17:29:52 UTC ---
Jason, could ou take a look at this please?

This looks like a G++ issue, doing something weird when nullptr_t is involved. 
Possibly assuming that since all nullptr_t values are the same there's no point
calling a function to get one and it's valid to just return nullptr
immediately.


future<int>::get() produces this:

_Res std::future<_Res>::get() [with _Res = int] (struct future * const this)
{
  struct __basic_future * D.117159;
  int D.117160;
  struct __basic_future * D.117161;
  struct _Result & D.117162;
  int & D.117163;
  type & D.117164;
  struct _Reset __reset;

  try
    {
      D.117159 = &this->D.52581;
      std::__basic_future<int>::_Reset::_Reset (&__reset, D.117159);
      try
        {
          D.117161 = &this->D.52581;
          D.117162 = std::__basic_future<int>::_M_get_result (D.117161);
          D.117163 = std::__future_base::_Result<int>::_M_value (D.117162);
          D.117164 = std::move<int&> (D.117163);
          D.117160 = *D.117164;
          return D.117160;
        }
      finally
        {
          std::__basic_future<int>::_Reset::~_Reset (&__reset);
        }
    }
  finally
    {
      __reset = {CLOBBER};
    }
}


But even though they're generated from the same template
future<nullptr_t>::get() looks like this, with no call to _M_get_result()
before returning:


_Res std::future<_Res>::get() [with _Res = std::nullptr_t] (struct future *
const this)
{ 
  struct __basic_future * D.117150;
  <<< Unknown tree: nullptr_type >>> D.117151;
  struct _Reset __reset;

  try 
    { 
      D.117150 = &this->D.52293;
      std::__basic_future<std::nullptr_t>::_Reset::_Reset (&__reset, D.117150);
      try
        {
          D.117151 = 0;
          return D.117151;
        }
      finally
        {
          std::__basic_future<std::nullptr_t>::_Reset::~_Reset (&__reset);
        }
    }
  finally
    { 
      __reset = {CLOBBER};
    }
}