[Bug c++/80593] New: GCC 7, aligned_storage and “dereferencing type-punned pointer will break strict-aliasing rules”

[freddie_chopin at op dot pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80593

            Bug ID: 80593
           Summary: GCC 7, aligned_storage and “dereferencing type-punned
                    pointer will break strict-aliasing rules”
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: freddie_chopin at op dot pl
  Target Milestone: ---

I've posted this info to the gcc mailing list. Richard Biener suggested opening
a bug report, so here it is.

https://gcc.gnu.org/ml/gcc/2017-05/msg00013.html

Following example code is warning-free on GCC 4.9, GCC 5 and GCC 6. It is also
warning free on some older GCC 7 snapshots (like 7-20170409).

-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --

$ cat punning.cpp 
template<unsigned _Len, unsigned _Align>
struct aligned_storage
{
  union type
    {
      unsigned char __data[_Len];
      struct __attribute__((__aligned__((_Align)))) { } __align;
    };
};

aligned_storage<sizeof(int), alignof(int)>::type storage;

int main()
{
  *reinterpret_cast<int*>(&storage) = 42;
}
$ g++ -Wall -O2 -c punning.cpp 
$

-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --

However in the most recent GCC 7 snapshots (including both RCs), it gives a
warning about type punning:

-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --

$ g++ -Wall -O2 -c punning.cpp 
punning.cpp: In function 'int main()':
punning.cpp:15:35: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
   *reinterpret_cast<int*>(&storage) = 42;
                                   ^
$

-- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 -- >8 --