[Bug middle-end/70054] New: GCC 6 gives a strict-aliasing warning on use of std::aligned_storage

[tavianator at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70054

            Bug ID: 70054
           Summary: GCC 6 gives a strict-aliasing warning on use of
                    std::aligned_storage
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tavianator at gmail dot com
  Target Milestone: ---

GCC 6 warns on this code; GCC 5 didn't.

boost::container::string uses this pattern as part of its SSO implementation,
so I'm getting a lot of new warnings with GCC 6 as a result.  As well, I'm
seeing what appears to be a miscompilation as a result, but I haven't reduced
that yet.

$ cat foo.cpp
#include <type_traits>

struct foo {
  std::aligned_storage<sizeof(long), alignof(long)>::type raw;

  long& cooked() {
    return *static_cast<long*>(static_cast<void*>(&raw));
  }
};
$ g++ -O2 -Wall -c foo.cpp
foo.cpp: In member function ‘long int& foo::cooked()’:
foo.cpp:7:56: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
     return *static_cast<long*>(static_cast<void*>(&raw));