GCC 7, aligned_storage and “dereferencing type-punned pointer will break strict-aliasing rules”

[Freddie Chopin <freddie_chopin at op dot pl>]

Hello!

A code that I wrote was warning-free in GCC 4.9, GCC 5 and GCC 6. It
was also warning-free with some older GCC 7 experimental snapshots (for
example 7-20170409). But in the most recent snapshot (including the
first RC), it started to produce a warning about aliasing. The code
basically boils down to this:

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

#include <type_traits>

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

int main()
{
    *reinterpret_cast<int*>(&storage) = 42;
}

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

Compilation with latest GCC 7 RC:

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

$ g++ -Wall -O2 -c main.cpp
main.cpp: In function 'int main()':
main.cpp:7:34: 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 --

(interesting observation is that the warning is not produced when
optimizations are disabled)

Compilation with GCC 6 gives no warnings at all.

Now I'm wondering, the code above definitely HAS type-punning, no
question about that, but isn't std::aligned_storage meant to be used
that way?

For instance the example code given here on cppreference generally
produces no warning with GCC 7 but only because:
- std::string somehow is not affected,
- std::aligned_storage is accessed with an offset.

http://en.cppreference.com/w/cpp/types/aligned_storage

By changing std::string into int, removing offset access to
std::aligned_storage and removing irrelevant parts you get this:

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

#include <iostream>
#include <type_traits>
#include <string>

template<class T, std::size_t N>
class static_vector
{
    // properly aligned uninitialized storage for N T's
    typename std::aligned_storage<sizeof(T), alignof(T)>::type data[N];
    std::size_t m_size = 0;

public:

    // Access an object in aligned storage
    const T& operator[](std::size_t pos) const
    {
        return *reinterpret_cast<const T*>(data/*+pos*/); // <- note
here, offset access disabled
    }
};

int main()
{
    static_vector<int, 10> v1;
    std::cout << v1[0] << '\n' << v1[1] << '\n';
}

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

And this produces exactly the same warning:

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

main.cpp: In instantiation of 'const T& static_vector<T,
N>::operator[](std::size_t) const [with T = int; unsigned int N = 10;
std::size_t = unsigned int]':
main.cpp:24:22:   required from here
main.cpp:17:16: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
         return *reinterpret_cast<const T*>(data/*+pos*/);
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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

So my question is - is this a bug or a feature?

Thanks in advance!

Regards,
FCh

BTW - I've also posted this question on stackoverflow
http://stackoverflow.com/questions/43711567/gcc-7-aligned-storage-and-dereferencing-type-punned-pointer-will-break-strict