[patch] libstdc++/65352 fix ubsan errors in std::array<T, 0>

[Jonathan Wakely]

On 28/05/15 15:52 +0200, Marc Glisse wrote:
>On Thu, 28 May 2015, Jonathan Wakely wrote:
>
>>Would there be a danger of an object compiled with gcc-5.1 that calls
>>array<T, 0>::data() finding the _S_ref from an object compiled with
>>gcc-5.2 and hitting the __builtin_unreachable in vali code?
>
>At -O0, maybe. To be safe you would need to give this _S_ref an 
>arbitrary abi_tag.

Or just rename it.

>You could also replace all uses of _S_ref with *_S_ptr.

I considered this, but I thought that changing _S_ref(_M_elems, n) to
_S_ptr(_M_elems)[n] would fail to give an error for out-of-range
accesses in constant expressions e.g.

    constexpr std::array<int, 1> a{};
    constexpr int i = a[1];

But it still seems to give an error, so maybe getting rid of _S_ref
entirely is the way to go.