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

[Jonathan Wakely]

On 28/05/15 14:38 +0100, Jonathan Wakely wrote:
>On 28/05/15 15:26 +0200, Marc Glisse wrote:
>>On Thu, 28 May 2015, Jonathan Wakely wrote:
>>
>>>Unsurprisingly ubsan doesn't like referencing a null pointer.
>>>
>>>With this change __array_traits::_S_ref is only used to access an
>>>element, which is invalid for std::array<T, 0> anyway.
>>
>>Should
>>
>>return *static_cast<_Tp*>(nullptr);
>>
>>be replaced with
>>
>>__builtin_unreachable();
>>
>>then? It seems strange to keep an implementation that is never 
>>supposed to be used.
>
>That's a good idea, I experimented with just not defining it but that
>fails for explicit instantiations of array<T, 0>.

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?