> I slightly prefer the constraining specification, so that you don't
> need to thinking about the fate of volatile tuple_element.
I believe you need to think about volatile in either case.
The subtlety with C-style array types is that there's no difference
between applying a cv-qualifier to the array or to the element type:
i.e. a constant array of N integers is the same as an array of N
constant integers. This is not the case with std::array.
My understanding is that the standard will drop the tuple-protocol
support for volatile-qualified std::array; not for std::array of
volatile-qualified elements. I.e.
//using namespace std;
tuple_element_t<0, array<int, 42> volatile> // Won't compile
tuple_element_t<0, array<int volatile, 42>> // Ok: int volatile