Getting the number of elements in std::array at compile time

Question

Is the following a valid C++ code, and why not?

std::array a1;
std::array a2;

It doesn\'t compile

Answer 1

It is not static but it is a constexpr http://www.cplusplus.com/reference/array/array/size/
EDIT: this may not be a bug, take a look at this Error using a constexpr as a template parameter within the same class