Why is std::array::size constexpr with simple types (int, double, …) but not std::vector (GCC)?

Question

The following code:

std::array arr1;
std::array arr2;

...compiles with both gcc and

Answer 1

I think this is a related to CWG issue 1684. Previously, the constexpr requirements included:

The class of which a constexpr function is a member shall be a literal type

And std::array, 4> is not a literal type, and hence its size() member function would not be constexpr. However, the new wording allows for a constexpr non-static member functions for non-literal types, assuming those functions meet all the other requirements of constexpr (which array::size() clearly does).

Per the new wording, this is a gcc bug. Previously filed as 66297.