c++ - Why is non-const std::array::operator[] not constexpr?

Question

I'm trying to fill a 2D array on compile time with a given function. Here is my code:

template<int H, int W>
struct Table
{
  int data[H][W];
  //std::array<std::array<int, H>, W> data;  // This does not work

  constexpr Table() : data{}
  {
    for (int i = 0; i < H; ++i)
      for (int j = 0; j < W; ++j)
        data[i][j] = i * 10 + j;  // This does not work with std::array
  }
};

constexpr Table<3, 5> table;  // I have table.data properly populated at compile time

It works just fine, table.data is properly populated at compile time.

However, if I change plain 2D array int[H][W] with std::array<std::array<int, H>, W>, I have an error in the loop body:

error: call to non-constexpr function 'std::array<_Tp, _Nm>::value_type& std::array<_Tp, _Nm>::operator[](std::array<_Tp, _Nm>::size_type) [with _Tp = int; long unsigned int _Nm = 3ul; std::array<_Tp, _Nm>::reference = int&; std::array<_Tp, _Nm>::value_type = int; std::array<_Tp, _Nm>::size_type = long unsigned int]'
data[i][j] = i * 10 + j;
^
Compilation failed

Obviously, I'm trying to call non-const overload of std::array::operator[], which is not constexpr. The question is, why it is not constexpr? If C++14 allows us to modify variables declared in constexpr scope, why this is not supported by std::array?

I used to think that std::array is just like plain array, only better. But here is an example, where I can use plain array, but cannot use std::array.

Answer 1

Ok, it is indeed an oversight in the standard. There even exists a proposal to fix this: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0107r0.pdf

[N3598] removed the implicit marking of constexpr member functions as const. However, the member functions of std::array were not revisited after this change, leading to a surprising lack of support for constexpr in std::array’s interface. This paper fixes this omission by adding constexpr to the member functions of std::array that can support it with a minimal amount of work.

UPD: Fixed in C++17: https://en.cppreference.com/w/cpp/container/array/operator_at