Mutating an int inside a constexpr function

Question

Why can I do this:

constexpr auto i_can() {    int a = 8;    a = 9;    //... } 

But I cannot do this:

constexpr auto i_cannot() {     std::array<int, 10> arr{};     //I cannot     arr[5] = 9; } 

My questions are:

1. If I can mutate an int, why can I not mutate an int that is inside the array?
2. Is this a language limitation (C++14) or a standard library spec problem? reference std::array<T, N>::operator[](size_t) is not currently constexpr.

Answer 1

Modification of objects inside constexpr functions has been introduced with C++14. However, while modifying e.g. a scalar by an assignment is fine, modifying a class object through a member function still needs that member function to be constexpr. And unfortunately, as you mentioned, the current std::array specification does not declare the non-const operator[] as constexpr.
Hence, §7.1.5/5 makes your definition ill-formed:

For a non-template, non-defaulted constexpr function […], if no argument values exist such that an invocation of the function […] could be an evaluated subexpression of a core constant expression (5.20), […], the program is ill-formed; no diagnostic required.

You can temporarily use a more modern implementation if you want full constexpr-ness. E.g. Constainer::Array.