Error(s):

1251009532/source.cpp:10:30: error: constexpr variable 'diff' must be initialized by a constant expression
    constexpr std::ptrdiff_t diff = *(&arr + 1) - arr;
                             ^      ~~~~~~~~~~~~~~~~~
1251009532/source.cpp:10:37: note: cannot access array element of pointer past the end of object
    constexpr std::ptrdiff_t diff = *(&arr + 1) - arr;
                                    ^
1 error generated.