Since the problem is the non-trivial destructor so if the destructor is removed from the std::string, it's possible to define a constexpr instance of that type. Like this
struct constexpr_str {
char const* str;
std::size_t size;
// can only construct from a char[] literal
template <std::size_t N>
constexpr constexpr_str(char const (&s)[N])
: str(s)
, size(N - 1) // not count the trailing nul
{}
};
int main()
{
constexpr constexpr_str s("constString");
// its .size is a constexpr
std::array<int, s.size> a;
return 0;
}