[Bug libstdc++/78156] constexpr basic_string_view::basic_string_view(const charT *) calls non-constexpr char_traits::length

[suokkos at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78156

--- Comment #3 from Pauli <suokkos at gmail dot com> ---
Could __builtin_constant_p help to select different code for compile time and
runtime?

Something like:

string_view(const _CharT* __str) :
  _M_len(/*null check*/__builtin_constant_p(__str /*or __str[0]?*/) ?
      recursive_strlen<_CharT>(__str) : /*or
traits_type::__constexpr_length()*/
      traits_type::length()),
  _M_str(__str)
{}