constant expression context for `std::string_view`

[Yu Sheng Oh]

Hi guys,

May I know why the compilation failed for the following code where the
compiler is complaining that `s` is not a constant expression? While
removing the `static_assert`, the compilation passed. Regards.

  1 #include <array>
  2 #include <string_view>
  3 #include <cstddef>
  4
  5 template<std::size_t N>
  6 constexpr std::array<int, N> test(std::string_view s)
  7 {
  8   static_assert(s.size() > N);
  9   return { 0 };
 10 }
 11
 12 int main(int argc, char *argv[])
 13 {
 14   using namespace std::literals;
 15   constexpr auto s = "123"sv;
 16   constexpr auto a = test<2>(s);
 17   return 0;
 18 }

The g++ version being used is g++-14 (Homebrew GCC 14.2.0_1) 14.2.0.

Error messages:
test.cpp: In instantiation of 'constexpr std::array<int, N>
test(std::string_view) [with long unsigned int N = 2; std::string_view
= std::basic_string_view<char>]':
test.cpp:16:29:   required from here
   16 |   constexpr auto a = test<2>(s);
      |                      ~~~~~~~^~~
test.cpp:8:26: error: non-constant condition for static assertion
    8 |   static_assert(s.size() > N);
      |                 ~~~~~~~~~^~~
In file included from test.cpp:2:
test.cpp:8:23:   in 'constexpr' expansion of
's.std::basic_string_view<char>::size()'
/opt/homebrew/Cellar/gcc/14.2.0_1/include/c++/14/string_view:230:22:
error: 's' is not a constant expression
  230 |       { return this->_M_len; }
      |                ~~~~~~^~~~~~