On Thu, 19 Dec 2024, 14:35 Yu Sheng Oh via Gcc-help, <gcc-help@xxxxxxxxxxx>
wrote:
> 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)
>
s is a function parameter, that is never a constant expression. It doesn't
matter that it's a constexpr function, and it even wouldn't matter if it
was consteval.
7 {
> 8 static_assert(s.size() > N);
>
You cannot use s here because it's not a constant expression.
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; }
> | ~~~~~~^~~~~~
>
