Cool, thanks. The original code was using `std::array` as the function
parameter, but when I changed it to `std::string_view`, it broke the
compilation. Both `size()` functions are non-static members, which
makes me think that if the context is a constant expression, then
static_assert should apply. To conclude, if an object is not
constant-evaluated syntactically in a local context (in this case as a
function parameter), the compiler still will look at the constexpr
member function definition to determine if it could be consteval in a
constant expression context, am i correct? Cheers.
On Fri, Dec 20, 2024 at 6:19 AM Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote:
>
>
>
> 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; }
>> | ~~~~~~^~~~~~
