#include <string>

int main(int argc, char *argv[])
{
    std::string str;
    char16_t c1; // error: unknown type named char16_t
    char32_t c2; // error: unknown type named char32_t
    std::u16string str16; // error: no type named 'u16string' in namespace 'std'.
    std::u32string str32; // error: no type named 'u32string' in namespace 'std'.
}