Don't use __builtin_strlen #4423
Description
When upgrading {fmt}, b4d1d7f8e broke some of our usage. We had a place where we were building up a format string using a type like this:
template <size_t N>
struct fixed_string {
char data[N] = {};
constexpr fixed_string(char const (&m)[N]) {
for (size_t i = 0; i != N; ++i) {
data[i] = m[i];
}
}
};And trying to do something like:
static constexpr auto f = fixed_string("x={}");
fmt::print(f.data, 42);This works on fmt 10.2.1 and 11.0, fails on trunk — after that commit, with:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:543:31: error: '__builtin_strlen(((const char*)(& f.fixed_string<5>::data)))' is not a constant expression
543 | size_ = __builtin_strlen(detail::narrow(s));
| ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~gcc says that __builtin_strlen isn't usable like this. Which... if they say so. But seems unlikely this will change, so I think {fmt} should also revert to previous usage.