constexpr function returning std::string

Question

Why does this program fails to compile in C++20? Checked with both GCC and Clang. Assignment to both str2 and str3 fails Compiler explorer link: https://gcc.godbolt.org/z/eEWf4avqW

#include <string>
constexpr std::string fun2() {
    return std::string("a string");
}
constexpr std::string fun3() {
    using namespace std::literals::string_literals;
    return "a string"s;
}
int main() {
    constexpr std::string str2 = fun2();
    constexpr std::string str3 = fun3();
    return 0;
}