https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113629
--- Comment #2 from 康桓瑋 <hewillk at gmail dot com> --- more reduced: struct Base { operator int(this auto&&) { return 42; } }; int main() { Base b; // return static_cast<int>(Base{}); // ok return static_cast<int>(b); // error } https://godbolt.org/z/qGrbf4rj7