Is it safe to cast arbitrary values of the underlying type to a strongly-typed enum type?

If I have a strongly-typed enum, with say, underlying type int, is it ok to cast an int value that does not match any enumerator to the enum type?

enum e1 : int { x = 0, y = 1 };
enum class e2 : int { x = 0, y = 1 };

int main() {
        e1 foo = static_cast<e1>(42); // is this UB?
        e2 bar = static_cast<e2>(42);
}