Why can't you just return a definition in c++?

Why can't you just return a definition?

pair<int,int>
get_char_index(const string &userStr){
    if (!userStr.empty()) {
        return pairType v{1,userStr.size()}; // Error
        // return pairType {1,userStr.size()}; // OK
    } else {
        return make_pair(1,0);
    }
}