View Compilation Error

0_0_21842566_27094.cpp: In function 'Mat query(int, int, int, int, int)':
0_0_21842566_27094.cpp:96:88: error: invalid initialization of non-const reference of type 'Mat&' from an rvalue of type 'Mat'
   return mul(query(lson[rt], l, mid, ql, mid), query(rson[rt], mid + 1, r, mid + 1, qr));
                                                                                        ^
0_0_21842566_27094.cpp:18:12: note: in passing argument 1 of 'Mat mul(Mat&, Mat&)'
 inline Mat mul(Mat &a, Mat &b)
            ^