foo.cc: In function 'Matrix3& operator+(Matrix3&, Matrix3&)':
foo.cc:34:9: warning: reference to local variable 'tmp' returned [-Wreturn-local-addr]
   34 |  return tmp;
      |         ^~~
foo.cc:28:10: note: declared here
   28 |  Matrix3 tmp;
      |          ^~~
foo.cc: In function 'Matrix3& operator-(Matrix3&, Matrix3&)':
foo.cc:43:9: warning: reference to local variable 'tmp' returned [-Wreturn-local-addr]
   43 |  return tmp;
      |         ^~~
foo.cc:37:10: note: declared here
   37 |  Matrix3 tmp;
      |          ^~~
foo.cc: In function 'Matrix3& operator*(Matrix3&, int)':
foo.cc:52:9: warning: reference to local variable 'tmp' returned [-Wreturn-local-addr]
   52 |  return tmp;
      |         ^~~
foo.cc:46:10: note: declared here
   46 |  Matrix3 tmp;
      |          ^~~
foo.cc: In function 'Matrix3& operator*(Matrix3&, Matrix3&)':
foo.cc:61:9: warning: reference to local variable 'tmp' returned [-Wreturn-local-addr]
   61 |  return tmp;
      |         ^~~
foo.cc:55:10: note: declared here
   55 |  Matrix3 tmp;
      |          ^~~

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