There are several operator= overloads and some compilers can be confused.

This is not present in 3.4. I cannot get my compiler options but here is the error I get:

error: use of overloaded operator '=' is ambiguous (with operand types 'cv::Mat' and 'int')
meta = 0;
~ ^ ~
./include/opencv2/core/mat.hpp:1070:10: note: candidate function
Mat& operator = (const Mat& m);
^
./include/opencv2/core/mat.hpp:1257:10: note: candidate function
Mat& operator = (const Scalar& s);
^
./include/opencv2/core/mat.hpp:2099:10: note: candidate function
Mat& operator = (Mat&& m);

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