Due to a runtime assert, it is currently not possible to extract fixed-sized blocks where one dimension is zero. (A use-case for that is to reduce trivial branches in user code). The local fix is trivial, but for proper unit-tests some other parts require adaption as well.
hm, actually a workaround is: MatrixXd m; m.block<0,N>(i,j, 0,N);
https://bitbucket.org/eigen/eigen/commits/c35c53729a53/
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