Perhaps I am missing something, but a quick look into SparseCore/SparseMatrixBase.h shows that there is no interface for mixed storage addition (there is for sparse/sparse through the plugin plugins/CommonCwiseBinaryOp.h) although the SparseCore/SparseCwiseBinaryOp.h seems to contain the implementation for that. It seems that the intention is there, but the execution is lacking...
*** This bug has been marked as a duplicate of bug 632 ***
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