The documentation of many SparseMatrixBase functions is shared with the corresponding dense functions -- including the code samples. That leads to some open questions, e.g., are block-expressions overwritable and are unaryExpressions called only for non-zeros?
*** Bug 1163 has been marked as a duplicate of this bug. ***
That's indeed an issue that has to be fixed asap. At last I clarified the manual pages for block-expressions:
Summary: Clarify documentation on the restrictions of writable sparse block expressions.
Summary: Bug 828: clarify documentation of SparseMatrixBase's unary methods.
For blocks: https://bitbucket.org/eigen/eigen/commits/d065e32c45f6/
Should be good enough.