SparseVector class has two function members:
internal::CompressedStorage<Scalar,Index>& _data() { return m_data; }
internal::CompressedStorage<Scalar,Index>& _data() const { return m_data; }

The second member is missing a const qualifier at the beginning of the definition.

I would suggest that the name of the above functions are changed to data(), as is the case in SparseMatrix class.

I would also add a public typedef:
typedef internal::CompressedStorage<Scalar,Index> Storage;

You're right, thank you for the finding.