I want to get the diagonal part of a sparse matrix as follows:

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typedef Eigen::SparseMatrix<std::complex<double>> SpMatcd;
SpMatcd A(4,4);
   A.insert(0,0)=1;
   A.insert(1,0)=1;
   A.insert(1,1)=-3;
   A.insert(2,2)=2;
SpMatcd  B=A.diagonal().asDiagonal();

but a compiler error occurs:
error: conversion from ‘const Eigen::DiagonalWrapper<const Eigen::Diagonal<const Eigen::SparseMatrix<std::complex<double> >, 0> >’ to non-scalar type ‘Eigen::Diagonal<Eigen::SparseMatrix<std::complex<double> >, 0>’

So how to fix it?