[eigen] lazy evaluation in sparse matrix multiply

[Frederik Heinz <Frederik.Heinz@xxxxxxxxxxxx>]

Hi,

in trying out some of the Eigen sparse-matrix functionality, I noticed, 
that unlike dense matrix multiplication, sparse matrices appear to 
default to lazy evaluation of matrix multiplication.

This causes an expression of the type

 M = M * M;

to yield a zero matrix when used with sparse matrices.

Forced evaluation as in

 M = (M * M).eval();

on the other hand, has the desired effect.

I suggest setting the evaluate-before-assigning flag for sparse 
matrix-product expressions.

Regards
Frederik

P.S.: What is the current status of complex Hermitian sparse (or block 
tridiagonal) eigenproblems in Eigen?