missing qr(A)' \ b for sparse

[stevengj]

If A is a sparse matrix, then we get

julia> qr(A)' \ c
ERROR: MethodError: no method matching adjoint(::SuiteSparse.SPQR.QRSparse{Float64,Int64})

It would be nice to have this implemented.

For example, when minimizing

(least-square + affine), as discussed on discourse, you need to solve systems with both A and A' so it would be nice to easily re-use the QR factorization.

Another missing method is qr(A'). A workaround is to use qr(oftype(A, A')), but that's a bit annoying.

[RoyiAvital]

Related to

• Missing ldiv! overload on sparse QR #242.
• LQ factorization is not implemented for sparse matrices #114.