SparseArrays.jl

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using SparseArrays, LinearAlgebra
A = rand(4,4)
S = sprand(4,4,0.1)
b = rand(4)
Aqr = qr(A)
Sqr = qr(S)
Aqr \ b # works!
Sqr \ b # works!
ldiv!(x, Aqr, b) # works!
ldiv!(x, Sqr, b) # fails!!!!!!!!!!!!!

is this even supported in SPQR? i couldn't find anything in the user doc

I will have to dig deeper into what SPQR is doing, I'm only really familiar with the UMFPACK and KLU code.

SPQR does not support ldiv! natively. What do you want ldiv! to do @ChrisRackauckas? We can define a copying form, that copies to x.

It should just give an error saying that the operation is not available, since you can't t do these things in-place.

There's a MethodError now, do you think it should be more specific?

Yes, I think an error message might make sense, since MethodError makes people think it is an unimplemented case.