Can't invoke solve methods on fixed-width matrices

[RReverser]

Simplified example:

let test_image_matrix = nalgebra::MatrixXx3::from_element(10, 0.0);
let mut db_x_column = nalgebra::MatrixXx1::from_element(10, 0.0);

anyhow::ensure!(test_image_matrix.qr().solve_mut(&mut db_x_column));

gives:

error[E0599]: no method named `solve_mut` found for struct `QR<{float}, Dynamic, nalgebra::U3>` in the current scope
   --> examples\polygons.rs:351:44
    |
351 |     anyhow::ensure!(test_image_matrix.qr().solve_mut(&mut db_x_column));
    |                                            ^^^^^^^^^ method not found in `QR<{float}, Dynamic, nalgebra::U3>`

If changing to fully qualified method path, I can get slightly more precise error:

let test_image_matrix = nalgebra::MatrixXx3::from_element(10, 0.0);
let mut db_x_column = nalgebra::MatrixXx1::from_element(10, 0.0);

anyhow::ensure!(nalgebra::QR::solve_mut(&test_image_matrix.qr(), &mut db_x_column));

error[E0308]: mismatched types
   --> examples\polygons.rs:351:45
    |
351 |     anyhow::ensure!(nalgebra::QR::solve_mut(&test_image_matrix.qr(), &mut db_x_column));
    |                                             ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `Dynamic`, found struct `nalgebra::U3`
    |
    = note: expected reference `&QR<_, _, _>`
               found reference `&QR<{float}, Dynamic, nalgebra::U3>`

So it appears that it wants the 2nd dimension to be Dynamic not fixed U3?

[RReverser]

And yes, changing to DMatrix does allow it to compile, but it's not great that I can't use compile-time dimensions where they make sense.

[sebcrozet]

It looks like a limitation originating from the impl block containing solve:

impl<N: ComplexField, D: DimMin<D, Output = D>> QR<N, D, D>
where
    DefaultAllocator: Allocator<N, D, D> + Allocator<N, D>,
{

This requires the type of row and column dimensions to be the same.
It should probably be changed to something like the following, to make it more flexible and accept your MatrixXx3:

impl<N: ComplexField, R: DimMin<C>, C: Dim> QR<N, R, C>
where
    R: DimEq<C>,
    DefaultAllocator: Allocator<N,R, C> + Allocator<N, DimMinimum<R, C>>,
{

[RReverser]

@sebcrozet Thanks. I'll probably also cc @estebank - I think the compiler errors above could be greatly improved, especially "method not found" in the first example.

[estebank]

I think the compiler errors above could be greatly improved, especially "method not found" in the first example.

I believe there are tickets about these cases already. I agree we can do a better job here.