confusing compiler error for dotting a reference with a non-reference

[imyxh]

Consider:

let a = array![
    [1., 2.],
    [3., 4.],
];
dbg!((&a).dot(a));

yielding

error[E0275]: overflow evaluating the requirement `&ArrayBase<_, _>: Not`
  --> src/main.rs:22:15
   |
22 |     dbg!((&a).dot(a));
   |               ^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate
   = note: required for `&ArrayBase<_, _>` to implement `Not`
   = note: 127 redundant requirements hidden
   = note: required for `&ArrayBase<OwnedRepr<{float}>, Dim<[usize; 2]>>` to implement `Not`

I find this error confusing—it took me a while (in real code) to realize that I just forgot to take a reference of the second matrix.

Possibly related to #1439?

[akern40]

Hrm, probably something broken in our Dot implementations - they can be a little tricky. What version of ndarray are you running?

[imyxh]

[dependencies.ndarray]
version = "0.16.1"
features = ["approx"]

[imyxh]

but reproducible on versions way back, e.g. 0.13.0, with no features enabled, so if it's a regression, it's probably not due to changes in this crate

[akern40]

I'll try to take a look soon - maybe there's something we can change about how we implement dot products to make this more readable.

[akern40]

On 0.17.*, this error is now much more interpretable

error[E0308]: mismatched types
   --> src/main.rs:5:19
    |
  5 |     dbg!((&a).dot(a));
    |               --- ^ expected `&_`, found `ArrayBase<..., ..., {float}>`
    |               |
    |               arguments to this method are incorrect
    |
    = note: expected reference `&_`
                  found struct `ArrayBase<OwnedRepr<{float}>, Dim<[usize; 2]>, {float}>`
note: method defined here
   --> /Users/adamkern/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ndarray-0.17.1/src/linalg/impl_linalg.rs:298:12
    |
298 |     pub fn dot<Rhs: ?Sized>(&self, rhs: &Rhs) -> <Self as Dot<Rhs>>::Output
    |            ^^^
help: consider borrowing here
    |
  5 |     dbg!((&a).dot(&a));
    |                   +

[imyxh]

much better!!