error[E0308]: mismatched types
  --> $DIR/issue-68295.rs:43:5
   |
LL | fn crash<R, C>(input: Matrix<R, C, ()>) -> Matrix<R, C, u32>
   |                                            ----------------- expected `Matrix<R, C, u32>` because of return type
...
LL |     input.into_owned()
   |     ^^^^^^^^^^^^^^^^^^ expected `Matrix<R, C, u32>`, found `Matrix<R, C, ...>`
   |
   = note: expected struct `Matrix<_, _, u32>`
              found struct `Matrix<_, _, <() as Allocator<R, C>>::Buffer>`
   = help: consider constraining the associated type `<() as Allocator<R, C>>::Buffer` to `u32`
   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.