could type inference fill in the `_` in `expr as Box<_>` when expecting `Box<Trait>` ?

[pnkfelix]

Could type inference fill in expr as Box<_> when expecting Box<Trait> ?

From what I have seen, it seems to make no attempt to use the surrounding context to fill in gaps like the _ in Box<_> above.

spawned from #22086

[pnkfelix]

cc #22181

[steveklabnik]

Triage: box stuff is still up for debate.

[Mark-Simulacrum]

I'm not quite sure how to triage this, since this doesn't compile at all today. Perhaps this should be closed until as Box<_> becomes possible?

error: non-scalar cast: `i32` as `std::boxed::Box<std::fmt::Display>`
 --> test.rs:4:27
  |
4 |     let t: Box<Display> = 10 as Box<_>;
  |                           ^^^^^^^^^^^^

error: aborting due to previous error

[Mark-Simulacrum]

Per discussion on IRC with @aturon I'm going to close. It doesn't seem helpful to track this, since in almost all or all cases, inferring the trait would likely lead to surprising behavior, and so we probably don't want to do this anyway.