Compiler panics in ambiguous arrayvec type situation

[grantslatton]

The compiler correctly errors about the ambiguous types, but also panics on this code:

use arrayvec::ArrayVec;

fn main() {
    let mut buffer = ArrayVec::new();
    let x = buffer.last().unwrap().0.clone();
    buffer.reverse(); 
}

error[E0282]: type annotations needed
 --> src/main.rs:5:13
  |
5 |     let x = buffer.last().unwrap().0.clone();
  |             ^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `T`
  |
  = note: type must be known at this point

error[E0609]: no field `0` on type `&_`
 --> src/main.rs:5:36
  |
5 |     let x = buffer.last().unwrap().0.clone();
  |                                    ^

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=d6120e5e2819deb7a546f61bae891657

[jonas-schievink]

Also reproduces on nightly. Panic happens here:

  16: rustc_typeck::check::method::confirm::ConfirmContext::confirm
  17: rustc_typeck::check::method::<impl rustc_typeck::check::FnCtxt>::lookup_method
  18: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  19: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  20: rustc_typeck::check::FnCtxt::check_stmt
  21: rustc_typeck::check::FnCtxt::check_block_with_expected
  22: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  23: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  24: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr

[pnkfelix]

triage: P-medium. removing nomination label.

reasoning: has a PR. tagging as P-medium (since it does produce useful error message before ICE'ing), and removing nomination label.