arbitrary self types causes unexpected error messages

[nikomatsakis]

This example:

#![feature(arbitrary_self_types)]
trait Test<T: core::ops::Deref<Target = Self>> {
    fn is_some(self: T);
}

fn f() {
    let x = Some(2);
    if x.is_some() {
        println!("Some");
    }
}

Playground link

gives the error

error[E0277]: the trait bound `std::option::Option<{integer}>: std::ops::Deref` is not satisfied
 --> src/lib.rs:8:10
  |
8 |     if x.is_some() {
  |          ^^^^^^^ the trait `std::ops::Deref` is not implemented for `std::option::Option<{integer}>`

error[E0308]: mismatched types
 --> src/lib.rs:8:8
  |
8 |     if x.is_some() {
  |        ^^^^^^^^^^^ expected `bool`, found `()`

error: aborting due to 2 previous errors

rather than an error because the Test trait is not implemented.

This is presumably just a case where improved diagnostics would be good. The inherent is_some method winds up not triggering because it requires auto-ref rather than being passed "by value".

Originally posted by @tguser402 in #66312 (comment)

[estebank]

This shouldn't be marked as [requires-nightly] because the same problem happens with Pin, which is stable.

[estebank]

Cc #66289

[estebank]

We currently provide a suggestion that is missing parens to be fully correct:

error[[E0277]](https://doc.rust-lang.org/nightly/error_codes/E0277.html): the trait bound `Option<{integer}>: Deref` is not satisfied
 --> src/lib.rs:8:10
  |
8 |     if x.is_some() {
  |          ^^^^^^^ the trait `Deref` is not implemented for `Option<{integer}>`
  |
note: required by a bound in `Test::is_some`
 --> src/lib.rs:2:15
  |
2 | trait Test<T: core::ops::Deref<Target = Self>> {
  |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Test::is_some`
3 |     fn is_some(self: T);
  |        ------- required by a bound in this associated function
help: consider borrowing here
  |
8 |     if &x.is_some() {
  |        +
8 |     if &mut x.is_some() {
  |        ++++

error[[E0308]](https://doc.rust-lang.org/nightly/error_codes/E0308.html): mismatched types
 --> src/lib.rs:8:8
  |
8 |     if x.is_some() {
  |        ^^^^^^^^^^^ expected `bool`, found `()`