Order of operands to equality expression matters when inferring a AsRef implementation

[shepmaster]

This may be related to or the same as #23673, but I wanted to file it anyway as it seems to have a slightly different flavor.

#![feature(convert)]

struct Container {
    i: u8,
    b: bool,
}

impl AsRef<u8> for Container {
    fn as_ref(&self) -> &u8 {
        &self.i
    }
}

// A second implementation to make the `as_ref` ambiguous without further information
impl AsRef<bool> for Container {
    fn as_ref(&self) -> &bool {
        &self.b
    }
}

fn main() {
    let c = Container { i: 42, b: true };

    // Succeeds
    &42u8 == c.as_ref();

    // Fails
    c.as_ref() == &42u8;
}

Fails with

type annotations required: cannot resolve `Container : core::convert::AsRef<_>` [E0283]

However, if you flip the order of the arguments to the equality operator, then the code compiles and the correct type is inferred. It seems as if both forms should work the same.

Originally from this Stack Overflow question

[hawkw]

I've seen what (appears to be) a similar issue when trying to use as_ref() in a pattern guard expression.

[brson]

Still repros.

[steveklabnik]

Triage: still reproduces

error[E0283]: type annotations needed for `Container`
  --> src/main.rs:28:7
   |
22 |     let c = Container { i: 42, b: true };
   |         - consider giving `c` a type
...
28 |     c.as_ref() == &42u8;
   |       ^^^^^^ cannot infer type for struct `Container`
   |
   = note: cannot resolve `Container: std::convert::AsRef<_>`

[krinistof]

Although the compiler emits a helpful message, this issue is still reproducible.

error[E0283]: type annotations needed
  --> test.rs:27:7
   |
27 |     c.as_ref() == &42u8;
   |       ^^^^^^
   |
note: multiple `impl`s satisfying `Container: AsRef<_>` found
  --> test.rs:7:1
   |
7  | impl AsRef<u8> for Container {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
14 | impl AsRef<bool> for Container {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: try using a fully qualified path to specify the expected types
   |
27 -     c.as_ref() == &42u8;
27 +     <Container as AsRef<T>>::as_ref(&c) == &42u8;
   |