Inherent associated type projection does not respect `where` clauses

[compiler-errors]

I tried this code:

#![feature(inherent_associated_types)]

struct Foo<T: ?Sized>(T);

impl<T> Foo<T> {
    type Bar = i32;
}

fn main() {
    let x: Foo<[u8]>::Bar = 1;
}

I expected to see it fail, since while Foo<[u8]> is WF, it does not satisfy the where-clauses of the impl that it's selecting the associated type Bar from.

[fmease]

For comparison, the analogous code containing an (inherent) associated constant instead leads to the following error:

error[E0599]: the associated item `BAR` exists for struct `Foo<[u8]>`, but its trait bounds were not satisfied
 --> src/main.rs:8:30
  |
1 | struct Foo<T: ?Sized>(T);
  | --------------------- associated item `BAR` not found for this struct
...
8 |     let _: () = Foo::<[u8]>::BAR;
  |                              ^^^ associated item cannot be called on `Foo<[u8]>` due to unsatisfied trait bounds
  |
  = note: the following trait bounds were not satisfied:
          `[u8]: Sized`

Code:

struct Foo<T: ?Sized>(T);

impl<T> Foo<T> {
    const BAR: () = ();
}

fn main() {
    let _: () = Foo::<[u8]>::BAR;
}

@rustbot claim

(edit: I've just noticed it says associated item cannot be called even though we're not calling anything ^^ well, that's a separate issue & I'm gonna create a GitHub issue for it sometime later)

[fmease]

Ah 😅 what's more, we don't actually take the concrete Self type into consideration during probing / resolution:

#![feature(inherent_associated_types)]

struct S<T>(T);

impl S<u8> {
    type P = ();
}

impl S<String> {
    type P = bool;
}

fn main() {
    let _: S<String>::P = false;
    //                    ^^^^^ expected `()`, found `bool` // <- this is incorrect!
    //                                                            the code should pass tycheck
}

I consider this to be part of this issue or at least strongly related to it. In any case, I am already looking into fixing this. Let's see if I can actually manage to do so!

Edit: I already have a working patch locally for which I am gonna open a PR after the weekend :)