Ambiguous associated type.

[Kesanov]

I tried this code:

use std::fmt::Debug;

pub mod a {
    pub type A = ();
    pub mod b {
        pub type B = ();
    }
}

fn build() {
    let a: a::A     = unimplemented!();
    let b: a::b::B  = unimplemented!(); 
    

    println!("{:?} {:?}", a, b);
}

// the same code as above, just implemented with traits

trait ModA {
    type A: Debug;
    type B: ModB;
}
trait ModB {
    type B: Debug;
}

fn build2<A:ModA>() {
    let a: A::A     = unimplemented!(); // works just fine
    let b: A::B::B  = unimplemented!(); // fails to compile

    println!("{:?} {:?}", a, b)
}

I expected to compile it without errors.

Instead, this happened:

error[E0223]: ambiguous associated type
  --> src/main.rs:28:12
   |
28 |     let b: A::B::B  = unimplemented!(); // fails to compile
   |            ^^^^^^^ help: use fully-qualified syntax: `<<A as ModA>::B as Trait>::B`

E0223 states:

The problem here is that we're attempting to take the type of X from MyTrait. Unfortunately, the type of X is not defined, because it's only made concrete in implementations of the trait.

This obviously doesn't apply here - B is as concrete as A. And the type A::B::B is just as specific as A::A and A::B, yet only the former fails to compile!

[jonas-schievink]

Caused by #22519, so closing in favor of that