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An attempt was made to retrieve an associated type, but the type was ambiguous.
Erroneous code example:
```compile_fail,E0223
trait MyTrait {type X; }
fn main() {
let foo: MyTrait::X;
}
```
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. A working version of this code might look like:
```
trait MyTrait {type X; }
struct MyStruct;
impl MyTrait for MyStruct {
type X = u32;
}
fn main() {
let foo: <MyStruct as MyTrait>::X;
}
```
This syntax specifies that we want the X type from MyTrait, as made concrete in
MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
might implement two different traits with identically-named associated types.
This syntax allows disambiguation between the two.
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