[rustc.git] / src / librustc_error_codes / error_codes / E0223.md

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.
Commit	Line	Data
60c5eb7d	1	An attempt was made to retrieve an associated type, but the type was ambiguous.
dfeec247 XL	2
dfeec247 XL	3	Erroneous code example:
60c5eb7d XL	4
	5	```compile_fail,E0223
	6	trait MyTrait {type X; }
	7
	8	fn main() {
	9	let foo: MyTrait::X;
	10	}
	11	```
	12
	13	The problem here is that we're attempting to take the type of X from MyTrait.
	14	Unfortunately, the type of X is not defined, because it's only made concrete in
	15	implementations of the trait. A working version of this code might look like:
	16
	17	```
	18	trait MyTrait {type X; }
	19	struct MyStruct;
	20
	21	impl MyTrait for MyStruct {
	22	type X = u32;
	23	}
	24
	25	fn main() {
	26	let foo: <MyStruct as MyTrait>::X;
	27	}
	28	```
	29
	30	This syntax specifies that we want the X type from MyTrait, as made concrete in
	31	MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
	32	might implement two different traits with identically-named associated types.
	33	This syntax allows disambiguation between the two.