Aliasing trait with associated types [duplicate]

问题:

This question already has answers here:

Is there any way to create a type alias for multiple traits? (2 answers)

Closed 3 years ago.

Is it possible to create a trait alias with specified associated types? I'm using a method from similar question Type alias for multiple traits

trait Trait {
    type Item;
}

fn print<T>(value: T) where T: Trait<Item=char> {
}

trait Alias: Trait {}
impl<T: Trait<Item=char>> Alias for T {}

fn print_alias<T: Alias>(value: T) {
    print(value)
}

fn main() {
}

However it fails to compile with following error:

<anon>:12:5: 12:10 error: type mismatch resolving `<T as Trait>::Item == char`:
 expected associated type,
    found char [E0271]
<anon>:12     print(value)
              ^~~~~
<anon>:12:5: 12:10 note: required by `print`
<anon>:12     print(value)
              ^~~~~
error: aborting due to previous error

Playpen link: http://is.gd/LE4h6a

回答1:

@Shepmaster's solution solves the problem locally; but you would have to specify the where T: Alias<Item=char> each time. Alternatively, you can solve it globally by requiring that all Alias implements Trait<Item=char>:

trait Alias: Trait<Item=char> {}
impl<T: Trait<Item=char>> Alias for T {}

Which of the global or local solutions is preferred is totally up to you.

回答2:

You have currently only required that all values of Alias must implement Trait, but not that the type of Item must be char. To do that you have to use the following:

trait Alias: Trait<Item=char> {}

回答3:

You still need to specify the associated type in your print_alias method:

fn print_alias<T>(value: T)
    where T: Alias<Item=char>
{
    print(value)
}

The problem is that you've specified that every Trait<Item=char> also implements Alias (and thus Alias<Item=char>), but that doesn't mean that every Alias implements Trait<Item=char>!