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|
A type mismatched an associated type of a trait.
Erroneous code example:
```compile_fail,E0271
trait Trait { type AssociatedType; }
fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
// ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
// | |
// This says `foo` can |
// only be used with |
// some type that |
// implements `Trait`. |
// |
// This says not only must
// `T` be an impl of `Trait`
// but also that the impl
// must assign the type `u32`
// to the associated type.
println!("in foo");
}
impl Trait for i8 { type AssociatedType = &'static str; }
//~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// | |
// `i8` does have |
// implementation |
// of `Trait`... |
// ... but it is an implementation
// that assigns `&'static str` to
// the associated type.
foo(3_i8);
// Here, we invoke `foo` with an `i8`, which does not satisfy
// the constraint `<i8 as Trait>::AssociatedType=u32`, and
// therefore the type-checker complains with this error code.
```
The issue can be resolved by changing the associated type:
1) in the `foo` implementation:
```
trait Trait { type AssociatedType; }
fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
println!("in foo");
}
impl Trait for i8 { type AssociatedType = &'static str; }
foo(3_i8);
```
2) in the `Trait` implementation for `i8`:
```
trait Trait { type AssociatedType; }
fn foo<T>(t: T) where T: Trait<AssociatedType = u32> {
println!("in foo");
}
impl Trait for i8 { type AssociatedType = u32; }
foo(3_i8);
```
|
