Error code E0792
A type alias impl trait can only have its hidden type assigned when used fully generically (and within their defining scope). This means
ⓘ
is not accepted. If it were accepted, one could create unsound situations like
#![feature(type_alias_impl_trait)]
type Foo<T> = impl Default;
fn foo() -> Foo<u32> {
5u32
}
fn main() {
let x = Foo::<&'static mut String>::default();
}
ⓘ
Instead you need to make the function generic:
#![feature(type_alias_impl_trait)]
type Foo<T> = impl std::fmt::Debug;
fn foo<U>() -> Foo<U> {
5u32
}
fn main() {}
This means that no matter the generic parameter to foo
,
the hidden type will always be u32
.
If you want to link the generic parameter to the hidden type,
you can do that, too:
#![feature(type_alias_impl_trait)]
use std::fmt::Debug;
type Foo<T: Debug> = impl Debug;
fn foo<U: Debug>() -> Foo<U> {
Vec::<U>::new()
}
fn main() {}