Closed
Description
I believe the following code should typecheck under nightly (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f3d45f199cadea62341a727f66c52d86):
#![feature(generic_associated_types)]
use std::marker::PhantomData;
struct Id<'id>(PhantomData<fn(&'id ()) -> &'id ()>);
fn new_id() -> Id<'static> {
Id(PhantomData)
}
pub trait HasLifetime where {
type AtLifetime<'a>;
}
pub struct ExistentialLifetime<S: HasLifetime>(S::AtLifetime<'static>);
impl<S: HasLifetime> ExistentialLifetime<S> {
pub fn new<F>(f: F) -> ExistentialLifetime<S>
where for<'id> F: FnOnce(Id<'id>) -> S::AtLifetime<'id> {
ExistentialLifetime(f(new_id()))
}
}
struct ExampleS<'id>(Id<'id>);
struct ExampleMarker;
impl HasLifetime for ExampleMarker {
type AtLifetime<'id> = ExampleS<'id>;
}
fn broken0() -> ExistentialLifetime<ExampleMarker> {
fn new_helper<'id>(id: Id<'id>) -> ExampleS<'id> {
ExampleS(id)
}
ExistentialLifetime::<ExampleMarker>::new(new_helper)
}
fn broken1() -> ExistentialLifetime<ExampleMarker> {
fn new_helper<'id>(id: Id<'id>) -> <ExampleMarker as HasLifetime>::AtLifetime<'id> {
ExampleS(id)
}
ExistentialLifetime::<ExampleMarker>::new(new_helper)
}
fn broken2() -> ExistentialLifetime<ExampleMarker> {
ExistentialLifetime::<ExampleMarker>::new(|id| ExampleS(id))
}Note that this uses generic associated types (AtLifetime<'a>) and it also uses a higher-rank trait bound in fn new.
However, rustc nightly rejects this program with the following error:
Compiling playground v0.0.1 (/playground)
error[E0271]: type mismatch resolving `for<'id> <for<'id> fn(Id<'id>) -> ExampleS<'id> {broken0::new_helper} as FnOnce<(Id<'id>,)>>::Output == <ExampleMarker as HasLifetime>::AtLifetime<'id>`
--> src/lib.rs:38:5
|
38 | ExistentialLifetime::<ExampleMarker>::new(new_helper)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found struct `ExampleS`
|
= note: expected associated type `<ExampleMarker as HasLifetime>::AtLifetime<'_>`
found struct `ExampleS<'_>`
= help: consider constraining the associated type `<ExampleMarker as HasLifetime>::AtLifetime<'_>` to `ExampleS<'_>` or calling a method that returns `<ExampleMarker as HasLifetime>::AtLifetime<'_>`
= note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
note: required by a bound in `ExistentialLifetime::<S>::new`
--> src/lib.rs:18:46
|
17 | pub fn new<F>(f: F) -> ExistentialLifetime<S>
| --- required by a bound in this
18 | where for<'id> F: FnOnce(Id<'id>) -> S::AtLifetime<'id> {
| ^^^^^^^^^^^^^^^^^^ required by this bound in `ExistentialLifetime::<S>::new`
error[E0271]: type mismatch resolving `for<'id> <for<'id> fn(Id<'id>) -> <ExampleMarker as HasLifetime>::AtLifetime<'id> {broken1::new_helper} as FnOnce<(Id<'id>,)>>::Output == <ExampleMarker as HasLifetime>::AtLifetime<'id>`
--> src/lib.rs:46:5
|
46 | ExistentialLifetime::<ExampleMarker>::new(new_helper)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found struct `ExampleS`
|
= note: expected associated type `<ExampleMarker as HasLifetime>::AtLifetime<'_>`
found struct `ExampleS<'_>`
= help: consider constraining the associated type `<ExampleMarker as HasLifetime>::AtLifetime<'_>` to `ExampleS<'_>` or calling a method that returns `<ExampleMarker as HasLifetime>::AtLifetime<'_>`
= note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
note: required by a bound in `ExistentialLifetime::<S>::new`
--> src/lib.rs:18:46
|
17 | pub fn new<F>(f: F) -> ExistentialLifetime<S>
| --- required by a bound in this
18 | where for<'id> F: FnOnce(Id<'id>) -> S::AtLifetime<'id> {
| ^^^^^^^^^^^^^^^^^^ required by this bound in `ExistentialLifetime::<S>::new`
error[E0271]: type mismatch resolving `for<'id> <[closure@src/lib.rs:50:47: 50:64] as FnOnce<(Id<'id>,)>>::Output == <ExampleMarker as HasLifetime>::AtLifetime<'id>`
--> src/lib.rs:50:5
|
50 | ExistentialLifetime::<ExampleMarker>::new(|id| ExampleS(id))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found struct `ExampleS`
|
= note: expected associated type `<ExampleMarker as HasLifetime>::AtLifetime<'_>`
found struct `ExampleS<'_>`
= help: consider constraining the associated type `<ExampleMarker as HasLifetime>::AtLifetime<'_>` to `ExampleS<'_>` or calling a method that returns `<ExampleMarker as HasLifetime>::AtLifetime<'_>`
= note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
note: required by a bound in `ExistentialLifetime::<S>::new`
--> src/lib.rs:18:46
|
17 | pub fn new<F>(f: F) -> ExistentialLifetime<S>
| --- required by a bound in this
18 | where for<'id> F: FnOnce(Id<'id>) -> S::AtLifetime<'id> {
| ^^^^^^^^^^^^^^^^^^ required by this bound in `ExistentialLifetime::<S>::new`
For more information about this error, try `rustc --explain E0271`.
error: could not compile `playground` due to 3 previous errors
I think rustc is wrong to reject this code. As you can see, it fails to unify type <ExampleMarker as HasLifetime>::AtLifetime<'_> with type ExampleS<'_>. But these types should unify!
Version of rustc: 1.60.0-nightly 2022-01-25 8cdb3cd94efece1e17cb.