Failed to unify types when using GATs and HRTBs #93341
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.