TAIT: Compiler struggles to infer the type

[toiglak]

I tried this code:

#![feature(type_alias_impl_trait)]

trait Foo {}

impl<T: Foo> Foo for &T {}

trait Bar {
    // I need this associated type to be implemented separately.
    // This is why I didn't put it inside `Baz` definition.
    type Bar<'a>: Foo;
}

trait Baz: Bar {
    fn baz<'a>(&'a self) -> Self::Bar<'a>;
}

// Problem:

struct Qux<T: Foo> {
    field: T,
}

impl<T: Foo> Bar for Qux<T> {
    type Bar<'a> = impl Foo + 'a; // Error: unconstrained opaque type.
                                  // `Bar` must be used in combination with a concrete type within
                                  // the same module.
}

impl<T: Foo> Baz for Qux<T> {
    fn baz<'a>(&'a self) -> Self::Bar<'a> {
        &self.field // Error: expected opaque type `<Qux<T> as Bar>::Bar<'a>`.
                    //        found reference `&T`
    }
}

fn main() {}

I got the exact errors visible in the comments.
I expected this to compile, but it didn't.

I found a following workaround:

trait Bar {
    type Bar<'a>: Foo where Self: 'a; // <--
}

// ...

struct Qux<T: Foo> {
    field: T,
}

type Temp<'a, T: Foo + 'a> = impl Foo + 'a; // <--

impl<T: Foo> Bar for Qux<T> {
    type Bar<'a> = Temp<'a, T> where Self : 'a; // <--
}

impl<T: Foo> Baz for Qux<T> {
    fn baz<'a>(&'a self) -> Self::Bar<'a> {
        &self.field
    }
}

Meta

rustc --version --verbose:

rustc 1.65.0-nightly (cf9ed0dd5 2022-09-15)
binary: rustc
commit-hash: cf9ed0dd5836201843d28bbad50abfbe1913af2a
commit-date: 2022-09-15
host: aarch64-apple-darwin
release: 1.65.0-nightly
LLVM version: 15.0.0

[jplatte]

@rustbot label F-type_alias_impl_trait T-compiler T-types

[oli-obk]

Yea... this is mostly expected. The impl Trait is inside one impl, and only that impl can constrain it.

I think we could allow impls for traits to restrict opaque types from supertrait's impls in the same crate.