Using associated types in async fn type break typing

[Ekleog]

With the following code:

trait Trait {
    type Assoc;
}

async fn foo<T: Trait<Assoc = ()>>() -> T::Assoc {
    ()
}

comes the following error:

error[E0271]: type mismatch resolving `<impl std::future::Future as std::future::Future>::Output == <T as Trait>::Assoc`
 --> src/lib.rs:7:41
  |
7 | async fn foo<T: Trait<Assoc = ()>>() -> T::Assoc {
  |                                         ^^^^^^^^ expected (), found associated type
  |
  = note: expected type `()`
             found type `<T as Trait>::Assoc`
  = note: the return type of a function must have a statically known size

error: aborting due to previous error

(playground link)

However, this function compiles correctly without the async keyword.

[Ekleog]

cc #50547

[cramertj]

You can trigger this without async/await using impl Trait:

struct Foo<T>(T);

trait FooLike { type Output; }
impl<T> FooLike for Foo<T> {
    type Output = T;
}


trait Trait {
    type Assoc;
}

fn foo<T: Trait<Assoc = ()>>() -> impl FooLike<Output = T::Assoc> {
    Foo(())
}

error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as Trait>::Assoc`
  --> src/main.rs:13:35
   |
13 | fn foo<T: Trait<Assoc = ()>>() -> impl FooLike<Output = T::Assoc> {
   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found associated type
   |
   = note: expected type `()`
              found type `<T as Trait>::Assoc`
   = note: the return type of a function must have a statically known size

cc @oli-obk @nikomatsakis

[oli-obk]

Some more reduction in genericity and clearing up a few things:

#![feature(impl_trait_in_bindings)]

struct Foo;

trait FooLike { type Output; }
impl FooLike for Foo {
    type Output = u32;
}

trait Trait {
    type Assoc;
}

fn foo<T: Trait<Assoc = i32>>() {
    let _: impl FooLike<Output = T::Assoc> = Foo;
}

error[E0271]: type mismatch resolving `<Foo as FooLike>::Output == <T as Trait>::Assoc`
  --> src/main.rs:15:12
   |
15 |     let _: impl FooLike<Output = T::Assoc> = Foo;
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected u32, found associated type
   |
   = note: expected type `u32`
              found type `<T as Trait>::Assoc`
   = note: the return type of a function must have a statically known size

The problem is that T::Assoc = i32 is not know in impl trait items. It's also not related to the return position, but refers to any impl trait in the function. Outside of impl trait T::Assoc = i32 can be resolved without a problem, so it must be something related to the way impl trait reuses the generics of its parent.

[nikomatsakis]

It seems to be a missing normalization, most likely.