Weird "cycle detected" error around type_alias_impl_trait and auto traits.

[steffahn]

#![feature(type_alias_impl_trait)]
#![feature(optin_builtin_traits)]

auto trait Auto {}

fn b() -> Box<dyn Auto> { Box::new(c()) }

type Type = impl Sized;

fn c() -> Type {}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0391]: cycle detected when processing `Type`
  --> src/lib.rs:8:1
   |
8  | type Type = impl Sized;
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
note: ...which requires type-checking `b`...
  --> src/lib.rs:6:1
   |
6  | fn b() -> Box<dyn Auto> { Box::new(c()) }
   | ^^^^^^^^^^^^^^^^^^^^^^^
   = note: ...which requires evaluating trait selection obligation `Type: Auto`...
   = note: ...which again requires processing `Type`, completing the cycle
note: cycle used when collecting item types in top-level module
  --> src/lib.rs:1:1
   |
1  | / #![feature(type_alias_impl_trait)]
2  | | #![feature(optin_builtin_traits)]
3  | |
4  | | auto trait Auto {}
...  |
9  | |
10 | | fn c() -> Type {}
   | |_________________^

error[E0391]: cycle detected when processing `Type`
  --> src/lib.rs:8:1
   |
8  | type Type = impl Sized;
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
note: ...which requires type-checking `b`...
  --> src/lib.rs:6:1
   |
6  | fn b() -> Box<dyn Auto> { Box::new(c()) }
   | ^^^^^^^^^^^^^^^^^^^^^^^
   = note: ...which again requires processing `Type`, completing the cycle
note: cycle used when collecting item types in top-level module
  --> src/lib.rs:1:1
   |
1  | / #![feature(type_alias_impl_trait)]
2  | | #![feature(optin_builtin_traits)]
3  | |
4  | | auto trait Auto {}
...  |
9  | |
10 | | fn c() -> Type {}
   | |_________________^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0391`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

@rustbot modify labels: C-bug, T-compiler, F-type_alias_impl_trait, requires-nightly, A-traits.
(Note: optin_builtin_traits only used to demonstrate that any auto trait works, for example Send.)

[taiki-e]

Looks like dup of #55997

[oli-obk]

Yes, it's a duplicate. Closing as such.

For anyone reaching this issue, at the time of writing this comment, the workaround is to put all defining uses into their own module together with the TAIT itself. For all further development on this, follow #55997

#![feature(type_alias_impl_trait)]
#![feature(auto_traits)]

auto trait Auto {}

fn b() -> Box<dyn Auto> { Box::new(foo::c()) }

mod foo {
    type Type = impl Sized;
    
    pub fn c() -> Type {}
}