error[E0391]: cycle detected when computing type of `m::Foo::{opaque#0}`
  --> $DIR/inference-cycle.rs:5:16
   |
LL |     type Foo = impl std::fmt::Debug;
   |                ^^^^^^^^^^^^^^^^^^^^
   |
note: ...which requires type-checking `m::bar`...
  --> $DIR/inference-cycle.rs:15:9
   |
LL |         is_send(foo()); // Today: error
   |         ^^^^^^^
   = note: ...which requires evaluating trait selection obligation `impl core::fmt::Debug: core::marker::Send`...
   = note: ...which again requires computing type of `m::Foo::{opaque#0}`, completing the cycle
note: cycle used when checking item types in module `m`
  --> $DIR/inference-cycle.rs:4:1
   |
LL | mod m {
   | ^^^^^

error[E0308]: mismatched types
  --> $DIR/inference-cycle.rs:19:22
   |
LL |     type Foo = impl std::fmt::Debug;
   |                -------------------- the expected opaque type
...
LL |         let f: Foo = 22_u32;
   |                ---   ^^^^^^ expected opaque type, found `u32`
   |                |
   |                expected due to this
   |
   = note: expected opaque type `impl Debug`
                     found type `u32`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0308, E0391.
For more information about an error, try `rustc --explain E0308`.