error[E0308]: mismatched types
  --> $DIR/issue-78722-2.rs:17:20
   |
LL | type F = impl core::future::Future<Output = u8>;
   |          -------------------------------------- the expected future
...
LL |         let f: F = async { 1 };
   |                -   ^^^^^^^^^^^ expected future, found `async` block
   |                |
   |                expected due to this
   |
   = note: expected opaque type `F`
            found `async` block `{async block@$DIR/issue-78722-2.rs:17:20: 17:25}`

error[E0271]: expected `{async block@$DIR/issue-78722-2.rs:14:13: 14:18}` to be a future that resolves to `u8`, but it resolves to `()`
  --> $DIR/issue-78722-2.rs:12:30
   |
LL |         fn concrete_use() -> F {
   |                              ^ expected `u8`, found `()`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0271, E0308.
For more information about an error, try `rustc --explain E0271`.