error[E0277]: expected a `FnOnce<()>` closure, found `()`
  --> $DIR/issue-63279.rs:8:11
   |
LL |     || -> Closure { || () }
   |           ^^^^^^^ expected an `FnOnce<()>` closure, found `()`
   |
   = help: the trait `FnOnce<()>` is not implemented for `()`
   = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`

error[E0308]: mismatched types
  --> $DIR/issue-63279.rs:8:21
   |
LL |     || -> Closure { || () }
   |                     ^^^^^ expected `()`, found closure
   |
   = note: expected unit type `()`
                found closure `[closure@$DIR/issue-63279.rs:8:21: 8:26]`

error[E0308]: mismatched types
  --> $DIR/issue-63279.rs:8:5
   |
LL | type Closure = impl FnOnce();
   |                ------------- the expected opaque type
LL | 
LL | fn c() -> Closure {
   |           ------- expected `Closure` because of return type
LL |     || -> Closure { || () }
   |     ^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found closure
   |
   = note: expected opaque type `Closure`
                  found closure `[closure@$DIR/issue-63279.rs:8:5: 8:28]`

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.