error[E0214]: parenthesized type parameters may only be used with a `Fn` trait
  --> $DIR/unboxed-closure-sugar-used-on-struct.rs:7:18
   |
LL | fn foo(b: Box<Bar()>) {
   |                  ^^ only `Fn` traits may use parentheses

error[E0107]: wrong number of type arguments: expected 1, found 0
  --> $DIR/unboxed-closure-sugar-used-on-struct.rs:7:15
   |
LL | fn foo(b: Box<Bar()>) {
   |               ^^^^^ expected 1 type argument

error: aborting due to 2 previous errors

Some errors occurred: E0107, E0214.
For more information about an error, try `rustc --explain E0107`.