error[E0214]: parenthesized type parameters may only be used with a `Fn` trait
  --> $DIR/unboxed-closure-sugar-used-on-struct.rs:7:15
   |
LL | fn foo(b: Box<Bar()>) {
   |               ^^^^^ only `Fn` traits may use parentheses

error[E0107]: this struct takes 1 generic argument but 0 generic arguments were supplied
  --> $DIR/unboxed-closure-sugar-used-on-struct.rs:7:15
   |
LL | fn foo(b: Box<Bar()>) {
   |               ^^^ expected 1 generic argument
   |
note: struct defined here, with 1 generic parameter: `A`
  --> $DIR/unboxed-closure-sugar-used-on-struct.rs:3:8
   |
LL | struct Bar<A> {
   |        ^^^ -
help: add missing generic argument
   |
LL | fn foo(b: Box<Bar(A)>) {
   |                   +

error: aborting due to 2 previous errors

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