error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
  --> $DIR/E0059.rs:3:41
   |
LL | fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
   |                                         ^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0059`.