error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
  --> $DIR/E0059.rs:13:41
   |
LL | fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) } //~ ERROR E0059
   |                                         ^^^^

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0059"