error[E0308]: mismatched types
  --> $DIR/if-typeck.rs:19:8
   |
LL |     if f { }
   |        ^ expected bool, found fn item
   |
   = note: expected type `bool`
              found type `fn() {f}`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.