error[E0277]: expected a `Fn<()>` closure, found `extern "C" fn() {f}`
  --> $DIR/extern-wrong-value-type.rs:9:11
   |
LL |     is_fn(f);
   |     ----- ^ expected an `Fn<()>` closure, found `extern "C" fn() {f}`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Fn<()>` is not implemented for fn item `extern "C" fn() {f}`
   = note: wrap the `extern "C" fn() {f}` in a closure with no arguments: `|| { /* code */ }`
note: required by a bound in `is_fn`
  --> $DIR/extern-wrong-value-type.rs:4:28
   |
LL | fn is_fn<F>(_: F) where F: Fn() {}
   |                            ^^^^ required by this bound in `is_fn`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.