error[E0277]: expected a `std::ops::FnMut<(_, char)>` closure, found `()`
  --> $DIR/issue-23966.rs:12:16
   |
LL |     "".chars().fold(|_, _| (), ());
   |                ^^^^ expected an `FnMut<(_, char)>` closure, found `()`
   |
   = help: the trait `std::ops::FnMut<(_, char)>` is not implemented for `()`

error: aborting due to previous error

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