error[E0277]: the trait bound `(): std::ops::FnMut<(_, char)>` is not satisfied
  --> $DIR/issue-23966.rs:12:16
   |
LL |     "".chars().fold(|_, _| (), ());
   |                ^^^^ 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`.