error[E0593]: function is expected to take a single 0-tuple as argument, but it takes 2 distinct arguments
  --> $DIR/issue-47706-trait.rs:3:24
   |
LL |     fn f(&self, _: ()) {
   |     ------------------ takes 2 distinct arguments
LL |         None::<()>.map(Self::f);
   |                    --- ^^^^^^^ expected function that takes a single 0-tuple as argument
   |                    |
   |                    required by a bound introduced by this call
   |
note: required by a bound in `Option::<T>::map`
  --> $SRC_DIR/core/src/option.rs:LL:COL

error: aborting due to previous error

For more information about this error, try `rustc --explain E0593`.