error[E0277]: the size for values of type `dyn for<'a> Fn(&'a isize) -> isize` cannot be known at compilation time --> $DIR/issue-18919.rs:3:15 | LL | fn ho_func(f: Option) { | ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `dyn for<'a> Fn(&'a isize) -> isize` note: required by an implicit `Sized` bound in `Option` --> $DIR/issue-18919.rs:7:13 | LL | enum Option { | ^ required by the implicit `Sized` requirement on this bound in `Option` help: you could relax the implicit `Sized` bound on `T` if it were used through indirection like `&T` or `Box` --> $DIR/issue-18919.rs:7:13 | LL | enum Option { | ^ this could be changed to `T: ?Sized`... LL | Some(T), | - ...if indirection were used here: `Box` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0277`.