error: the type `&dyn Fn(i32) -> _` is not well-formed (chalk)
  --> $DIR/trait-objects.rs:11:12
   |
LL |     let f: &dyn Fn(i32) -> _ = &|x| x + x;
   |            ^^^^^^^^^^^^^^^^^

error[E0277]: `(i32,)` is not a tuple
  --> $DIR/trait-objects.rs:12:5
   |
LL |     f(2);
   |     ^^^^ the trait `Tuple` is not implemented for `(i32,)`
   |
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
   |
LL | fn main() where (i32,): Tuple {
   |           +++++++++++++++++++

error[E0277]: expected a `Fn<(i32,)>` closure, found `dyn Fn(i32) -> i32`
  --> $DIR/trait-objects.rs:12:5
   |
LL |     f(2);
   |     ^^^^ expected an `Fn<(i32,)>` closure, found `dyn Fn(i32) -> i32`
   |
   = help: the trait `Fn<(i32,)>` is not implemented for `dyn Fn(i32) -> i32`

error: aborting due to 3 previous errors

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