error[E0277]: expected a `Fn<_>` closure, found `fn() -> str`
  --> $DIR/builtin-fn-must-return-sized.rs:15:27
   |
LL |     foo::<fn() -> str, _>(None, ());
   |     --------------------- ^^^^ expected an `Fn<_>` closure, found `fn() -> str`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Fn<_>` is not implemented for `fn() -> str`
note: required by a bound in `foo`
  --> $DIR/builtin-fn-must-return-sized.rs:10:11
   |
LL | fn foo<F: Fn<T>, T: Tuple>(f: Option<F>, t: T) {
   |           ^^^^^ required by this bound in `foo`

error: aborting due to previous error

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