error[E0277]: `Rc<Foo>` cannot be sent between threads safely
  --> $DIR/issue-40827.rs:14:5
   |
LL | fn f<T: Send>(_: T) {}
   |         ---- required by this bound in `f`
...
LL |     f(Foo(Arc::new(Bar::B(None))));
   |     ^ `Rc<Foo>` cannot be sent between threads safely
   |
   = help: within `Bar`, the trait `Send` is not implemented for `Rc<Foo>`
   = note: required because it appears within the type `Bar`
   = note: required because of the requirements on the impl of `Send` for `Arc<Bar>`
   = note: required because it appears within the type `Foo`

error[E0277]: `Rc<Foo>` cannot be shared between threads safely
  --> $DIR/issue-40827.rs:14:5
   |
LL | fn f<T: Send>(_: T) {}
   |         ---- required by this bound in `f`
...
LL |     f(Foo(Arc::new(Bar::B(None))));
   |     ^ `Rc<Foo>` cannot be shared between threads safely
   |
   = help: within `Bar`, the trait `Sync` is not implemented for `Rc<Foo>`
   = note: required because it appears within the type `Bar`
   = note: required because of the requirements on the impl of `Send` for `Arc<Bar>`
   = note: required because it appears within the type `Foo`

error: aborting due to 2 previous errors

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