It means you can't just write `impl Bar for MyType` and get Foo pulled in automatically. You have to write both `impl`s yourself.

The inheritance-like syntax is shorthand for `trait Bar where Self: Foo`, and bounds like this can show up in lots of other places, where they follow the same rules: `MyType` has to impl all of them, and the traits and bounds can have fairly arbitrary (or at least un-tree-like) relationships.

The upcasting thing is a rare exception to the idea that this `Self: Foo` bound is just like any other bound.