Misc 5 @WaffleLapkin @BoxyUwU

Warning: this quiz is still "work-in-progress", some questions might not have good explanations (or any at all), formatting/structure/titles/etc are not final, and so on. You might want to return here on a later date.

trait Super {
    fn assoc() -> Self;
}

trait Sub: Super {}

fn f<T: Sub>() -> T {
    Sub::assoc()
}

fn main() {}
Solution 
error[E0782]: expected a type, found a trait
 --> examples/misc_5.rs:8:5
  |
8 |     Sub::assoc()
  |     ^^^
  |
help: you can add the `dyn` keyword if you want a trait object
  |
8 |     <dyn Sub>::assoc()
  |     ++++    +

For more information about this error, try `rustc --explain E0782`.
error: could not compile `code` (example "misc_5") due to 1 previous error

The diagnostic can be confusing, but the issue here is that you can't refer to items from super traits through sub traits. Even though Sub has a super trait Super, you can't use Sub::assoc().

The diagnostic assumes that you meant to use Sub as a type, not a trait, which requires the dyn keyword. (In this specific case this assumption doesn't make sense, since Sub is not dyn compatible)

You can use Super::assoc() though, i.e. this compiles just fine:

trait Super {
    fn assoc() -> Self;
}

trait Sub: Super {}

fn f<T: Sub>() -> T {
    Super::assoc()
}

Trait::assoc is a shorter version of <_ as Trait>::assoc (aka fully qualified path).