Associated types of a subtrait must always be explicit even when a supertrait implementation implies the subtrait with specific associated types

[jimmycuadra]

Imagine the following scenario:

There is a trait A that has many associated types. There are many functions in a program that take a Box<A> only with specific associated types. To avoid having to write out all this boilerplate (including importing all the associated types), I want to create an alias for A with these specific associated types. A type alias cannot work, because you can't do impl Foo for Bar where Foo is a type alias. Let's try subtraits:

trait A {
    type Assoc;
    // Pretend there are several more associated types here to make uses of `Box<A>` inconvenient.
}

trait B: A<Assoc=()> {}

fn takes_b(b: Box<B>) {}

This results in:

error[E0191]: the value of the associated type `Assoc` (from the trait `A`) must be specified
 --> src/lib.rs:8:19
  |
8 | fn takes_b(b: Box<B>) {}
  |                   ^ missing associated type `Assoc` value

This seems like it should work. Note that declaring B this way is valid, as the program compiles if takes_b is commented out.

[durka]

see #24010 and #29328

[jimmycuadra]

Thank you for linking those issues! I wasn't sure how to search for this issue cause I wasn't sure how to phrase the title/question. Closing this as a duplicate.