Bounds on associated types of super-traits not propagated

[gnzlbg]

MWE on the playground:

pub trait WithAssoc {
  type Assoc;
  fn wa(&self) {}
}

pub trait Baz {}

pub trait Moo: WithAssoc
  where Self::Assoc: Baz // this is allowed
{}

pub trait Foo: Moo
// where Self::Assoc: Baz // but bound is not propagated
{
  fn foo(&self) {
    self.wa();
  }
}

fn main() {}

[arielb1]

duplicate of #20671

[gnzlbg]

@arielb1 I want to write a trait like this:

pub trait Point: std::ops::Index<usize> where Self::Output: Float {};

but if I do that I need to repeat that where clause everywhere where I use the trait.

If I could write my own index trait I would just do:

pub trait Index<T> {
  type Output: Float;
  fn index...
}

and then I can just require that and never have to write the bound. Is there a way to add that bound externally so that I don't have to repeat it everywhere?

[arielb1]

and then I can just require that and never have to write the bound. Is there a way to add that bound externally so that I don't have to repeat it everywhere?

Not that I know of. The implied bounds RFC might improve the situation in the future - got to talk about that with @nikomatsakis .

[gnzlbg]

The following works:

pub trait Point
  : std::ops::Index<usize, Output = <Self as Point>::Output> {
     type Output: Float;
}