This almost works (in nightly): https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=332ca47a9d3618a8427ae618f0a8b188

#![feature(non_lifetime_binders)]

trait Foo {}

trait HasAssoc {
    type Assoc<T>;
}

trait HasFooAssoc: for<T> HasAssoc<Assoc<T>: Foo> {}

struct X;
struct Y<T>(T);

impl<T> Foo for Y<T> {}
impl HasAssoc for X {
    type Assoc<T> = Y<T>;
}
impl HasFooAssoc for X {}

I’m trying to define HasFooAssoc, which is a HasAssoc where Assoc: Foo is known to hold. From my reading of the errors, it seems like the problem is that for<T> has to also hold for T: ?Sized, and I’m not syntactically allowed to type for<T: Sized> (T actually should be Sized for my use case).

Note that if the Foo bound is hardcoded into HasAssoc, there is no problem: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=e6f7ec36babd27e0e160c023052be4c3

trait Foo {}

trait HasAssoc {
    type Assoc<T>: Foo;
}

struct X;
struct Y<T>(T);

impl<T> Foo for Y<T> {}
impl HasAssoc for X {
    type Assoc<T> = Y<T>;
}

but I would really like to add that bound in afterwards for abstraction purposes (I want e.g. both HasFooAssoc and HasBarAssoc, and I’d like to have the HasAssoc part factored out, because in the real use case it’s more complex).