simplified.rs

trait A {}
trait B {}

// The impl we want to allow to be added without breakage
impl<T: A> B for T {}

trait Assoc {
    type Out;
}

impl<T> Assoc for T where T: A + B {
    type Out = ();
}

impl<T> Foo for T where T: A, T: Assoc {}

// The `Out = ()` constraint here is basically a proxy for `T: A + B`
impl<T> Foo for T where T: A, T: Assoc, T: Assoc<Out = ()> {}

This crate would be broken by impl line 5 regardless of impls line 15 and 18.

struct Foo;
impl A for Foo { }
impl Assoc for Foo {
    type Out = String;
}

This crate would be broken by impl line 5 regardless of impls line 15 and 18.

struct Foo;
impl A for Foo { }
impl Assoc for Foo {
    type Out = String;
}