auto_traits + negative_impls not working as expected with associated types

[gbj]

It's possible that this is working as designed or that I'm taking the wrong approach here, so apologies up front if that's the case.

I seem to be able to use the combination of auto_traits and negative_impls to implement traits in a way that is generic but polymorphic for generic types on concrete types, but not for associated types on traits.

#![feature(auto_traits, negative_impls)]

use std::future::Future;

trait MyTrait {}

struct MyStruct {}

struct GenericStruct<T> {
    value: T,
}

auto trait NotString {}
impl !NotString for String {}

// Generics on a concrete type work as expected
// I can specify T: NotString and there is no conflict
impl MyTrait for GenericStruct<String> {}
impl<T: NotString> MyTrait for GenericStruct<T> {}

// Associated Types don't work as I'd expect
impl<S> MyTrait for S where S: Future<Output = String> {}

// Even though T is NotString there's a conflict between Future<Output = String> and Future<Output = T>
impl<S, T: NotString> MyTrait for S where S: Future<Output = T> {}

I would've expected that the compiler could tell that Future<Output = String> and Future<Output = T> were mutually exclusive, given that T: NotString.

Meta

rustc --version --verbose:

rustc 1.50.0-nightly (7efc097c4 2020-12-12)
binary: rustc
commit-hash: 7efc097c4fe6e97f54a44cee91c56189e9ddb41c
commit-date: 2020-12-12
host: x86_64-apple-darwin
release: 1.50.0-nightly