Fails to infer associated type bounds with for

[jonhoo]

The following code does not compile (playground):

trait Bar<T> {
    type Assoc;
}
impl<'a> Bar<&'a ()> for () {
    type Assoc = Box<Send>;
}

fn oops<C>()
where
    for<'a> C: Bar<&'a ()>,
    for<'a> <C as Bar<&'a ()>>::Assoc: Send,
{
}

fn main() {
    oops::<()>();
}

with

error[E0277]: `<() as Bar<&'a ()>>::Assoc` cannot be sent between threads safely
  --> src/lib.rs:16:5
   |
16 |     oops::<()>();
   |     ^^^^^^^^^^ `<() as Bar<&'a ()>>::Assoc` cannot be sent between threads safely
   |
   = help: the trait `for<'a> std::marker::Send` is not implemented for `<() as Bar<&'a ()>>::Assoc`
note: required by `oops`
  --> src/lib.rs:8:1
   |
8  | / fn oops<C>()
9  | | where
10 | |     for<'a> C: Bar<&'a ()>,
11 | |     for<'a> <C as Bar<&'a ()>>::Assoc: Send,
12 | | {
13 | | }
   | |_^

The issue seems to be that the for<'a> does not "carry along" the Send bound for Assoc in the impl Bar for (). This code compiles on the other hand compiles just fine:

trait Bar<T> {
    type Assoc;
}
impl<'a> Bar<&'a ()> for () {
    type Assoc = Box<Send>;
}

fn oops<'a, C>()
where
    C: Bar<&'a ()>,
    <C as Bar<&'a ()>>::Assoc: Send,
{
}

fn main() {
    oops::<()>();
}

Notice that the only change is that 'a is now a lifetime parameter of oops, which removes the need for the for<'a>. I believe the for<'a> version should also be correct though?

[sfackler]

This looks like a duplicate of #56556.

[jonhoo]

@sfackler hmm, yes, I believe that's right! Closing in favor of #56556, and then posting this example code there to add a test case.