I'm struggling to understand a "downstream crates may implement trait" error that reduces to the following example (playground):

/// Some trait dependent on a tuple of arguments.
pub trait Foo<Args> {}

/// Blanket implementation for async closures.
impl<F, Args> Foo<Args> for F
where
    Args: Tuple,
    F: AsyncFnOnce<Args>,
{
}

/// We also define our own private struct that implements the trait.
struct Bar {}

impl<Args> Foo<Args> for Bar where Args: Tuple {}

error[E0119]: conflicting implementations of trait `Foo<_>` for type `Bar`
  --> src/lib.rs:22:1
   |
11 | / impl<F, Args> Foo<Args> for F
12 | | where
13 | |     Args: Tuple,
14 | |     F: AsyncFnOnce<Args>,
   | |_________________________- first implementation here
...
22 |   impl<Args> Foo<Args> for Bar where Args: Tuple {}
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Bar`
   |
   = note: downstream crates may implement trait `std::ops::AsyncFnOnce<_>` for type `Bar`

It's not true that downstream crates can implement AsyncFnOnce for Bar, right? They can't mention that type, which is private, and even if they could it is not a local type for them so they shouldn't be able to implement AsyncFnOnce for it.

How do I make this work? I feel like I should be able to use sealed traits, even though that seems like it should be unnecessary, but I can't find an arrangement that works.

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