incomplete projection candidate selection for trait objects

[lcnr]

rust-lang/rust#107887

trait Trait<T> {
    type Assoc: ?Sized;
}

impl Trait<u32> for u32 {
    type Assoc = u16;
}

// This would trigger the check for overlap between automatic and custom impl
// They actually don't overlap so an impl like this should remain possible
// forever.
//
// impl Trait<u64> for dyn Trait<u32, Assoc = u32> {
//     type Assoc =  dyn Trait<u32, Assoc = u32>;
// }
trait Indirect<T: ?Sized> {}
impl Indirect<dyn Trait<u32, Assoc = u32>> for () {}
impl<T: ?Sized> Trait<u64> for T
where
    (): Indirect<T>,
{
    type Assoc = dyn Trait<u32, Assoc = u32>;
}

fn yay<T: Trait<U> + ?Sized, U>(x: &'static T) -> &'static <T as Trait<U>>::Assoc {
    todo!();
}

fn unconstrained<T>() -> T {
    todo!()
}

fn should_be_ambig() {
    let y: &'static dyn Trait<u32, Assoc = u32> = unconstrained();
    let _ = yay::<dyn Trait<u32, Assoc = u32>, _>(y);
    // The current solver incorrectly constrains `_` to `u32` here.
}

fn main() {
    let y: &'static dyn Trait<u32, Assoc = u32> = unconstrained();
    let mut x = yay::<_, u64>(y); // compiles
    // let mut x = yay::<_, _>(y); // errors
    x = y;
}

This example works on stable but will break with the new solver.

[lcnr]

results in coherence unsoundness by allowing overlap: rust-lang/rust#107887 (comment)