normalization no longer prefers object bound over user written impls

[lcnr]

pub trait Hook {
    type Value;
    fn use_hook(&mut self);
}

pub trait ValueGat {
    type Value;
}

pub trait ErasedHook {
    type ValueGat: ?Sized + ValueGat;
    fn erased_use_hook(&mut self) -> <Self::ValueGat as ValueGat>::Value {
        unimplemented!()
    }
}

impl<H: ?Sized + Hook> ErasedHook for H {
    type ValueGat = dyn ValueGat<Value = <H as Hook>::Value>;
}

impl<V: ?Sized + ValueGat> Hook for dyn '_ + ErasedHook<ValueGat = V> {
    type Value = <V as ValueGat>::Value;
    fn use_hook(&mut self) {
        Self::erased_use_hook(self);
    }
}

compiles with the old solver by preferring the builtin object impl over the user written impl for ErasedHook.

The new solver fails with ambiguity. Affects hooks-core, thx @lqd. https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/crater.20triage.3A.20.60hooks-core.60.20cannot.20normalize/with/547829505

[lcnr]

jup, definitely unsound :3

// depends on hooks-core = "1.0.0-alpha.10"
use hooks_core::{
    Hook, HookLifetime,
    erased_hook::{ErasedHook, ValueGat},
};

struct Whatever;
impl ValueGat<'_> for Whatever {
    type Value = ();
}

fn foo<H: ?Sized + Hook<Args>, Args>(
    x: Box<<H as ErasedHook<Args>>::ValueGat>,
) -> Box<dyn for<'hook> ValueGat<'hook, Value = <H as HookLifetime<'hook, Args>>::Value>> {
    x
}

fn bar<V: for<'hook> ValueGat<'hook>, Args>(
    x: Box<V>,
) -> Box<dyn for<'hook> ValueGat<'hook, Value = <V as ValueGat<'hook>>::Value>> {
    foo::<dyn ErasedHook<Args, ValueGat = V> + '_, Args>(x)
}

fn main() {
    bar::<Whatever, ()>(Box::new(Whatever));
}