discard_impls_shadowed_by_env forces unnecessary ambiguity

[lcnr]

the following works with the old solver but breaks with new:

trait Outer {
    type Assoc;
}

trait Id<U: ?Sized> {
    type Id;
}
impl<T, U: ?Sized + Copy> Id<U> for T {
    type Id = T;
}

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

fn create<T: Outer, U: Copy>(
    u: U,
) -> <<T as Id<U>>::Id as Outer>::Assoc {
    todo!()
}

fn foo<T: Outer<Assoc = i32>>() {
    let u = unconstrained();
    let assoc = create::<T, _>(u);
    assoc.abs();
    let _: i32 = u;
}

The type of assoc is <<T as Id<?u>>::Id as Outer>::Assoc and we have a T: Outer<Assoc = i32> where bound. <T as Id<?u>>::Id uses the impl candidate and normalizes to T, doing so tries to prove U: Copy, resulting in Certainty::Ambiguity. We now use the ParamEnv candidate for <<T as Id<?u>>::Id as Outer>::Assoc (<T as Outer>::Assoc after normalizing the self type). In discard_impls_shadowed_by_env there exists an ambiguous ParamEnv trait candidate, causing us to discard all constraints.

We should somehow only limit "discard all constraints" to the case where there are multiple ParamEnv candidates and we're unable to choose, not if we have an applicable ParamEnv candidate which is ambiguous.