associated_type_defaults are unsound in the new solver

[compiler-errors]

rust-lang/rust#110673 attempted to make alias bounds sound in the new solver by only assembling alias bounds for a projection (such as <A as B>::C) when the trait that the projection comes from (<A as B>) can be satisfied via a param-env candidate.

This is problematic when checking whether an associated type default is well-formed, because we assume within the trait that Self: Trait<..> holds.

// compile-flags: -Ztrait-solver=next
#![feature(associated_type_defaults)]

trait Tr {
    type Ty: Copy = String;

    fn clone(x: &Self::Ty) -> Self::Ty {
        *x
    }
}

impl Tr for () {}

fn main() {
    let x = String::from("hello, world");
    let y = <() as Tr>::clone(&x);
    drop(x);
    println!("{y}");
    // ^^
}

During check_type_bounds in the above program, we must prove:

[
    Obligation(predicate=Binder { value: TraitPredicate(<<Self as Tr>::Ty as std::marker::Sized>, polarity:Positive), bound_vars: [] }, depth=0),
    Obligation(predicate=Binder { value: TraitPredicate(<<Self as Tr>::Ty as std::marker::Copy>, polarity:Positive), bound_vars: [] }, depth=0),
]

given the caller bounds of:

[
    Binder { value: TraitPredicate(<Self as Tr>, polarity:Positive), bound_vars: [] },
]

Which means that the check implemented in EvalCtxt::validate_alias_bound_self_from_param_env succeeds.

[lcnr]

This feels somewhat unsolvable until we fully enable coinductive trait goals and check impl where clauses instead of the current hack which disables alias bounds candidates for stuff proven via impls