object-safe traits can have associated types with unchecked bounds

[arielb1]

STR

trait Foo { type Assoc: PartialEq<Box<Self>>; }
impl Foo for u32 { type Assoc = Box<u32>; }

fn foo<T: Foo+?Sized>(u: Box<T>, v: T::Assoc) -> bool {
    &v == &u
}

fn main() {
    let bar: Box<Foo<Assoc=Box<u32>>> = Box::new(4);
    foo(bar, Box::new(5));
}

Result

<anon>:5:5: 5:13 error: internal compiler error: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<Box<u32> as core::cmp::PartialEq<Box<Foo<Assoc=Box<u32>> + 'static>>>)),depth=1),Unimplemented)]` fulfilling during trans

I guess this should be banned.

cc @nikomatsakis

[arielb1]

Related:

use std::fmt;

trait Foo {
    type Assoc: 'static;
}

fn foo<T: Foo+?Sized>(t: T::Assoc) -> Box<fmt::Display+'static>
        where T::Assoc: fmt::Display {
    Box::new(t)
}

fn wat() -> Box<fmt::Display+'static> {
    let x = 42;
    foo::<Foo<Assoc=&u32>>(&x)
}

fn main() {
    println!("{}", wat());
}

[nikomatsakis]

Interesting. I agree that it should be illegal and we should amend WF relations for object types to check that the bindings cover the requirements of the trait -- the RFC was incomplete on this topic.

[pnkfelix]

The example from the first comment (#27675 (comment)) is actually scarier than the original bug description; at least, I usually can think "oh, an ICE, well that might just be something that won't actually ever compile"; but the first comment is showing code that is indeed a case of "wat"!

[pnkfelix]

triage: P-medium