Ambiguity, differing outlives constraints with types equal modulo normalization

[compiler-errors]

fn main() {
    let _: &dyn Iterator<Item=i32> = &vec![1].into_iter();
}

When coercing &'?0 <Vec<{integer}> as IntoIterator>::IntoIter into &'?1 dyn Iterator<Item=i32>, we end up with two responses.

The first has a type-outlives constraint like <Vec<{integer}> as IntoIterator>::IntoIter: &'?1, and the second has one like std::vec::IntoIter<{integer}>: &'?1. These are identical except for their types being normalized, but that causes us to flounder.

[compiler-errors]

Interestingly, the code with Box doesn't fail because the Box::new constructor causes the type to be normalized due to the argument being a coercion point:

fn main() {
    let _: Box<dyn Iterator<Item=i32> + '_> = Box::new(vec![1].into_iter());
}

[compiler-errors]

This may be a problem somewhat specific to T -> dyn Trait unsizing -- we could perhaps consider those goals separately within assemble_blanket_impl_candidates, and not in assemble_candidates_via_self_ty.

[lcnr]

I think we should consider the three way split of "assemble blanket candidates", "assemble_normalization_step_candidates" and "assemble_candidates_via_self_ty" or whatever