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coherence incorrectly considers unnormalizable_projection: Trait to not hold even if it could #114061
Description
// crate dep
trait IsUnit {}
impl IsUnit for () {}
pub trait WithAssoc<'a> {
type Assoc;
}
// The two impls of `Trait` overlap
pub trait Trait {}
impl<T> Trait for T
where
T: 'static,
for<'a> T: WithAssoc<'a>,
for<'a> <T as WithAssoc<'a>>::Assoc: IsUnit,
{
}
impl<T> Trait for Box<T> {}
// root crate
use dep::*;
struct Local;
impl WithAssoc<'_> for Box<Local> {
type Assoc = ();
}
fn impls_trait<T: Trait>() {}
fn main() {
impls_trait::<Box<Local>>();
}During coherence for<'a> <Box<T> as WithAssoc<'a>>::Assoc: IsUnit is considered to not hold.
This allows overlapping impls in coherence which is generally considered to be unsound™.
Coherence considers the impls to be disjoint because for<'a> <Box<T> as WithAssoc<'a>>::Assoc: IsUnit, does not hold. When trying to normalize <Box<T> as WithAssoc<'a>>::Assoc ends up ambiguous, we keep the projection around. Coherence then considers for<'a> <Box<T> as WithAssoc<'a>>::Assoc: IsUnit to not have any impl as IsUnit is a local trait.
Reacted by Ali MJ Al-Nasrawy
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lcnr commentedon Oct 2, 2023
lcnr commentedon Oct 9, 2023
figure out why this detects overlap in the new solver, it is strongly related to #99554
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Auto merge of rust-lang#121848 - lcnr:stabilize-next-solver, r=compil…
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Auto merge of rust-lang#121848 - lcnr:stabilize-next-solver, r=<try>
Auto merge of rust-lang#121848 - lcnr:stabilize-next-solver, r=<try>
Auto merge of rust-lang#121848 - lcnr:stabilize-next-solver, r=compil…
Auto merge of #121848 - lcnr:stabilize-next-solver, r=compiler-errors
Auto merge of rust-lang#130654 - lcnr:stabilize-coherence-again, r=<try>
Auto merge of #121848 - lcnr:stabilize-next-solver, r=compiler-errors
Auto merge of rust-lang#130654 - lcnr:stabilize-coherence-again, r=<try>
Auto merge of rust-lang#130654 - lcnr:stabilize-coherence-again, r=<try>
Auto merge of rust-lang#130654 - lcnr:stabilize-coherence-again, r=co…
Auto merge of #130654 - lcnr:stabilize-coherence-again, r=compiler-er…