ICE with specialization + associated types: &[T] was a subtype of &[u8] but now is not?

[oli-obk]

A combination of specialization and an associated type yields this error. Removing the Error associated type and replacing all uses with a concrete type removes the error:

#![feature(specialization)]

trait Deserializer {
    type Error;
    fn deserialize_bytes<V: Visitor>(self, visitor: V) -> Result<V::Value, Self::Error>;
    fn deserialize<V: Visitor>(self, visitor: V) -> Result<V::Value, Self::Error>;
}

trait Visitor {
    type Value;
    type Error;
    fn visit_bytes(self, &[u8]) -> Result<Self::Value, Self::Error>;
}

impl<'a, T> Deserializer for &'a [T] {
    type Error = ();
    default fn deserialize_bytes<V: Visitor>(self, visitor: V) -> Result<V::Value, Self::Error> {
        self.deserialize(visitor)
    }
    fn deserialize<V: Visitor>(self, visitor: V) -> Result<V::Value, Self::Error> {
        unimplemented!()
    }
}

impl<'a> Deserializer for &'a [u8] {
    fn deserialize_bytes<V: Visitor>(self, visitor: V) -> Result<V::Value, Self::Error> {
        visitor.visit_bytes(self)
    }
}

fn main() {}

this occurs on the playground nightly

[istankovic]

With rustc 1.26.0-nightly (adf2135ad 2018-03-17) this is not an ICE, I get this:

error[E0308]: mismatched types
  --> a.rs:27:9
   |
26 |     fn deserialize_bytes<V: Visitor>(self, visitor: V) -> Result<V::Value, Self::Error> {
   |                                                           ----------------------------- expected `std::result::Result<<V as Visitor>::Value, ()>` because of return type
27 |         visitor.visit_bytes(self)
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found associated type
   |
   = note: expected type `std::result::Result<_, ()>`
              found type `std::result::Result<_, <V as Visitor>::Error>`

error: aborting due to previous error

[pnkfelix]

@oli-obk this does not seem like a bug anymore. Or at least, the behavior reported by @istankovic sounds plausibly correct?

Shall we close this?

[oli-obk]

Jup, looks fixed to me, too