ICE with using a trait as a type alias

[erickt]

@graydon's workcache.rs file was trying to use a trait to make a type alias, which triggers an ICE:

extern mod std;

use send_map::linear;
use std::json;
use std::serialization::{Deserializable, deserialize};

trait JD : Deserializable<json::Deserializer> { }
//type JD = Deserializable<json::Deserializer>;

fn exec<T: JD>() {
    let doc = result::unwrap(json::from_str(""));
    let _v: T = deserialize(&json::Deserializer(move doc));
    fail
}

fn main() {}

It errors with:

error: internal compiler error: substitution a had opt_region &static and b had opt_region none with variance Some(rv_invariant)

If you use a proper type type alias, it compiles fine.

[catamorphism]

Now I get a proper borrow checker error instead of an ICE:

../src/test/run-pass/issue-4016.rs:12:29: 12:58 error: cannot infer an appropriate lifetime due to conflicting requirements
../src/test/run-pass/issue-4016.rs:12     let _v: T = deserialize(&json::Deserializer(move doc));
                                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: first, the lifetime cannot outlive lifetime re_bound(br_self)...
../src/test/run-pass/issue-4016.rs:12:16: 12:27 note: ...due to the following expression
../src/test/run-pass/issue-4016.rs:12     let _v: T = deserialize(&json::Deserializer(move doc));
                                                      ^~~~~~~~~~~
../src/test/run-pass/issue-4016.rs:12:16: 12:59 note: but, the lifetime must be valid for the call at 12:16...
../src/test/run-pass/issue-4016.rs:12     let _v: T = deserialize(&json::Deserializer(move doc));
                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/test/run-pass/issue-4016.rs:12:29: 12:58 note: ...due to the following expression
../src/test/run-pass/issue-4016.rs:12     let _v: T = deserialize(&json::Deserializer(move doc));
                                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

but it compiles with no errors if I change the trait to be an actual type. So there's still something fishy.

[catamorphism]

Bumping to 0.7

[catamorphism]

This compiles now. Updated test case:

extern mod std;

use std::json;
use std::serialize::Decodable;

trait JD : Decodable<json::Decoder> { }

fn exec<T: JD>() {
    let doc = result::unwrap(json::from_str(""));
    let mut decoder = json::Decoder(doc);
    let _v: T = Decodable::decode(&mut decoder);
    fail!()
}

fn main() {}

I'll check it in.

[catamorphism]

Test case is src/test/run-pass/issue-4016.rs