ICE: resolve_trait_associated_const: unexpected vtable type

[oli-obk]

the following code ices on nightly:

#![feature(const_fn, associated_consts)]

trait Trait {
    const N: usize;
}

impl Trait {
    const fn n() -> usize { Self::N }
}

It should give an error instead noting that you can't do Self::N.

[bltavares]

Triaging: this issue is already being tracked on https://github.com/steveklabnik/glacier

Reference: rust-lang/glacier#144

[niconii]

Still happens as of rustc 1.16.0-nightly (4ecc85beb 2016-12-28).

[aiwenar]

As of rustc 1.17.0-nightly (ea7a6486a 2017-02-04) this can also be hit with <Trait as Trait>::CONST

#![feature(associated_consts)]

trait Trait {
  const N: usize;
}

fn n() -> usize {
  <Trait as Trait>::N
}

[Mark-Simulacrum]

The first example doesn't compile due to the trait not being object safe and the last example also doesn't compile today. Neither ICEs; E-needstest.

[kvark]

@Mark-Simulacrum
Here is a new case:

pub enum Foo {}
pub trait Food {
    const FOO: Foo;
}
pub trait Bard {
    type Foo: Food;
    fn get_foo() -> Foo { Self::Foo::FOO }
}

Error on Rust-1.20 (playground):

error: internal compiler error: /checkout/src/librustc_const_eval/eval.rs:529: resolve_trait_associated_const: unexpected vtable type

Note: this is sort of a blocker for us to get use of the associated consts.

[oli-obk]

@kvark this is already fixed on beta/nightly

[kvark]

@oli-obk great, but what about stable? No hot patches planned?

[Mark-Simulacrum]

We generally do not backport patches to stable unless they are critical bug fixes (for which I don't think I can give a concrete definition). We should have a stable out with the fix in 6 weeks, though.