ICE: Region parameter out of range when substituting in region...

[cynecx]

#![feature(generic_associated_types)]

struct S;

trait Trait {
    type Associated<'a>;
}

impl Trait for S {
    type Associated<'a> = &'a ();
}

fn main() {}

error: internal compiler error: librustc/ty/subst.rs:439: Region parameter out of range when substituting in region 'a (root type=Some(TestIterItem<'a>)) (index=0)

thread 'rustc' panicked at 'Box<Any>', librustc_errors/lib.rs:488:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to 2 previous errors

Rust playground

Probably related: #47309

I can reproduce this with rustc 1.26.0-nightly (f5631d9ac 2018-03-24).

[matthewjasper]

Generic associated types aren't implemented past syntax yet, so this is probably expected. cc #44265

[0ndorio]

This does not only happen with generic associated lifetimes but with any generic associated type:

#![feature(generic_associated_types)]

pub trait MyTrait {
    type InternalType<TypeA>;
}

pub struct GenericType<TypeA> {
    _dummy: TypeA,
}

pub struct MyStruct {}

impl MyTrait for MyStruct {
    type InternalType<TypeA> = GenericType<TypeA>;
}

// -----------------------------------------------------------------------------
fn main() {}

Compiling playground v0.0.1 (file:///playground)
error: internal compiler error: librustc/ty/subst.rs:493: Type parameter `TypeA/#0` (TypeA/0) out of range when substituting (root type=Some(GenericType<TypeA>)) substs=[]

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:499:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to previous error

note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: rustc 1.27.0-nightly (565235ee7 2018-05-07) running on x86_64-unknown-linux-gnu
note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin
note: some of the compiler flags provided by cargo are hidden

Rust playground: https://play.rust-lang.org/?gist=c36f51038511f3e6568b1b2c866f2ca3&version=nightly&mode=debug

Maybe its worth to disable the feature flag or add a compiler error warning if the flag is used, until the feature implementation progressed further.