GAT ICE: dtorck encountered internal error

The issue appeared when following and using the TypeFamily pattern:

Code

#![feature(generic_associated_types)]


pub trait Trait1 {
    type Associated: Ord;
}

pub trait Trait2 {
    type Associated: Clone;
}

pub trait GatTrait {
    type Gat<T: Clone>;
}

pub struct GatStruct;

impl GatTrait for GatStruct {
    type Gat<T: Clone> = Box<T>;
}

pub struct OuterStruct<T1: Trait1, T2: Trait2> {
    inner: InnerStruct<T2, GatStruct>,
    t1:    T1,
}

pub struct InnerStruct<T: Trait2, G: GatTrait> {
    pub gat: G::Gat<T::Associated>,
}

impl<T1, T2> OuterStruct<T1, T2>
where
    T1: Trait1,
    T2: Trait2<Associated = T1::Associated>,
{
    pub fn new() -> Self {
        todo!()
    }
}

Playground link

Error output

error: internal compiler error: dtorck encountered internal error
  --> src/source.rs:16:26
   |
16 |       pub fn new() -> Self {
   |  __________________________^
17 | |         todo!()
18 | |     }
   | |_____^
   |
   = note: delayed at compiler/rustc_trait_selection/src/traits/query/dropck_outlives.rs:67:18

error: internal compiler error: dtorck encountered internal error
  --> src/source.rs:17:9
   |
17 |         todo!()
   |         ^^^^^^^
   |
   = note: delayed at compiler/rustc_trait_selection/src/traits/query/dropck_outlives.rs:67:18
   = note: this error: internal compiler error originates in the macro `$crate::panic::panic_2015` (in Nightly builds, run with -Z macro-backtrace for more info)

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:1189:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

Backtrace

error: internal compiler error: dtorck encountered internal error
  --> src/source.rs:16:26
   |
16 |       pub fn new() -> Self {
   |  __________________________^
17 | |         todo!()
18 | |     }
   | |_____^
   |
   = note: delayed at compiler/rustc_trait_selection/src/traits/query/dropck_outlives.rs:67:18

error: internal compiler error: dtorck encountered internal error
  --> src/source.rs:17:9
   |
17 |         todo!()
   |         ^^^^^^^
   |
   = note: delayed at compiler/rustc_trait_selection/src/traits/query/dropck_outlives.rs:67:18
   = note: this error: internal compiler error originates in the macro `$crate::panic::panic_2015` (in Nightly builds, run with -Z macro-backtrace for more info)

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:1189:13
stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic_display::<&str>
   3: <rustc_errors::HandlerInner>::flush_delayed
   4: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   5: core::ptr::drop_in_place::<rustc_session::parse::ParseSess>
   6: <alloc::rc::Rc<rustc_session::session::Session> as core::ops::drop::Drop>::drop
   7: core::ptr::drop_in_place::<rustc_interface::interface::Compiler>
   8: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
   9: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.59.0-nightly (efec54529 2021-12-04) running on x86_64-apple-darwin

note: compiler flags: -C embed-bitcode=no -C split-debuginfo=unpacked -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
warning: `cozal` (lib) generated 1 warning
error: could not compile `cozal`; 1 warning emitted

playground with the issue

additional notes:

adding T1::Associated: Clone to the where for the impl for OuterStruct makes the playground compile. It looks like the compiler should either complain that the associated type is under-specified or implicitly apply the bound.

So...uh yeah. Something is wrong here. This playground doesn't work: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d4ac99ec4e25ef340002727b453c5d5c

That doesn't use GATs, just normal associated types. The impl definition doesn't error for a missing T1::Associated: Clone bound, but we also can't use that in the new body.

I was scared for a bit that this would be in someway unsound, but it's not.

Okay, so probably the right solution is to make T1::Associated: Clone be implied here (rather than requiring that bound). I have a POC branch here: https://github.com/jackh726/rust/tree/issue-91985

Unfortunately, it's not as simple as "just make this change". I think either way we run into potential issues. With the above changes, this test passes. But 2 other tests start failing and one has additional errors:

issue-67684.rs goes from pass to fail. We end up with additional obligations, because now we know that A::Error = A::Item, but propagate that A::Item: ParseError (because A::Error: ParseError). Kind of needs the reverse: when we see X: Trait1<Assoc = <Y as Trait2>::Assoc>, we need to also be able to elaborate that any bounds on <Y as Trait2>::Assoc also hold for <X as Trait1>::Assoc.

trait-with-supertraits-needing-sized-self.rs goes from fail to pass. The gist of this is basically when we have something like trait ArithmeticOps: Add<Output=Self> {}, we currently error because Self: Sized doesn't hold. But, with the above branch, we actually imply that Self: Sized holds because Output=Self.

issue-47715.rs ends up with new "type annotations needed" errors. I don't think these errors are correct.

GATs issue triage: not blocking. From above, this is a more general bug with associated types. Not an backwards-incompatibility hazard.

Update tests for dropck normalization errors

This was fixed by #136539

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GAT ICE: dtorck encountered internal error #91985

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maboesanman commented on Dec 18, 2021

maboesanman commented on Dec 18, 2021

jackh726 commented on Jan 8, 2022

jackh726 commented on Jan 8, 2022

jackh726 commented on Feb 7, 2022

spastorino commented on Oct 17, 2025

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