ICE when using constraint on Generic Associated Type

[Nashenas88]

The following code

#![feature(generic_associated_types)]

trait Fut {
    fn do_the_thing() -> usize;
}

trait WantsToReturnFuture<F> where F: Fut {
    fn returns_future() -> F;
}

trait HasAssociatedTypes {
    type Generic<F: Fut>: WantsToReturnFuture<F>;

    fn get_generic() -> Self::Generic;
}

fn main() {
    println!("Hello, world!");
}

causes this ICE:

$ RUST_BACKTRACE=1 cargo check
    Checking bug_case v0.1.0 (file:///Users/pfaria/bug_case)
error: internal compiler error: librustc/ty/subst.rs:479: Type parameter `F/#1` (F/1) out of range when substituting (root type=Some(F)) substs=[Self]

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:499:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::span_bug
   8: rustc::session::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: rustc::ty::context::tls::with_context_opt
  11: rustc::ty::context::tls::with_opt
  12: rustc::session::opt_span_bug_fmt
  13: rustc::session::span_bug_fmt
  14: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  15: rustc::ty::fold::TypeFoldable::fold_with
  16: rustc::ty::Predicate::subst_supertrait
  17: <core::iter::Map<I, F> as core::iter::iterator::Iterator>::try_fold
  18: rustc::traits::object_safety::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::predicates_reference_self
  19: rustc::traits::object_safety::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::object_safety_violations_for_trait
  20: <core::iter::FlatMap<I, U, F> as core::iter::iterator::Iterator>::next
  21: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  22: rustc::traits::object_safety::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::object_safety_violations
  23: rustc::traits::object_safety::is_object_safe_provider
  24: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::is_object_safe<'tcx>>::compute
  25: rustc::ty::context::tls::with_context
  26: rustc::dep_graph::graph::DepGraph::with_task_impl
  27: rustc::ty::context::tls::with_related_context
  28: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  30: rustc::traits::select::SelectionContext::assemble_candidates
  31: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  32: rustc::ty::context::tls::with_context
  33: rustc::dep_graph::graph::DepGraph::with_anon_task
  34: rustc::traits::select::SelectionContext::candidate_from_obligation
  35: rustc::traits::select::SelectionContext::select
  36: rustc::infer::InferCtxt::commit_if_ok
  37: rustc::traits::project::opt_normalize_projection_type
  38: rustc::traits::project::normalize_projection_type
  39: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  40: rustc::ty::fold::TypeFoldable::fold_with
  41: rustc::ty::fold::TypeFoldable::fold_with
  42: rustc::ty::fold::TypeFoldable::fold_with
  43: rustc::traits::project::normalize
  44: rustc::traits::fully_normalize
  45: rustc::ty::context::tls::with_context::{{closure}}
  46: rustc::ty::context::tls::with_related_context
  47: rustc::ty::context::GlobalCtxt::enter_local
  48: rustc::traits::normalize_param_env_or_error
  49: rustc::ty::param_env
  50: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::param_env<'tcx>>::compute
  51: rustc::ty::context::tls::with_context
  52: rustc::dep_graph::graph::DepGraph::with_task_impl
  53: rustc::ty::context::tls::with_related_context
  54: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  55: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  56: rustc_typeck::check::wfcheck::for_id
  57: rustc_typeck::check::wfcheck::check_item_well_formed
  58: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::check_item_well_formed<'tcx>>::compute
  59: rustc::ty::context::tls::with_context
  60: rustc::dep_graph::graph::DepGraph::with_task_impl
  61: rustc::ty::context::tls::with_related_context
  62: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  63: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  64: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::ensure_query
  65: rustc::hir::Crate::visit_all_item_likes
  66: rustc::session::Session::track_errors
  67: rustc::util::common::time
  68: rustc_typeck::check_crate
  69: rustc::ty::context::tls::enter_context
  70: <std::thread::local::LocalKey<T>>::with
  71: rustc::ty::context::TyCtxt::create_and_enter
  72: rustc_driver::driver::compile_input
  73: rustc_driver::run_compiler_with_pool
  74: <scoped_tls::ScopedKey<T>>::set
  75: syntax::with_globals
  76: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  77: __rust_maybe_catch_panic
  78: rustc_driver::run
  79: rustc_driver::main
  80: std::rt::lang_start::{{closure}}
  81: std::panicking::try::do_call
  82: __rust_maybe_catch_panic
  83: std::rt::lang_start_internal
  84: main
query stack during panic:
#0 [is_object_safe] determine object safety of trait `HasAssociatedTypes`
#1 [param_env] processing `HasAssociatedTypes`
#2 [check_item_well_formed] processing `HasAssociatedTypes`
end of query stack
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 1.28.0-nightly (e3bf634e0 2018-06-28) running on x86_64-apple-darwin

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

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

error: Could not compile `bug_case`.

To learn more, run the command again with --verbose.

Version info:

$ rustc --version --verbose
rustc 1.28.0-nightly (e3bf634e0 2018-06-28)
binary: rustc
commit-hash: e3bf634e060bc2f8665878288bcea02008ca346e
commit-date: 2018-06-28
host: x86_64-apple-darwin
release: 1.28.0-nightly
LLVM version: 6.0

As a workaround I can instead implement this as:

trait Fut {
    fn do_the_thing() -> usize;
}

trait WantsToReturnFuture<F> where F: Fut {
    fn returns_future() -> F;
}

trait HasAssociatedTypes<F: Fut> {
    type Generic: WantsToReturnFuture<F>;

    fn get_generic() -> Self::Generic;
}

fn main() {
    println!("Hello, world!");
}

with the downside being I need to require users of the trait to work with PhantomData 😦 .

[kennytm]

GATs is still not implemented yet, so this ICE is kinda expected.

Closing as duplicate of #49362.