error: internal compiler error: src/librustc/ty/subst.rs:611: type parameter `B/#2` (B/2) out of range when substituting (root type=Some((B,))) substs=[ReEarlyBound(0, 'a)]

[steffahn]

Uncommenting the features and running in Nightly makes the complaints about unstable features go away while the panick stays.

I’m not sure yet how to make a smaller example out of this. smaller examples in comments below

This code was a step of me trying out if I could turn this working code (link) into a trait to get method syntax.

//#![feature(unboxed_closures)]
//#![feature(type_alias_impl_trait)]

type Curried<'a, A: 'a, B, C, F: Fn(A, B) -> C>
 = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;

fn curry<'a, A: 'a, B, C, F: Fn(A, B) -> C> (f: &'a F)
    -> Curried<'a, A, B, C, F>
{
    move |a| move |b| f(a,b)
}

fn main() {}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0658]: `impl Trait` in type aliases is unstable
 --> src/main.rs:4:1
  |
4 | / type Curried<'a, A: 'a, B, C, F: Fn(A, B) -> C>
5 | |  = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;
  | |_________________________________________________________^
  |
  = note: for more information, see https://github.com/rust-lang/rust/issues/63063

error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change. Use parenthetical notation (Fn(Foo, Bar) -> Baz) instead
 --> src/main.rs:5:9
  |
5 |  = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: for more information, see https://github.com/rust-lang/rust/issues/29625

error: internal compiler error: src/librustc/ty/subst.rs:611: type parameter `B/#2` (B/2) out of range when substituting (root type=Some((B,))) substs=[ReEarlyBound(0, 'a)]

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:890:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:61
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1028
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1412
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:65
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:50
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:188
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:205
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:468
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::span_bug
  14: rustc_errors::Handler::span_bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_context_opt
  18: rustc::ty::context::tls::with_opt
  19: rustc::util::bug::opt_span_bug_fmt
  20: rustc::util::bug::span_bug_fmt
  21: <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_ty
  22: rustc::ty::fold::TypeFoldable::fold_with
  23: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable for &rustc::ty::TyS>::super_fold_with
  24: <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_ty
  25: rustc::ty::fold::TypeFoldable::fold_with
  26: rustc::ty::subst::Subst::subst
  27: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  28: rustc::ty::GenericPredicates::instantiate_into
  29: rustc::infer::opaque_types::Instantiator::fold_opaque_ty
  30: <rustc::ty::fold::BottomUpFolder<F,G,H> as rustc::ty::fold::TypeFolder>::fold_ty
  31: rustc::infer::opaque_types::Instantiator::fold_opaque_ty
  32: <rustc::ty::fold::BottomUpFolder<F,G,H> as rustc::ty::fold::TypeFolder>::fold_ty
  33: rustc_typeck::check::FnCtxt::instantiate_opaque_types_from_value
  34: rustc_typeck::check::check_fn
  35: rustc::ty::context::tls::with_context::{{closure}}
  36: rustc_typeck::check::typeck_tables_of
  37: rustc::ty::query::__query_compute::typeck_tables_of
  38: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  39: rustc::dep_graph::graph::DepGraph::with_task_impl
  40: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  41: rustc_typeck::collect::find_opaque_ty_constraints::ConstraintLocator::check
  42: rustc::hir::intravisit::Visitor::visit_nested_item
  43: rustc_typeck::collect::find_opaque_ty_constraints
  44: rustc_typeck::collect::checked_type_of
  45: rustc_typeck::collect::type_of
  46: rustc::ty::query::__query_compute::type_of
  47: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  48: rustc::dep_graph::graph::DepGraph::with_task_impl
  49: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  50: <rustc_typeck::collect::CollectItemTypesVisitor as rustc::hir::intravisit::Visitor>::visit_item
  51: rustc::hir::map::Map::visit_item_likes_in_module
  52: rustc_typeck::collect::collect_mod_item_types
  53: rustc::ty::query::__query_compute::collect_mod_item_types
  54: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_mod_item_types>::compute
  55: rustc::dep_graph::graph::DepGraph::with_task_impl
  56: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  57: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
  58: rustc_typeck::check_crate::{{closure}}::{{closure}}
  59: rustc::util::common::time
  60: rustc_typeck::check_crate
  61: rustc_interface::passes::analysis
  62: rustc::ty::query::__query_compute::analysis
  63: rustc::dep_graph::graph::DepGraph::with_task_impl
  64: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  65: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  66: rustc_interface::passes::create_global_ctxt::{{closure}}
  67: rustc_interface::passes::BoxedGlobalCtxt::enter
  68: rustc_interface::interface::run_compiler_in_existing_thread_pool
  69: std::thread::local::LocalKey<T>::with
  70: scoped_tls::ScopedKey<T>::set
  71: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.40.0 (73528e339 2019-12-16) 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

query stack during panic:
#0 [typeck_tables_of] processing `curry`
#1 [type_of] processing `Curried`
#2 [collect_mod_item_types] collecting item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0658`.
error: could not compile `playground`.

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

[matthewjasper]

Minimized:

type Curried<A> = impl std::ops::Deref<Target = impl FnOnce(A)>;

fn curry() -> Curried<()> {
    Box::new(drop)
}

fn main() {}

[steffahn]

Ah, neat, so it has nothing to do with closures or Fn traits.

//#![feature(type_alias_impl_trait)]

trait WithAssoc { type AssocType; }

trait WithParam<A> {}

type Return<A> = impl WithAssoc<AssocType = impl WithParam<A>>;

fn my_fun() -> Return<()> {}

fn main() {}

[pnkfelix]

T-compiler triage: I'm going to assume from @matthewjasper 's self-assignment that this does not require discussion at T-compiler meeting. Removing I-nominated label.