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)]

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.life-white.uk-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.life-white.uk-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.

Minimized:

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

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

fn main() {}

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() {}

@matthewjasper

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.

Are nested type-alias-impl-traits even allowed by the RFC? If not, we should check and emit an error we we construct the type.

They are

Properly use parent generics for opaque types

Rollup merge of rust-lang#69008 - Aaron1011:fix/opaque-ty-parent, r=m…

@Aaron1011

@Aaron1011 any ideas how I can get my original example running though?
I was starting with this function (which does compile)

fn curry<'a, A: 'a, B, C, F: Fn(A, B) -> C> (f: &'a F)
    -> impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a
{
    move |a| move |b| f(a,b)
}

and wanted to give a name to its return type.
The straightforward approach seems to be

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)
}

But the compiler is still unhappy with lifetimes currently, after your fix. I'm getting the following.

error: cannot infer an appropriate lifetime
  --> src/main.rs:11:5
   |
6  |  = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;
   |                           ------------------------ this return type evaluates to the `'static` lifetime...
...
11 |     move |a| move |b| f(a,b)
   |     ^^^^^^^^^^^^^^^^^^^^^^^^ ...but this borrow...
   |
note: ...can't outlive the lifetime `'a` as defined on the function body at 8:10
  --> src/main.rs:8:10
   |
8  | fn curry<'a, A: 'a, B, C, F: Fn(A, B) -> C> (f: &'a F)
   |          ^^
help: you can add a bound to the return type to make it last less than `'static` and match the lifetime `'a` as defined on the function body at 8:10
   |
9  |     -> Curried<'a, A, B, C, F> + '_
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: cannot infer an appropriate lifetime
  --> src/main.rs:11:14
   |
6  |  = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;
   |                           ------------------------ this return type evaluates to the `'static` lifetime...
...
11 |     move |a| move |b| f(a,b)
   |              ^^^^^^^^^^^^^^^ ...but this borrow...
   |
note: ...can't outlive the lifetime `'a` as defined on the function body at 8:10
  --> src/main.rs:8:10
   |
8  | fn curry<'a, A: 'a, B, C, F: Fn(A, B) -> C> (f: &'a F)
   |          ^^
help: you can add a bound to the return type to make it last less than `'static` and match the lifetime `'a` as defined on the function body at 8:10
   |
9  |     -> Curried<'a, A, B, C, F> + '_
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

At least it works without the lifetimes (i.e. after removing parametrization over 'a and replacing all occurrences of 'a with 'static).

@Aaron1011

@Aaron1011
Also I'm now (self-compiled master with the fix) getting a panic on this (admittedly incorrect) program:

#![feature(type_alias_impl_trait)]

trait SomeTrait where {}

trait WithAssoc<A> where {
	type AssocType;
}

type Return<A> // 'a is not in scope
 = impl WithAssoc<A, AssocType = impl SomeTrait + 'a>;

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

fn main() {}

which didn't panic on the latest nightly (2020-02-13).
Edit: I double checked: the behavior changes precisely over your 2 commits.

Message is:

error[E0261]: use of undeclared lifetime name `'a`
  --> src/main.rs:11:51
   |
10 | type Return<A> // 'a is not in scope
   |             - help: consider introducing lifetime `'a` here: `'a,`
11 |  = impl WithAssoc<A, AssocType = impl SomeTrait + 'a>;
   |                                                   ^^ undeclared lifetime

error: internal compiler error: src/librustc/ty/subst.rs:496: Region parameter out of range when substituting in region 'a (root type=None) (index=0)

thread 'rustc' panicked at 'Box<Any>', <::std::macros::panic macros>:2:4
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::sys_common::backtrace::print
   4: std::panicking::default_hook::{{closure}}
   5: std::panicking::default_hook
   6: rustc_driver::report_ice
   7: std::panicking::rust_panic_with_hook
   8: std::panicking::begin_panic
   9: rustc_errors::HandlerInner::span_bug
  10: rustc_errors::Handler::span_bug
  11: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  12: rustc::ty::context::tls::with_opt::{{closure}}
  13: rustc::ty::context::tls::with_opt
  14: rustc::util::bug::opt_span_bug_fmt
  15: rustc::util::bug::span_bug_fmt
  16: <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_region
  17: rustc::ty::fold::TypeFoldable::fold_with
  18: <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_binder
  19: rustc::ty::subst::Subst::subst
  20: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  21: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::spec_extend
  22: rustc::ty::GenericPredicates::instantiate_into
  23: rustc::ty::GenericPredicates::instantiate
  24: rustc::infer::opaque_types::Instantiator::fold_opaque_ty
  25: <rustc::ty::fold::BottomUpFolder<F,G,H> as rustc::ty::fold::TypeFolder>::fold_ty
  26: rustc::ty::fold::TypeFoldable::fold_with
  27: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable for rustc::ty::sty::Binder<T>>::fold_with
  28: rustc::ty::fold::TypeFoldable::fold_with
  29: rustc::infer::opaque_types::Instantiator::instantiate_opaque_types_in_map
  30: rustc::infer::opaque_types::Instantiator::fold_opaque_ty
  31: <rustc::ty::fold::BottomUpFolder<F,G,H> as rustc::ty::fold::TypeFolder>::fold_ty
  32: rustc::infer::opaque_types::Instantiator::instantiate_opaque_types_in_map
  33: rustc::infer::opaque_types::<impl rustc::infer::InferCtxt>::instantiate_opaque_types
  34: rustc_typeck::check::FnCtxt::instantiate_opaque_types_from_value
  35: rustc_typeck::check::check_fn
  36: rustc::ty::context::GlobalCtxt::enter_local
  37: rustc::infer::InferCtxtBuilder::enter
  38: rustc_typeck::check::typeck_tables_of_with_fallback
  39: rustc_typeck::check::typeck_tables_of
  40: rustc::ty::query::__query_compute::typeck_tables_of
  41: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  42: rustc::dep_graph::graph::DepGraph::with_task_impl
  43: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  44: rustc_typeck::collect::find_opaque_ty_constraints::ConstraintLocator::check
  45: <rustc_typeck::collect::find_opaque_ty_constraints::ConstraintLocator as rustc_hir::intravisit::Visitor>::visit_item
  46: rustc_hir::intravisit::walk_crate
  47: rustc_typeck::collect::find_opaque_ty_constraints
  48: rustc_typeck::collect::type_of
  49: rustc::ty::query::__query_compute::type_of
  50: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  51: rustc::dep_graph::graph::DepGraph::with_task_impl
  52: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  53: <rustc_typeck::collect::CollectItemTypesVisitor as rustc_hir::intravisit::Visitor>::visit_item
  54: rustc::hir::map::Map::visit_item_likes_in_module
  55: rustc_typeck::collect::collect_mod_item_types
  56: rustc::ty::query::__query_compute::collect_mod_item_types
  57: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_mod_item_types>::compute
  58: rustc::dep_graph::graph::DepGraph::with_task_impl
  59: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  60: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
  61: rustc_session::session::Session::track_errors
  62: rustc_typeck::check_crate
  63: rustc_interface::passes::analysis
  64: rustc::ty::query::__query_compute::analysis
  65: rustc::dep_graph::graph::DepGraph::with_eval_always_task
  66: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  67: rustc::ty::context::tls::enter_global
  68: rustc_interface::interface::run_compiler_in_existing_thread_pool
  69: scoped_tls::ScopedKey<T>::set
  70: syntax::attr::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 1.43.0-dev running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [typeck_tables_of] processing `my_fun`
#1 [type_of] processing `Return`
#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 2 previous errors

For more information about this error, try `rustc --explain E0261`.

@steffahn

@steffahn: Can you open separate issues for each of those problems?

@Aaron1011

@Aaron1011 Okay, done ^^ (#69136, #69137)

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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)] #67844

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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)] #67844

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matthewjasper commented on Jan 3, 2020

steffahn commented on Jan 3, 2020

pnkfelix commented on Jan 16, 2020

Aaron1011 commented on Jan 23, 2020

matthewjasper commented on Jan 23, 2020

steffahn commented on Feb 13, 2020

steffahn commented on Feb 13, 2020

Aaron1011 commented on Feb 13, 2020

steffahn commented on Feb 13, 2020

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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)] #67844