ICE: type parameter `E/#2` (E/2) out of range when substituting

[redgoldlace]

Code

trait ExampleTrait<I> {}
type BlahResult<I, E = impl ExampleTrait<I>> = Result<I, E>;

fn example<'a, T>() -> BlahResult<()> {}

Meta

rustc --version --verbose:

rustc 1.50.0 (cb75ad5db 2021-02-10)
binary: rustc
commit-hash: cb75ad5db02783e8b0222fee363c5f63f7e2cf5b
commit-date: 2021-02-10
host: x86_64-pc-windows-gnu
release: 1.50.0

I also had the same error on nightly, with the following version details:

rustc 1.52.0-nightly (45b3c2851 2021-03-04)
binary: rustc
commit-hash: 45b3c28518e4c45dfd12bc2c4400c0d0e9639927
commit-date: 2021-03-04
host: x86_64-pc-windows-gnu
release: 1.52.0-nightly
LLVM version: 12.0.0

Error output

error: internal compiler error: compiler\rustc_middle\src\ty\subst.rs:529:17: type parameter `E/#1` (E/1) out of range when substituting, substs=[()]
 --> src\lib.rs:4:24
  |
4 | fn example<'a, T>() -> BlahResult<()> {}
  |                        ^^^^^^^^^^^^^^

thread 'rustc' panicked at 'Box<Any>', /rustc/cb75ad5db02783e8b0222fee363c5f63f7e2cf5b\compiler\rustc_errors\src\lib.rs:904:9

query stack during panic:
#0 [fn_sig] computing function signature of `example`
#1 [collect_mod_item_types] collecting item types in top-level module
end of query stack

Backtrace

   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::span_bug
   2: rustc_errors::Handler::span_bug
   3: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
   4: rustc_middle::ty::context::tls::with_opt::{{closure}}
   5: rustc_middle::ty::context::tls::with_opt
   6: rustc_middle::util::bug::opt_span_bug_fmt
   7: rustc_middle::util::bug::span_bug_fmt
   8: <rustc_middle::ty::subst::SubstFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty
   9: rustc_middle::ty::fold::TypeFoldable::fold_with
  10: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_fold_with
  11: <<dyn rustc_typeck::astconv::AstConv>::create_substs_for_ast_path::SubstsForAstPathCtxt as rustc_typeck::astconv::CreateSubstsForGenericArgsCtxt>::inferred_kind
  12: <dyn rustc_typeck::astconv::AstConv>::create_substs_for_ast_path
  13: <dyn rustc_typeck::astconv::AstConv>::ast_path_substs_for_ty
  14: <dyn rustc_typeck::astconv::AstConv>::res_to_ty
  15: <dyn rustc_typeck::astconv::AstConv>::ast_ty_to_ty_inner
  16: <dyn rustc_typeck::astconv::AstConv>::ty_of_fn
  17: rustc_typeck::collect::fn_sig
  18: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::fn_sig>::compute
  19: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  20: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  21: rustc_data_structures::stack::ensure_sufficient_stack
  22: rustc_query_system::query::plumbing::get_query_impl
  23: rustc_query_system::query::plumbing::ensure_query_impl
  24: <rustc_typeck::collect::CollectItemTypesVisitor as rustc_hir::intravisit::Visitor>::visit_item
  25: rustc_middle::hir::map::Map::visit_item_likes_in_module
  26: rustc_typeck::collect::collect_mod_item_types
  27: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::collect_mod_item_types>::compute
  28: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  29: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  30: rustc_data_structures::stack::ensure_sufficient_stack
  31: rustc_query_system::query::plumbing::get_query_impl
  32: rustc_query_system::query::plumbing::ensure_query_impl
  33: rustc_typeck::check_crate
  34: rustc_interface::passes::analysis
  35: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  36: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  37: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  38: rustc_data_structures::stack::ensure_sufficient_stack
  39: rustc_query_system::query::plumbing::get_query_impl
  40: rustc_interface::passes::QueryContext::enter
  41: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  42: rustc_span::with_source_map
  43: rustc_interface::interface::create_compiler_and_run

[SNCPlay42]

Output on 1.36:

error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
 --> <source>:2:24
  |
2 | type BlahResult<I, E = impl ExampleTrait<I>> = Result<I, E>;
  |                        ^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

@rustbot label regression-from-stable-to-stable A-impl-trait

[matthiaskrgr]

Probably a duplicate of #78671

[JohnTitor]

@matthiaskrgr #78671 isn't ICE anymore but this one still is on nightly.

[apiraino]

bisection fails to identify a specific nightly because we dont have them anymore in the CI

********************************************************************************
Regression in nightly-2019-06-07
********************************************************************************
found 7 bors merge commits in the specified range
  commit[0] 2019-06-05UTC: Auto merge of #61548 - Centril:rollup-5t6cvbk, r=Centril
  commit[1] 2019-06-06UTC: Auto merge of #59331 - varkor:TypeVariableOrigin-refactor, r=eddyb
  commit[2] 2019-06-06UTC: Auto merge of #57428 - alexreg:associated_type_bounds, r=nikomatsakis,Centril
  commit[3] 2019-06-06UTC: Auto merge of #61373 - tmandry:emit-storagedead-along-unwind, r=eddyb
  commit[4] 2019-06-06UTC: Auto merge of #61494 - Mark-Simulacrum:move-to-cfg-bootstrap, r=alexcrichton
  commit[5] 2019-06-06UTC: Auto merge of #61583 - Centril:rollup-ug2cbfd, r=Centril
  commit[6] 2019-06-06UTC: Auto merge of #61601 - Centril:rollup-uegbsns, r=Centril
ERROR: no commits between 7cdaffd7962c4aae0cadd82baa241901b03f9458 and 5eeb567a27eba18420a620ca7d0c007e29d8bc0c within last 167 days

[apiraino]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

@rustbot label -I-prioritize +P-medium