ICE(expected defining type in borrowck) when existential type causes type error

[meithecatte]

This might be related to #74280, but I am reporting this issue separately as the panic message and location are entirely different. When I try to minimize it further, the panic message changes.

Code

#![feature(type_alias_impl_trait)]

type Closure = impl Fn() -> u64;
struct Anonymous(Closure);

fn test() {
    let y = || -> Closure { || 3 };
    Anonymous(|| {
        3
    })
}

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (346aec9b0 2020-07-11)
binary: rustc
commit-hash: 346aec9b02f3c74f3fce97fd6bda24709d220e49
commit-date: 2020-07-11
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

Error output

error[E0308]: mismatched types
  --> rust-ice2.rs:8:15
   |
3  |   type Closure = impl Fn() -> u64;
   |                  ---------------- the expected opaque type
...
8  |       Anonymous(|| {
   |  _______________^
9  | |         3
10 | |     })
   | |_____^ expected opaque type, found closure
   |
   = note: expected opaque type `impl std::ops::Fn<()>`
                  found closure `[[email protected]:8:15: 10:6]`

error[E0308]: mismatched types
  --> rust-ice2.rs:8:5
   |
8  | /     Anonymous(|| {
9  | |         3
10 | |     })
   | |______^ expected `()`, found struct `Anonymous`
   |
help: try adding a semicolon
   |
10 |     });
   |       ^
help: try adding a return type
   |
6  | fn test() -> Anonymous {
   |           ^^^^^^^^^^^^

error: internal compiler error: src/librustc_mir/borrow_check/universal_regions.rs:532:26: expected defining type for `DefId(0:11 ~ rust_ice2[317d]::test[0]::{{closure}}[1])`: `[type error]`
  --> rust-ice2.rs:8:15
   |
8  |       Anonymous(|| {
   |  _______________^
9  | |         3
10 | |     })
   | |_____^

Backtrace

thread 'rustc' panicked at 'Box<Any>', /rustc/346aec9b02f3c74f3fce97fd6bda24709d220e49/src/libstd/macros.rs:13:23
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:530
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::span_bug
  14: rustc_errors::Handler::span_bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::span_bug_fmt
  20: rustc_mir::borrow_check::universal_regions::UniversalRegions::new
  21: rustc_mir::borrow_check::do_mir_borrowck
  22: rustc_infer::infer::InferCtxtBuilder::enter
  23: rustc_mir::borrow_check::mir_borrowck
  24: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  25: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  26: rustc_data_structures::stack::ensure_sufficient_stack
  27: rustc_query_system::query::plumbing::get_query_impl
  28: rustc_typeck::collect::type_of::find_opaque_ty_constraints::ConstraintLocator::check
  29: rustc_hir::intravisit::walk_expr
  30: rustc_hir::intravisit::walk_expr
  31: rustc_hir::intravisit::walk_fn
  32: rustc_hir::intravisit::walk_item
  33: rustc_hir::intravisit::Visitor::visit_nested_item
  34: rustc_typeck::collect::type_of::type_of
  35: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::type_of>::compute
  36: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  37: rustc_data_structures::stack::ensure_sufficient_stack
  38: rustc_query_system::query::plumbing::get_query_impl
  39: rustc_query_system::query::plumbing::ensure_query_impl
  40: <rustc_typeck::collect::CollectItemTypesVisitor as rustc_hir::intravisit::Visitor>::visit_item
  41: rustc_middle::hir::map::Map::visit_item_likes_in_module
  42: rustc_typeck::collect::collect_mod_item_types
  43: 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
  44: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  45: rustc_data_structures::stack::ensure_sufficient_stack
  46: rustc_query_system::query::plumbing::get_query_impl
  47: rustc_query_system::query::plumbing::ensure_query_impl
  48: rustc_session::session::Session::track_errors
  49: rustc_typeck::check_crate
  50: rustc_interface::passes::analysis
  51: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  52: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  53: rustc_query_system::query::plumbing::get_query_impl
  54: rustc_middle::ty::context::tls::enter_global
  55: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  56: rustc_span::with_source_map
  57: rustc_interface::interface::create_compiler_and_run
  58: scoped_tls::ScopedKey<T>::set
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 1.46.0-nightly (346aec9b0 2020-07-11) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_borrowck] borrow-checking `test::{{closure}}#1`
#1 [type_of] computing type of `Closure::{{opaque}}#0`
#2 [collect_mod_item_types] collecting item types in top-level module
#3 [analysis] running analysis passes on this crate
end of query stack

[matthiaskrgr]

This ICEs since 1.43
@rustbot modify labels: +regression-from-stable-to-stable

[camelid]

Assigning P-low and removing I-prioritize as discussed in the prioritization working group.