ICE: tuple_fields called on non-tuple with type_alias_impl_trait, async block, and non-static trait lifetime

[samlich]

Code

Removing or making the lifetime static fixes it, as does removing the early return (but not making it conditional). Moving the async part to a standalone function also does not crash. Undef can be defined, but doesn't have to be.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=de619bbd48328b8854fd77e3338dfb3c

#![feature(type_alias_impl_trait)]
#![no_std]

pub trait Tr<'x> {
    type Fut: core::future::Future<Output = ()>;

    fn f() -> Self::Fut;
}

impl<'x> Tr<'x> for () {
    type Fut = impl core::future::Future<Output = ()>;

    fn f() -> Self::Fut {
        async {
            //if false {
            return ();
            //}
            let res: Undef = ();
            res
        }
    }
}

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (ffa2e7ae8 2020-10-24)
binary: rustc
commit-hash: ffa2e7ae8fbf9badc035740db949b9dae271c29f
commit-date: 2020-10-24
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly
LLVM version: 11.0

Error output

    Running `rustc --crate-name tuple_fields --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C metadata=c6835f7100680813 -C extra-filename=-c6835f7100680813 --out-dir /home/s/.cargo_target/debug/deps -C incremental=/home/s/.cargo_target/debug/incremental -L dependency=/home/s/.cargo_target/debug/deps -C link-arg=-fuse-ld=lld`
error[E0412]: cannot find type `Undef` in this scope
  --> src/lib.rs:18:22
   |
18 |             let res: Undef = ();
   |                      ^^^^^ not found in this scope

warning: unreachable statement
  --> src/lib.rs:18:13
   |
16 |             return ();
   |             --------- any code following this expression is unreachable
17 |             //}
18 |             let res: Undef = ();
   |             ^^^^^^^^^^^^^^^^^^^^ unreachable statement
   |
   = note: `#[warn(unreachable_code)]` on by default

error: internal compiler error: compiler/rustc_middle/src/ty/sty.rs:2150:18: tuple_fields called on non-tuple

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:945:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.49.0-nightly (ffa2e7ae8 2020-10-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental -C link-arg=-fuse-ld=lld --crate-type lib

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

query stack during panic:
#0 [typeck] type-checking `<() as Tr<'x>>::f`
#1 [type_of] computing type of `<() as Tr<'x>>::Fut::{opaque#0}`
end of query stack
error: aborting due to 2 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0412`.
error: could not compile `tuple_fields`

Caused by:
  process didn't exit successfully: `rustc --crate-name tuple_fields --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C metadata=c6835f7100680813 -C extra-filename=-c6835f7100680813 --out-dir /home/s/.cargo_target/debug/deps -C incremental=/home/s/.cargo_target/debug/incremental -L dependency=/home/s/.cargo_target/debug/deps -C link-arg=-fuse-ld=lld` (exit code: 101)

Backtrace

``` thread 'rustc' panicked at 'Box', compiler/rustc_errors/src/lib.rs:945:9 [24/44328] stack backtrace: 0: std::panicking::begin_panic 1: rustc_errors::HandlerInner::bug 2: rustc_errors::Handler::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::bug_fmt 8: rustc_middle::ty::sty::::tuple_fields 9: as rustc_middle::ty::fold::TypeVisitor>::visit_ty 10: rustc_middle::ty::fold::TypeFoldable::visit_with 11: rustc_middle::ty::structural_impls::::super_visit_with 12: as rustc_middle::ty::fold::TypeVisitor>::visit_ty 13: ::constrain_opaque_types 14: rustc_typeck::check::regionck::RegionCtxt::visit_fn_body 15: rustc_hir::intravisit::walk_expr 16: ::visit_expr 17: rustc_hir::intravisit::walk_expr 18: ::visit_expr 19: ::visit_expr 20: rustc_typeck::check::regionck::RegionCtxt::visit_fn_body 21: rustc_typeck::check::regionck::::regionck_fn 22: rustc_infer::infer::InferCtxtBuilder::enter 23: rustc_typeck::check::typeck 24: rustc_middle::ty::query:: for rustc_middle::ty::query::queries::typeck>::compute 25: rustc_middle::dep_graph::::with_deps 26: rustc_query_system::dep_graph::graph::DepGraph::with_task_impl 27: rustc_data_structures::stack::ensure_sufficient_stack 28: rustc_query_system::query::plumbing::get_query_impl 29: rustc_typeck::collect::type_of::find_opaque_ty_constraints::ConstraintLocator::check 30: rustc_hir::intravisit::walk_impl_item_ref 31: rustc_hir::intravisit::walk_item 32: rustc_typeck::collect::type_of::type_of 33: rustc_middle::ty::query:: for rustc_middle::ty::query::queries::type_of>::compute 34: rustc_middle::dep_graph::::with_deps 35: rustc_query_system::dep_graph::graph::DepGraph::with_task_impl 36: rustc_data_structures::stack::ensure_sufficient_stack 37: rustc_query_system::query::plumbing::get_query_impl 38: rustc_typeck::check::check::check_item_type 39: rustc_middle::hir::map::Map::visit_item_likes_in_module 40: rustc_typeck::check::check::check_mod_item_types 41: rustc_middle::ty::query:: for rustc_middle::ty::query::queries::check_mod_item_types>::compute 42: rustc_middle::dep_graph::::with_deps 43: rustc_query_system::dep_graph::graph::DepGraph::with_task_impl 44: rustc_data_structures::stack::ensure_sufficient_stack 45: rustc_query_system::query::plumbing::get_query_impl 46: rustc_query_system::query::plumbing::ensure_query_impl 47: rustc_session::utils::::time 48: rustc_typeck::check_crate 49: rustc_interface::passes::analysis 50: rustc_middle::ty::query:: for rustc_middle::ty::query::queries::analysis>::compute 51: rustc_middle::dep_graph::::with_deps 52: rustc_query_system::dep_graph::graph::DepGraph::with_task_impl 53: rustc_middle::ty::query::plumbing::::start_query::{{closure}}::{{closure}}::{{closure}} 54: rustc_query_system::query::plumbing::get_query_impl 55: rustc_interface::passes::QueryContext::enter 56: rustc_interface::queries::::enter 57: rustc_span::with_source_map 58: rustc_interface::interface::create_compiler_and_run 59: scoped_tls::ScopedKey::set note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace. ```

[camelid]

Note that it ICEs without #[no_std] as well.

[matthiaskrgr]

duplicate of https://github.com/rust-lang/rust/issues/77993 ?

[samlich]

duplicate of #77993 ?

I thought this looked different, but it doesn't break on 2020-10-06 nightly, so likely a duplicate.