ICE due to RBV not handling RTN where trait is behind trait alias

[fmease]

Context: It's HIR ty lowering's job to resolve+lower RTN. However, it needs the lifetime resolutions from RBV which runs before HIR ty lowering and which (unfortunately) sort of duplicates some of HIR ty lowering code to resolve RTN itself. Now, while HIR ty lowering can handle RTN where the trait that defines the assoc fn is behind a trait alias, RBV can't yet. As a result, HIR ty lowering lacks the necessary resolutions and ICEs.

Example reproducer:

#![feature(return_type_notation, trait_alias)]

trait Al = Tr;
trait Tr { fn f() -> impl Sized; }

fn f<T: Al<f(..): Copy>>() {}

Compiler ICE message:

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: bad return type notation here
 --> src/lib.rs:6:12
  |
6 | fn f<T: Al<f(..): Copy>>() {}
  |            ^
  |

Full compiler output (includes backtrace):

Details

warning: trait `Tr` is never used
 --> src/lib.rs:4:7
  |
4 | trait Tr { fn f() -> impl Sized; }
  |       ^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: function `f` is never used
 --> src/lib.rs:6:4
  |
6 | fn f<T: Al<f(..): Copy>>() {}
  |    ^

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: bad return type notation here
 --> src/lib.rs:6:12
  |
6 | fn f<T: Al<f(..): Copy>>() {}
  |            ^
  |
note: delayed at /rustc-dev/db3e99bbab28c6ca778b13222becdea54533d908/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs:1889:26
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, &str>
         4: <rustc_hir_analysis::collect::resolve_bound_vars::BoundVarContext as rustc_hir::intravisit::Visitor>::visit_path
         5: <rustc_hir_analysis::collect::resolve_bound_vars::BoundVarContext>::visit_poly_trait_ref_inner
         6: <rustc_hir_analysis::collect::resolve_bound_vars::BoundVarContext as rustc_hir::intravisit::Visitor>::visit_where_predicate
         7: <rustc_hir_analysis::collect::resolve_bound_vars::BoundVarContext as rustc_hir::intravisit::Visitor>::visit_generics
         8: <rustc_hir_analysis::collect::resolve_bound_vars::BoundVarContext as rustc_hir::intravisit::Visitor>::visit_fn
         9: rustc_hir::intravisit::walk_item::<rustc_hir_analysis::collect::resolve_bound_vars::BoundVarContext>
        10: <rustc_hir_analysis::collect::resolve_bound_vars::BoundVarContext as rustc_hir::intravisit::Visitor>::visit_item
        11: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::resolve_bound_vars::make_query_vtable::{closure#1}::{closure#0}, rustc_middle::query::erase::ErasedData<[u8; 8]>>
        12: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::SemiDynamicQueryDispatcher<rustc_data_structures::vec_cache::VecCache<rustc_hir_id::OwnerId, rustc_middle::query::erase::ErasedData<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, {rustc_query_impl::QueryFlags { is_anon: false, is_depth_limit: false, is_feedable: false }}>, false>
        13: rustc_query_impl::query_impl::resolve_bound_vars::get_query_non_incr::__rust_end_short_backtrace
        14: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::named_variable_map::make_query_vtable::{closure#1}::{closure#0}, rustc_middle::query::erase::ErasedData<[u8; 8]>>
        15: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::SemiDynamicQueryDispatcher<rustc_data_structures::vec_cache::VecCache<rustc_hir_id::OwnerId, rustc_middle::query::erase::ErasedData<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, {rustc_query_impl::QueryFlags { is_anon: false, is_depth_limit: false, is_feedable: false }}>, false>
        16: rustc_query_impl::query_impl::named_variable_map::get_query_non_incr::__rust_end_short_backtrace
        17: <rustc_middle::ty::context::TyCtxt>::named_bound_var
        18: <dyn rustc_hir_analysis::hir_ty_lowering::HirTyLowerer>::lower_ty_param
        19: rustc_hir_analysis::collect::predicates_of::gather_explicit_predicates_of
        20: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::explicit_predicates_of::make_query_vtable::{closure#1}::{closure#0}, rustc_middle::query::erase::ErasedData<[u8; 24]>>
        21: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::SemiDynamicQueryDispatcher<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::ErasedData<[u8; 24]>>, {rustc_query_impl::QueryFlags { is_anon: false, is_depth_limit: false, is_feedable: true }}>, false>
        22: rustc_query_impl::query_impl::explicit_predicates_of::get_query_non_incr::__rust_end_short_backtrace
        23: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::predicates_of::make_query_vtable::{closure#1}::{closure#0}, rustc_middle::query::erase::ErasedData<[u8; 24]>>
        24: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::SemiDynamicQueryDispatcher<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::ErasedData<[u8; 24]>>, {rustc_query_impl::QueryFlags { is_anon: false, is_depth_limit: false, is_feedable: false }}>, false>
        25: rustc_query_impl::query_impl::predicates_of::get_query_non_incr::__rust_end_short_backtrace
        26: rustc_hir_analysis::check::check::check_item_type
        27: rustc_hir_analysis::check::wfcheck::check_well_formed
        28: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_well_formed::make_query_vtable::{closure#1}::{closure#0}, rustc_middle::query::erase::ErasedData<[u8; 1]>>
        29: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::SemiDynamicQueryDispatcher<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 1]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, {rustc_query_impl::QueryFlags { is_anon: false, is_depth_limit: false, is_feedable: false }}>, false>
        30: rustc_query_impl::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
        31: rustc_hir_analysis::check::wfcheck::check_type_wf
        32: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_type_wf::make_query_vtable::{closure#1}::{closure#0}, rustc_middle::query::erase::ErasedData<[u8; 1]>>
        33: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::SemiDynamicQueryDispatcher<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 1]>>, {rustc_query_impl::QueryFlags { is_anon: false, is_depth_limit: false, is_feedable: false }}>, false>
        34: rustc_query_impl::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
        35: rustc_hir_analysis::check_crate
        36: rustc_interface::passes::analysis
        37: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::make_query_vtable::{closure#1}::{closure#0}, rustc_middle::query::erase::ErasedData<[u8; 0]>>
        38: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::SemiDynamicQueryDispatcher<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, {rustc_query_impl::QueryFlags { is_anon: false, is_depth_limit: false, is_feedable: false }}>, false>
        39: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
        40: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
        41: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        42: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
        43: <std::thread::lifecycle::spawn_unchecked<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        44: <std::sys::thread::unix::Thread>::new::thread_start
        45: <unknown>
        46: clone
      
 --> src/lib.rs:6:12
  |
6 | fn f<T: Al<f(..): Copy>>() {}
  |            ^

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: please make sure that you have updated to the latest nightly

note: please attach the file at `/playground/rustc-ice-2026-02-05T08_31_41-26.txt` to your bug report

note: rustc 1.95.0-nightly (db3e99bba 2026-02-04) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

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

query stack during panic:
end of query stack
warning: `playground` (lib) generated 2 warnings
error: could not compile `playground` (lib); 2 warnings emitted