Panic with GATs during lexical region resolve

[hlb8122]

Code

#![feature(generic_associated_types)]

trait GatTrait {
    type Gat<'a>;
    
    fn test(&self) -> Self::Gat<'_>;
}

trait SuperTrait<T>
where
    for<'a> Self: GatTrait<Gat<'a> = &'a T>,
{
    fn copy(&self) -> Self::Gat<'_> where T: Copy {
        *self.test()
    }
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=3ee6652c9d7ef9f1bd9658a44e0e1215

Note that this does not panic if we actually write correct Rust and switch to fn copy(&self) -> T.

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (0afc20860 2021-08-25)

Error output

error: internal compiler error: compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs:498:17: cannot relate region: LUB(ReErased, ReEmpty(U0))

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1145:9
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 1.56.0-nightly (0afc20860 2021-08-25) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [typeck] type-checking `SuperTrait::copy`
#1 [typeck_item_bodies] type-checking all item bodies
end of query stack
For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

Backtrace

stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_infer::infer::lexical_region_resolve::LexicalResolver::lub_concrete_regions
   8: rustc_infer::infer::lexical_region_resolve::LexicalResolver::expand_node
   9: rustc_infer::infer::lexical_region_resolve::LexicalResolver::infer_variable_values
  10: rustc_infer::infer::lexical_region_resolve::resolve
  11: rustc_infer::infer::InferCtxt::resolve_regions_and_report_errors
  12: rustc_typeck::check::regionck::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::regionck_fn
  13: rustc_infer::infer::InferCtxtBuilder::enter
  14: rustc_typeck::check::typeck
  15: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  16: rustc_data_structures::stack::ensure_sufficient_stack
  17: rustc_query_system::query::plumbing::force_query_with_job
  18: rustc_query_system::query::plumbing::get_query_impl
  19: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::typeck
  20: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::par_body_owners
  21: rustc_typeck::check::typeck_item_bodies
  22: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  23: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  24: rustc_query_system::query::plumbing::force_query_with_job
  25: rustc_query_system::query::plumbing::get_query_impl
  26: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::typeck_item_bodies
  27: rustc_session::utils::<impl rustc_session::session::Session>::time
  28: rustc_typeck::check_crate
  29: rustc_interface::passes::analysis
  30: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  31: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  32: rustc_query_system::dep_graph::graph::DepGraph<K>::with_eval_always_task
  33: rustc_data_structures::stack::ensure_sufficient_stack
  34: rustc_query_system::query::plumbing::force_query_with_job
  35: rustc_query_system::query::plumbing::get_query_impl
  36: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  37: rustc_interface::passes::QueryContext::enter
  38: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  39: rustc_span::with_source_map
  40: scoped_tls::ScopedKey<T>::set