ICE in type inference

[dlblv]

Code

struct S<T: Sized>(T);

impl<T: Sized> Iterator for S<T>{
    type Item = T;
    fn next(&mut self) -> Option<<Self as Iterator>::Item> {
        union U {
            a: <Self as Iterator>::Item,
        }
        None
    }
}

Meta

rustc --version --verbose:

rustc 1.53.0 (53cb7b09b 2021-06-17)
binary: rustc
commit-hash: 53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b
commit-date: 2021-06-17
host: x86_64-unknown-linux-gnu
release: 1.53.0
LLVM version: 12.0.1

Error output

error: internal compiler error: compiler/rustc_traits/src/normalize_erasing_regions.rs:54:32: could not fully normalize `<<S<T> as std::iter::Iterator>::next::U as std::iter::Iterator>::Item`

thread 'rustc' panicked at 'Box<Any>', /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/std/src/panic.rs:59:5
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 1.53.0 (53cb7b09b 2021-06-17) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

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

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<<impl at src/lib.rs:3:1: 12:2>::next::U as std::iter::Iterator>::Item`
#1 [check_mod_unstable_api_usage] checking for unstable API usage in top-level module
end of query stack

Backtrace

error: internal compiler error: compiler/rustc_traits/src/normalize_erasing_regions.rs:54:32: could not fully normalize `<<S<T> as std::iter::Iterator>::next::U as std::iter::Iterator>::Item`

thread 'rustc' panicked at 'Box<Any>', /rustc/53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b/library/std/src/panic.rs:59:5
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::InferCtxtBuilder::enter
   8: core::ops::function::FnOnce::call_once
   9: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  10: rustc_data_structures::stack::ensure_sufficient_stack
  11: rustc_query_system::query::plumbing::force_query_with_job
  12: rustc_query_system::query::plumbing::get_query_impl
  13: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::normalize_generic_arg_after_erasing_regions
  14: <rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty
  15: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::normalize_erasing_regions
  16: <rustc_passes::stability::Checker as rustc_hir::intravisit::Visitor>::visit_item
  17: rustc_middle::hir::map::Map::visit_item_likes_in_module
  18: rustc_passes::stability::check_mod_unstable_api_usage
  19: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  20: rustc_query_system::query::plumbing::force_query_with_job
  21: rustc_query_system::query::plumbing::get_query_impl
  22: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_mod_unstable_api_usage
  23: std::panic::catch_unwind
  24: rustc_session::utils::<impl rustc_session::session::Session>::time
  25: rustc_interface::passes::analysis
  26: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  27: rustc_data_structures::stack::ensure_sufficient_stack
  28: rustc_query_system::query::plumbing::force_query_with_job
  29: rustc_query_system::query::plumbing::get_query_impl
  30: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  31: rustc_interface::passes::QueryContext::enter
  32: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  33: rustc_span::with_source_map
  34: rustc_interface::interface::create_compiler_and_run
  35: 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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.53.0 (53cb7b09b 2021-06-17) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

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

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<<impl at src/lib.rs:3:1: 12:2>::next::U as std::iter::Iterator>::Item`
#1 [check_mod_unstable_api_usage] checking for unstable API usage in top-level module
#2 [analysis] running analysis passes on this crate
end of query stack

[jyn514]

Simplified:

union U {
    a: <Self as Iterator>::Item,
}

[apiraino]

I wonder if this could relate to this comment from a similar issue (there's a bunch of them).

[jyn514]

Yup, this is a duplicate of #81199, good catch :)