rust-lang / rust-lang/rust

[ICE]: `it should not be possible to encounter unnormalized aliases in borrowck`

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C-bug F-async_fn_traits F-explicit_tail_calls F-type_alias_impl_trait F-unboxed_closures I-ICE needs-triage T-compiler
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

type MyClosure = impl AsyncFnOnce<()>;
type MyFuture = <MyClosure as AsyncFnOnce<()>>::CallOnceFuture;
impl MyClosure {
    async fn foo(self) -> MyFuture {
        become self()
    }
}

original:

#![feature(explicit_tail_calls)]
#![feature(type_alias_impl_trait)]
#![feature(async_fn_traits)]
#![feature(unboxed_closures)]
type MyClosure = impl AsyncFnOnce<()>;
type MyFuture = <MyClosure as AsyncFnOnce<()>>::CallOnceFuture;
impl MyClosure {
    async fn foo(self) -> MyFuture {
        become self()
    }
}

fn main() {}

Version information

rustc 1.100.0-nightly (74b3f086e 2026-09-12)
binary: rustc
commit-hash: 74b3f086e27d20f04611a52addc3781576f3c749
commit-date: 2026-09-12
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

Possibly related line of code:
https://github.com/rust-lang/rust/blob/74b3f086e27d20f04611a52addc3781576f3c749/compiler/rustc_borrowck/src/type_check/relate_tys.rs#L407-L419

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output 

error[E0670]: `async fn` is not permitted in Rust 2015
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:4:5
  |
4 |     async fn foo(self) -> MyFuture {
  |     ^^^^^ to use `async fn`, switch to Rust 2018 or later
  |
  = help: pass `--edition 2024` to `rustc`
  = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error[E0658]: `become` expression is experimental
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:5:9
  |
5 |         become self()
  |         ^^^^^^^^^^^^^
  |
  = note: see issue #112788 <https://github.com/rust-lang/rust/issues/112788> for more information
  = help: add `#![feature(explicit_tail_calls)]` to the crate attributes to enable
  = note: this compiler was built on 2026-09-12; consider upgrading it if it is out of date

error[E0658]: `impl Trait` in type aliases is unstable
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:1:18
  |
1 | type MyClosure = impl AsyncFnOnce<()>;
  |                  ^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
  = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
  = note: this compiler was built on 2026-09-12; consider upgrading it if it is out of date

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:7:2
  |
7 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs`

error[E0658]: use of unstable library feature `async_fn_traits`
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:2:17
  |
2 | type MyFuture = <MyClosure as AsyncFnOnce<()>>::CallOnceFuture;
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add `#![feature(async_fn_traits)]` to the crate attributes to enable
  = note: this compiler was built on 2026-09-12; consider upgrading it if it is out of date

error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:2:17
  |
2 | type MyFuture = <MyClosure as AsyncFnOnce<()>>::CallOnceFuture;
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `AsyncFnOnce() -> ()`
  |
  = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
  = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
  = note: this compiler was built on 2026-09-12; consider upgrading it if it is out of date

error: unconstrained opaque type
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:1:18
  |
1 | type MyClosure = impl AsyncFnOnce<()>;
  |                  ^^^^^^^^^^^^^^^^^^^^
  |
  = note: `MyClosure` must be used in combination with a concrete type within the same crate

error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:1:23
  |
1 | type MyClosure = impl AsyncFnOnce<()>;
  |                       ^^^^^^^^^^^^^^^ help: use parenthetical notation instead: `AsyncFnOnce() -> ()`
  |
  = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
  = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable
  = note: this compiler was built on 2026-09-12; consider upgrading it if it is out of date

error: `become` is not allowed in closures
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:5:9
  |
5 |         become self()
  |         ^^^^^^^^^^^^^

error: internal compiler error: /rustc-dev/74b3f086e27d20f04611a52addc3781576f3c749/compiler/rustc_borrowck/src/type_check/relate_tys.rs:413:17: it should not be possible to encounter unnormalized aliases in borrowck
 --> /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:4:36
  |
4 |       async fn foo(self) -> MyFuture {
  |  ____________________________________^
5 | |         become self()
6 | |     }
  | |_____^


thread 'rustc' (744594) panicked at /rustc-dev/74b3f086e27d20f04611a52addc3781576f3c749/compiler/rustc_borrowck/src/type_check/relate_tys.rs:413:17:
Box<dyn Any>
stack backtrace:
   0:     0x7ff8ac43d6f6 - <<std[3528600ba3f9fe66]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[9253e626ef1be868]::fmt::Display>::fmt
   1:     0x7ff8aca0988f - core[9253e626ef1be868]::fmt::write
   2:     0x7ff8ac4526bc - <std[3528600ba3f9fe66]::sys::stdio::unix::Stderr as core[9253e626ef1be868]::io::write::Write>::write_fmt
   3:     0x7ff8ac4103da - std[3528600ba3f9fe66]::panicking::default_hook::{closure#0}
   4:     0x7ff8ac431e53 - std[3528600ba3f9fe66]::panicking::default_hook
   5:     0x7ff8ab16269f - std[3528600ba3f9fe66]::panicking::update_hook::<alloc[19f73542e5485bc1]::boxed::Box<rustc_driver_impl[4014d772c3bdc445]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7ff8ac4322f2 - std[3528600ba3f9fe66]::panicking::panic_with_hook
   7:     0x7ff8ab191624 - std[3528600ba3f9fe66]::panicking::begin_panic::<rustc_errors[ece56505557c1b0d]::ExplicitBug>::{closure#0}
   8:     0x7ff8ab18a216 - std[3528600ba3f9fe66]::sys::backtrace::__rust_end_short_backtrace::<std[3528600ba3f9fe66]::panicking::begin_panic<rustc_errors[ece56505557c1b0d]::ExplicitBug>::{closure#0}, !>
   9:     0x7ff8ab18819e - std[3528600ba3f9fe66]::panicking::begin_panic::<rustc_errors[ece56505557c1b0d]::ExplicitBug>
  10:     0x7ff8ab19cda1 - <rustc_errors[ece56505557c1b0d]::diagnostic::BugAbort as rustc_errors[ece56505557c1b0d]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7ff8ab7ea78c - <rustc_errors[ece56505557c1b0d]::DiagCtxtHandle>::span_bug::<rustc_span[2d93c3ee6901fd4d]::span_encoding::Span, alloc[19f73542e5485bc1]::string::String>
  12:     0x7ff8ab818856 - rustc_middle[5a5f5798a8f38072]::util::bug::opt_span_bug_fmt::<rustc_span[2d93c3ee6901fd4d]::span_encoding::Span>::{closure#0}
  13:     0x7ff8ab818cb2 - rustc_middle[5a5f5798a8f38072]::ty::context::tls::with_opt::<rustc_middle[5a5f5798a8f38072]::util::bug::opt_span_bug_fmt<rustc_span[2d93c3ee6901fd4d]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7ff8ab803a40 - rustc_middle[5a5f5798a8f38072]::ty::context::tls::with_context_opt::<rustc_middle[5a5f5798a8f38072]::ty::context::tls::with_opt<rustc_middle[5a5f5798a8f38072]::util::bug::opt_span_bug_fmt<rustc_span[2d93c3ee6901fd4d]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7ff8a95c9fc8 - rustc_middle[5a5f5798a8f38072]::util::bug::span_bug_fmt::<rustc_span[2d93c3ee6901fd4d]::span_encoding::Span>
  16:     0x7ff8ad5596f4 - <rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg as rustc_type_ir[e133639d9cb9d719]::relate::Relate<rustc_middle[5a5f5798a8f38072]::ty::context::TyCtxt>>::relate::<rustc_borrowck[fb2d02eb91b19704]::type_check::relate_tys::NllTypeRelating>
  17:     0x7ff8ad557184 - <core[9253e626ef1be868]::result::Result<rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg, rustc_type_ir[e133639d9cb9d719]::error::TypeError<rustc_middle[5a5f5798a8f38072]::ty::context::TyCtxt>> as rustc_type_ir[e133639d9cb9d719]::interner::CollectAndApply<rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg, &rustc_middle[5a5f5798a8f38072]::ty::list::RawList<(), rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg>>>::collect_and_apply::<core[9253e626ef1be868]::iter::adapters::map::Map<core[9253e626ef1be868]::iter::adapters::zip::Zip<core[9253e626ef1be868]::iter::adapters::copied::Copied<core[9253e626ef1be868]::slice::iter::Iter<rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg>>, core[9253e626ef1be868]::iter::adapters::copied::Copied<core[9253e626ef1be868]::slice::iter::Iter<rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg>>>, rustc_type_ir[e133639d9cb9d719]::relate::relate_args_invariantly<rustc_middle[5a5f5798a8f38072]::ty::context::TyCtxt, rustc_borrowck[fb2d02eb91b19704]::type_check::relate_tys::NllTypeRelating>::{closure#0}>, <rustc_middle[5a5f5798a8f38072]::ty::context::TyCtxt>::mk_args_from_iter<core[9253e626ef1be868]::iter::adapters::map::Map<core[9253e626ef1be868]::iter::adapters::zip::Zip<core[9253e626ef1be868]::iter::adapters::copied::Copied<core[9253e626ef1be868]::slice::iter::Iter<rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg>>, core[9253e626ef1be868]::iter::adapters::copied::Copied<core[9253e626ef1be868]::slice::iter::Iter<rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg>>>, rustc_type_ir[e133639d9cb9d719]::relate::relate_args_invariantly<rustc_middle[5a5f5798a8f38072]::ty::context::TyCtxt, rustc_borrowck[fb2d02eb91b19704]::type_check::relate_tys::NllTypeRelating>::{closure#0}>, core[9253e626ef1be868]::result::Result<rustc_middle[5a5f5798a8f38072]::ty::generic_args::GenericArg, rustc_type_ir[e133639d9cb9d719]::error::TypeError<rustc_middle[5a5f5798a8f38072]::ty::context::TyCtxt>>>::{closure#0}>
  18:     0x7ff8ad562b47 - rustc_type_ir[e133639d9cb9d719]::relate::structurally_relate_tys::<rustc_middle[5a5f5798a8f38072]::ty::context::TyCtxt, rustc_borrowck[fb2d02eb91b19704]::type_check::relate_tys::NllTypeRelating>::{closure#0}
  19:     0x7ff8a911d8b3 - rustc_borrowck[fb2d02eb91b19704]::type_check::type_check
  20:     0x7ff8add4af75 - rustc_borrowck[fb2d02eb91b19704]::borrowck_collect_region_constraints
  21:     0x7ff8add5586a - <rustc_borrowck[fb2d02eb91b19704]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  22:     0x7ff8add54704 - rustc_borrowck[fb2d02eb91b19704]::mir_borrowck
  23:     0x7ff8add54419 - rustc_query_impl[d5422e89ac02de23]::query_vtables::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
  24:     0x7ff8ad0cdc75 - rustc_query_impl[d5422e89ac02de23]::execution::try_execute_query::<rustc_data_structures[1609676847358c32]::vec_cache::VecCache<rustc_span[2d93c3ee6901fd4d]::def_id::LocalDefId, rustc_middle[5a5f5798a8f38072]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[5a5f5798a8f38072]::dep_graph::graph::DepNodeIndex>, false>
  25:     0x7ff8ad0cd7e7 - rustc_query_impl[d5422e89ac02de23]::query_vtables::mir_borrowck::execute_query_non_incr::__rust_end_short_backtrace
  26:     0x7ff8ad8d52ce - rustc_hir_analysis[a3e24397d7d8df45]::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
  27:     0x7ff8ad8d4faa - rustc_hir_analysis[a3e24397d7d8df45]::collect::type_of::type_of_opaque
  28:     0x7ff8aca47c3d - rustc_query_impl[d5422e89ac02de23]::execution::try_execute_query::<rustc_middle[5a5f5798a8f38072]::query::caches::DefIdCache<rustc_middle[5a5f5798a8f38072]::query::erase::ErasedData<[u8; 8usize]>>, false>
  29:     0x7ff8adcd072a - rustc_query_impl[d5422e89ac02de23]::query_vtables::type_of_opaque::execute_query_non_incr::__rust_end_short_backtrace
  30:     0x7ff8ace23ded - rustc_hir_analysis[a3e24397d7d8df45]::collect::type_of::type_of
  31:     0x7ff8ace265a6 - rustc_query_impl[d5422e89ac02de23]::query_vtables::type_of::invoke_provider_fn::__rust_begin_short_backtrace
  32:     0x7ff8aca47c3d - rustc_query_impl[d5422e89ac02de23]::execution::try_execute_query::<rustc_middle[5a5f5798a8f38072]::query::caches::DefIdCache<rustc_middle[5a5f5798a8f38072]::query::erase::ErasedData<[u8; 8usize]>>, false>
  33:     0x7ff8aca478e3 - rustc_query_impl[d5422e89ac02de23]::query_vtables::type_of::execute_query_non_incr::__rust_end_short_backtrace
  34:     0x7ff8ad8d57e8 - rustc_hir_analysis[a3e24397d7d8df45]::check::check::check_opaque
  35:     0x7ff8ad7b987b - rustc_hir_analysis[a3e24397d7d8df45]::check::check::check_item_type
  36:     0x7ff8ad7b04dc - rustc_hir_analysis[a3e24397d7d8df45]::check::wfcheck::check_well_formed
  37:     0x7ff8ad7b049f - rustc_query_impl[d5422e89ac02de23]::query_vtables::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  38:     0x7ff8ad7afafb - rustc_query_impl[d5422e89ac02de23]::execution::try_execute_query::<rustc_data_structures[1609676847358c32]::vec_cache::VecCache<rustc_span[2d93c3ee6901fd4d]::def_id::LocalDefId, rustc_middle[5a5f5798a8f38072]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[5a5f5798a8f38072]::dep_graph::graph::DepNodeIndex>, false>
  39:     0x7ff8ad7af895 - rustc_query_impl[d5422e89ac02de23]::query_vtables::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
  40:     0x7ff8ad7a8576 - <rustc_middle[5a5f5798a8f38072]::hir::ModuleItems>::par_opaques::<rustc_hir_analysis[a3e24397d7d8df45]::check::wfcheck::check_type_wf::{closure#5}>
  41:     0x7ff8ad7ad0bb - rustc_hir_analysis[a3e24397d7d8df45]::check::wfcheck::check_type_wf
  42:     0x7ff8ad7acc27 - rustc_query_impl[d5422e89ac02de23]::query_vtables::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  43:     0x7ff8ada76689 - rustc_query_impl[d5422e89ac02de23]::execution::try_execute_query::<rustc_middle[5a5f5798a8f38072]::query::caches::SingleCache<rustc_middle[5a5f5798a8f38072]::query::erase::ErasedData<[u8; 1usize]>>, false>
  44:     0x7ff8ada7642d - rustc_query_impl[d5422e89ac02de23]::query_vtables::check_type_wf::execute_query_non_incr::__rust_end_short_backtrace
  45:     0x7ff8ad1c711a - rustc_hir_analysis[a3e24397d7d8df45]::check_crate
  46:     0x7ff8ad0cf233 - rustc_interface[a9b0b26ac5f97aef]::passes::analysis
  47:     0x7ff8ada75dca - rustc_query_impl[d5422e89ac02de23]::execution::try_execute_query::<rustc_middle[5a5f5798a8f38072]::query::caches::SingleCache<rustc_middle[5a5f5798a8f38072]::query::erase::ErasedData<[u8; 0usize]>>, false>
  48:     0x7ff8ada75b6b - rustc_query_impl[d5422e89ac02de23]::query_vtables::analysis::execute_query_non_incr::__rust_end_short_backtrace
  49:     0x7ff8adce7fad - rustc_interface[a9b0b26ac5f97aef]::interface::run_compiler::<(), rustc_driver_impl[4014d772c3bdc445]::run_compiler::{closure#0}>::{closure#2}
  50:     0x7ff8addae664 - std[3528600ba3f9fe66]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[a9b0b26ac5f97aef]::util::run_in_thread_with_globals<rustc_interface[a9b0b26ac5f97aef]::util::run_in_thread_pool_with_globals<rustc_interface[a9b0b26ac5f97aef]::interface::run_compiler<(), rustc_driver_impl[4014d772c3bdc445]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  51:     0x7ff8addae40b - <std[3528600ba3f9fe66]::thread::lifecycle::spawn_unchecked<rustc_interface[a9b0b26ac5f97aef]::util::run_in_thread_with_globals<rustc_interface[a9b0b26ac5f97aef]::util::run_in_thread_pool_with_globals<rustc_interface[a9b0b26ac5f97aef]::interface::run_compiler<(), rustc_driver_impl[4014d772c3bdc445]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[9253e626ef1be868]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  52:     0x7ff8addb77db - <std[3528600ba3f9fe66]::sys::thread::unix::Thread>::new::thread_start
  53:     0x7ff8a6c980a2 - <unknown>
  54:     0x7ff8a6d2080c - <unknown>
  55:                0x0 - <unknown>

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: rustc 1.100.0-nightly (74b3f086e 2026-09-12) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_borrowck] borrow-checking `<impl at /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:3:1: 3:15>::foo`
#1 [type_of_opaque] computing type of opaque `<impl at /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:3:1: 3:15>::foo::{opaque#0}`
#2 [type_of] computing type of `<impl at /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:3:1: 3:15>::foo::{opaque#0}`
#3 [check_well_formed] checking that `<impl at /tmp/icemaker_global_tempdir.iQ7cEy1zH8N7/rustc_testrunner_tmpdir_reporting.f4c8fWrUpSCp/mvce.rs:3:1: 3:15>::foo::{opaque#0}` is well-formed
#4 [check_type_wf] checking that types are well-formed
#5 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 10 previous errors

Some errors have detailed explanations: E0601, E0658, E0670.
For more information about an error, try `rustc --explain E0601`.

@rustbot label +F-explicit_tail_calls +F-type_alias_impl_trait +F-async_fn_traits +F-unboxed_closures

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by compiling the auto-reduced example with the listed nightly rustc command and enabled features, then inspect compiler/rustc_borrowck/src/type_check/relate_tys.rs around line 413. Compare the borrow-checking path reached by the example with the ICE output; done means the reproducer no longer triggers an internal compiler error and reports a normal result.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Active
Clarity
Needs clarification
Newbie friendliness
42/100

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