ICE: `When translating generic parameters from DefId, the expected specialization failed to hold`

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(specialization)]

trait Spec {
    type Assoc;
}

default impl<T, U> Spec for T
where
    T: IntoIterator<Item = U>,
{
    type Assoc = U;
}

impl<T> Spec for [T; 0] {}

fn main() {
    let x: <[_; 0] as Spec>::Assoc = 1;
}

original:

//@ revisions: current next
//@ ignore-compare-mode-next-solver (explicit revisions)
//@[next] compile-flags: -Znext-solver
//@[next] check-pass
//@[current] known-bug: unknown
//@[current] failure-status: 101
//~^ ERROR defaults for generic parameters are not allowed in `for<...>` binders

// Tests that rebasing from the concrete impl to the default impl also processes the
// `[u32; 0]: IntoIterator<Item = ?U>` predicate to constrain the `?U` impl arg.
// This test also makes sure that we don't do anything weird when rebasing the args
// is ambiguous.

#![feature(specialization)]
//[next]~^ WARN the feature `specialization` is incomplete

trait Spec {
    type Assoc;
}

default impl<T, U> Spec for T where T: IntoIterator<Item = U> {
    type Assoc = U;
}

impl<T> Spec for [T; 0] {}

fn main() {
    let x: <[_; 0] as Spec>::Assoc = 1;
}

Version information

rustc 1.84.0-nightly (ef972a346 2024-11-02)
binary: rustc
commit-hash: ef972a346668ed4234d1a43ed4ad7ca4e9c58d51
commit-date: 2024-11-02
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1

Possibly related line of code:

rust/compiler/rustc_trait_selection/src/traits/specialize/mod.rs

Lines 121 to 133 in ef972a3

	if source_impl == target_impl {
	return source_args;
	}
	fulfill_implication(infcx, param_env, source_trait_ref, source_impl, target_impl, cause)
	.unwrap_or_else(|()| {
	bug!(
	"When translating generic parameters from {source_impl:?} to \
	{target_impl:?}, the expected specialization failed to hold"
	)
	})
	}
	specialization_graph::Node::Trait(..) => source_trait_ref.args,

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

Program output

warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.jxbDO1GMyC8v/rustc_testrunner_tmpdir_reporting.X0a6Q55Ph3Zn/mvce.rs:1:12
  |
1 | #![feature(specialization)]
  |            ^^^^^^^^^^^^^^
  |
  = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
  = help: consider using `min_specialization` instead, which is more stable and complete
  = note: `#[warn(incomplete_features)]` on by default

error: internal compiler error: compiler/rustc_trait_selection/src/traits/specialize/mod.rs:127:21: When translating generic parameters from DefId(0:9 ~ mvce[4f49]::{impl#1}) to DefId(0:5 ~ mvce[4f49]::{impl#0}), the expected specialization failed to hold

thread 'rustc' panicked at compiler/rustc_trait_selection/src/traits/specialize/mod.rs:127:21:
Box<dyn Any>
stack backtrace:
   0:     0x7737134584aa - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hce429173f4835cf6
   1:     0x773713c040ca - core::fmt::write::hd474fbdfe63c8008
   2:     0x77371507b011 - std::io::Write::write_fmt::ha6b1be51a74d6c82
   3:     0x773713458302 - std::sys::backtrace::BacktraceLock::print::h9a37e75f7192ac21
   4:     0x77371345a806 - std::panicking::default_hook::{{closure}}::hd9ac8cf20a0b7897
   5:     0x77371345a650 - std::panicking::default_hook::hdde16dcfa738c4aa
   6:     0x7737124ddb49 - std[85c38ff02e03f0e6]::panicking::update_hook::<alloc[39d8aefd6e9d2d6]::boxed::Box<rustc_driver_impl[8ea1370c87ac4c52]::install_ice_hook::{closure#0}>>::{closure#0}
   7:     0x77371345af18 - std::panicking::rust_panic_with_hook::h6eef9cc88a2d4e79
   8:     0x773712517231 - std[85c38ff02e03f0e6]::panicking::begin_panic::<rustc_errors[a3358f633eebb703]::ExplicitBug>::{closure#0}
   9:     0x77371250a206 - std[85c38ff02e03f0e6]::sys::backtrace::__rust_end_short_backtrace::<std[85c38ff02e03f0e6]::panicking::begin_panic<rustc_errors[a3358f633eebb703]::ExplicitBug>::{closure#0}, !>
  10:     0x773712505829 - std[85c38ff02e03f0e6]::panicking::begin_panic::<rustc_errors[a3358f633eebb703]::ExplicitBug>
  11:     0x773712520e01 - <rustc_errors[a3358f633eebb703]::diagnostic::BugAbort as rustc_errors[a3358f633eebb703]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x773712b9a363 - rustc_middle[b16c40c3cb132ea9]::util::bug::opt_span_bug_fmt::<rustc_span[94bdfa138cad1541]::span_encoding::Span>::{closure#0}
  13:     0x773712b8095a - rustc_middle[b16c40c3cb132ea9]::ty::context::tls::with_opt::<rustc_middle[b16c40c3cb132ea9]::util::bug::opt_span_bug_fmt<rustc_span[94bdfa138cad1541]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x773712b807eb - rustc_middle[b16c40c3cb132ea9]::ty::context::tls::with_context_opt::<rustc_middle[b16c40c3cb132ea9]::ty::context::tls::with_opt<rustc_middle[b16c40c3cb132ea9]::util::bug::opt_span_bug_fmt<rustc_span[94bdfa138cad1541]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x773710ccf260 - rustc_middle[b16c40c3cb132ea9]::util::bug::bug_fmt
  16:     0x773713c65283 - rustc_trait_selection[7d6948a7a05ac95]::traits::specialize::translate_args_with_cause::<rustc_trait_selection[7d6948a7a05ac95]::traits::specialize::translate_args::{closure#0}>
  17:     0x77371469a013 - rustc_trait_selection[7d6948a7a05ac95]::traits::project::opt_normalize_projection_term
  18:     0x7737146931df - <rustc_trait_selection[7d6948a7a05ac95]::traits::normalize::AssocTypeNormalizer as rustc_type_ir[45f01c5603667543]::fold::TypeFolder<rustc_middle[b16c40c3cb132ea9]::ty::context::TyCtxt>>::fold_ty
  19:     0x773713f06b53 - <rustc_hir_typeck[643602d1ca0da9fa]::fn_ctxt::FnCtxt>::normalize::<rustc_middle[b16c40c3cb132ea9]::ty::Ty>
  20:     0x7737145c13fb - <dyn rustc_hir_analysis[614b5c894f29c678]::hir_ty_lowering::HirTyLowerer>::lower_ty
  21:     0x7737147b4fa9 - <rustc_hir_typeck[643602d1ca0da9fa]::gather_locals::GatherLocalsVisitor>::declare
  22:     0x773713f2336d - <rustc_hir_typeck[643602d1ca0da9fa]::gather_locals::GatherLocalsVisitor as rustc_hir[885137f5f5b6a9f]::intravisit::Visitor>::visit_expr
  23:     0x773713f24f19 - rustc_hir_typeck[643602d1ca0da9fa]::check::check_fn
  24:     0x773713f1b2f5 - rustc_hir_typeck[643602d1ca0da9fa]::typeck
  25:     0x773713f1ac93 - rustc_query_impl[c0318c643aee4f05]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c0318c643aee4f05]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b16c40c3cb132ea9]::query::erase::Erased<[u8; 8usize]>>
  26:     0x773714321d81 - rustc_query_system[708ef94f36d60214]::query::plumbing::try_execute_query::<rustc_query_impl[c0318c643aee4f05]::DynamicConfig<rustc_query_system[708ef94f36d60214]::query::caches::VecCache<rustc_span[94bdfa138cad1541]::def_id::LocalDefId, rustc_middle[b16c40c3cb132ea9]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[c0318c643aee4f05]::plumbing::QueryCtxt, false>
  27:     0x77371432024d - rustc_query_impl[c0318c643aee4f05]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  28:     0x77371431fec7 - <rustc_middle[b16c40c3cb132ea9]::hir::map::Map>::par_body_owners::<rustc_hir_analysis[614b5c894f29c678]::check_crate::{closure#4}>::{closure#0}
  29:     0x77371431de99 - rustc_hir_analysis[614b5c894f29c678]::check_crate
  30:     0x7737140e384a - rustc_interface[46946d42a40348f7]::passes::run_required_analyses
  31:     0x7737147d451e - rustc_interface[46946d42a40348f7]::passes::analysis
  32:     0x7737147d44ef - rustc_query_impl[c0318c643aee4f05]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c0318c643aee4f05]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b16c40c3cb132ea9]::query::erase::Erased<[u8; 1usize]>>
  33:     0x773714ba1bae - rustc_query_system[708ef94f36d60214]::query::plumbing::try_execute_query::<rustc_query_impl[c0318c643aee4f05]::DynamicConfig<rustc_query_system[708ef94f36d60214]::query::caches::SingleCache<rustc_middle[b16c40c3cb132ea9]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[c0318c643aee4f05]::plumbing::QueryCtxt, false>
  34:     0x773714ba188e - rustc_query_impl[c0318c643aee4f05]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  35:     0x773714a65292 - rustc_interface[46946d42a40348f7]::interface::run_compiler::<core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>, rustc_driver_impl[8ea1370c87ac4c52]::run_compiler::{closure#0}>::{closure#1}
  36:     0x773714af0590 - std[85c38ff02e03f0e6]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[46946d42a40348f7]::util::run_in_thread_with_globals<rustc_interface[46946d42a40348f7]::util::run_in_thread_pool_with_globals<rustc_interface[46946d42a40348f7]::interface::run_compiler<core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>, rustc_driver_impl[8ea1370c87ac4c52]::run_compiler::{closure#0}>::{closure#1}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#0}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>
  37:     0x773714af09ab - <<std[85c38ff02e03f0e6]::thread::Builder>::spawn_unchecked_<rustc_interface[46946d42a40348f7]::util::run_in_thread_with_globals<rustc_interface[46946d42a40348f7]::util::run_in_thread_pool_with_globals<rustc_interface[46946d42a40348f7]::interface::run_compiler<core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>, rustc_driver_impl[8ea1370c87ac4c52]::run_compiler::{closure#0}>::{closure#1}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#0}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d9ceb9bc2a384707]::result::Result<(), rustc_span[94bdfa138cad1541]::ErrorGuaranteed>>::{closure#1} as core[d9ceb9bc2a384707]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  38:     0x773714af1479 - std::sys::pal::unix::thread::Thread::new::thread_start::h00969398ad9e9953
  39:     0x77371631139d - <unknown>
  40:     0x77371639649c - <unknown>
  41:                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 1.84.0-nightly (ef972a346 2024-11-02) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [typeck] type-checking `main`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 1 previous error; 1 warning emitted

@rustbot label +F-specialization

[matthiaskrgr]

Regression in nightly-2021-04-26
commit[0] 2021-04-24: Auto merge of #84511 - davidhewitt:revert-84445, r=GuillaumeGomez
commit[1] 2021-04-24: Auto merge of #84310 - RalfJung:const-fn-feature-flags, r=oli-obk
commit[2] 2021-04-25: Auto merge of #80339 - jyn514:no-span, r=GuillaumeGomez
commit[3] 2021-04-25: Auto merge of #84115 - CDirkx:rt, r=m-ou-se
commit[4] 2021-04-25: Auto merge of #84147 - cuviper:array-method-dispatch, r=nikomatsakis,m-ou-se
commit[5] 2021-04-25: Auto merge of #84216 - RalfJung:black-box, r=Mark-Simulacrum
commit[6] 2021-04-25: Auto merge of #84299 - lcnr:const-generics-defaults-name-res, r=varkor
commit[7] 2021-04-25: Auto merge of #84481 - RalfJung:miri, r=RalfJung
commit[8] 2021-04-25: Auto merge of #84325 - jsha:ephemeral-collapse, r=GuillaumeGomez