[ICE]: internal compiler error: errors selecting obligation during MIR typeck: query type op DefId(..)

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally --crate-type=lib
pub trait Trait {
    type Assoc: Copy + 'static;
}

pub fn foo<T: Trait<Assoc = i32> + Trait<Assoc = i64>>() {
    const {}
}

original:

pub trait Trait {
    type Assoc: Copy + 'static;
}

const fn conjure<T>() -> T {
    panic!()
}

const fn get_assoc<T: Trait>() -> impl Copy {
    }

pub fn foo<T: Trait<Assoc = i32> + Trait<Assoc = i64>>() {
    const {
        get_assoc::<T>();
    }
}

Version information

rustc 1.95.0-nightly (621d76794 2026-01-18)
binary: rustc
commit-hash: 621d76794c76fc21c0a6151fbc090120e0230a91
commit-date: 2026-01-18
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 21.1.8

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Znext-solver=globally --crate-type=lib

Program output

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: errors selecting obligation during MIR typeck: query type op DefId(0:5 ~ mvce[9ef6]::foo) [Ambiguity, Ambiguity]
  |
  = note: delayed at /rustc-dev/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs:103:29
             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>::delayed_bug::<alloc::string::String>
             4: <rustc_borrowck::type_check::TypeChecker>::ascribe_user_type
             5: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_const_operand
             6: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_body
             7: rustc_borrowck::type_check::type_check
             8: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
             9: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
            10: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            11: rustc_query_impl::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
            12: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
            13: rustc_interface::passes::analysis
            14: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
            15: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            16: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
            17: <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}
            18: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
            19: 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}, ()>
            20: <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}
            21: <std::sys::thread::unix::Thread>::new::thread_start
            22: <unknown>
            23: <unknown>
          

error: internal compiler error: errors selecting obligation during MIR typeck: query type op DefId(0:5 ~ mvce[9ef6]::foo) [Ambiguity]
  |
  = note: delayed at /rustc-dev/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs:103:29
             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>::delayed_bug::<alloc::string::String>
             4: <rustc_borrowck::type_check::TypeChecker>::normalize_and_prove_instantiated_predicates
             5: <rustc_borrowck::type_check::TypeChecker as rustc_middle::mir::visit::Visitor>::visit_body
             6: rustc_borrowck::type_check::type_check
             7: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
             8: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
             9: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            10: rustc_query_impl::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
            11: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
            12: rustc_interface::passes::analysis
            13: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
            14: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            15: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
            16: <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}
            17: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
            18: 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}, ()>
            19: <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}
            20: <std::sys::thread::unix::Thread>::new::thread_start
            21: <unknown>
            22: <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 1.95.0-nightly (621d76794 2026-01-18) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z next-solver=globally --crate-type lib -Z dump-mir-dir=dir

query stack during panic:
end of query stack

@rustbot label +WG-trait-system-refactor

[matthiaskrgr]

this one goes form compiles (old solver) to ICE (new solver)

[matthiaskrgr]

Regression in nightly-2024-04-25

  commit[0] 2024-04-23: Auto merge of #123126 - oli-obk:feed_crate_num, r=davidtwco
  commit[1] 2024-04-23: Auto merge of #121557 - RalfJung:const-fn-call-promotion, r=oli-obk
  commit[2] 2024-04-24: Auto merge of #124312 - weihanglo:update-cargo, r=weihanglo
  commit[3] 2024-04-24: Auto merge of #122053 - erikdesjardins:alloca, r=nikic
  commit[4] 2024-04-24: Auto merge of #124175 - Kobzol:ci-dynamic-job, r=pietroalbini
  commit[5] 2024-04-24: Auto merge of #123792 - oli-obk:coroutine_closures, r=compiler-errors
  commit[6] 2024-04-24: Auto merge of #122500 - petrochenkov:deleg, r=fmease
  commit[7] 2024-04-24: Auto merge of #104087 - nbdd0121:const, r=scottmcm
  commit[8] 2024-04-24: Auto merge of #124330 - fmease:rollup-a98y7jf, r=fmease

[matthiaskrgr]

this one goes form compiles (old solver) to ICE (new solver)

just checked and this is stll crashing

cc @lcnr

[lcnr]

Unlike for impls, we don't check that the where-clauses of functions can be used to prove themselves, however, MIR borrowck requires proving the where-clauses of inline consts.

As proving T: Trait<Assoc = i32> + Trait<Assoc = i64> is ambiguous, this then fails, causing the ICE

[lcnr]

mentoring instructions in https://rust-lang.zulipchat.com/#narrow/channel/618216-t-types.2Fcall-for-participation/topic/make.20sure.20fn.20where-clauses.20can.20prove.20themselves/with/615617969

[lumi-me-not]

Could I take this one? (Also responded on the Zulip)