[ICE]: next solver: missing generic: `called 'Result::unwrap_err()' on an 'Ok' value: {const error}`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
trait Foo {}
struct BarType<const N: usize>;
impl<const N: usize> Foo for BarType {}

fn a(x: &Foo) {
    let bar = BarType;
    a(bar);
}

original:

trait Foo {}
struct BarType<const N: usize>;
impl<const N: usize> Foo for BarType {} // expected 1 generic argument

fn a(x: &Foo) {
    let bar = BarType;
    a(bar);
}

fn main() {}

Version information

rustc 1.99.0-nightly (b9ee8d8bc 2026-07-21)
binary: rustc
commit-hash: b9ee8d8bcb1e4c7905a183a37e6d984c4b9a8468
commit-date: 2026-07-21
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8

Possibly related line of code:

rust/compiler/rustc_infer/src/infer/context.rs

Lines 518 to 530 in b9ee8d8

	return c;
	}
	match c.kind() {
	ty::ConstKind::Infer(ty::InferConst::Var(vid)) => {
	let vid = self.infcx.root_const_var(vid);
	let universe = self.infcx.try_resolve_const_var(vid).unwrap_err();
	if self.for_universe.can_name(universe) {
	c
	} else {
	let origin = self.infcx.const_var_origin(vid).unwrap();
	let new_var_id = self
	.infcx

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

Program output

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.Jl1slngaS2Bx/rustc_testrunner_tmpdir_reporting.3T320K0DEjES/mvce.rs:8:2
  |
8 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.Jl1slngaS2Bx/rustc_testrunner_tmpdir_reporting.3T320K0DEjES/mvce.rs`

error[E0107]: missing generics for struct `BarType`
 --> /tmp/icemaker_global_tempdir.Jl1slngaS2Bx/rustc_testrunner_tmpdir_reporting.3T320K0DEjES/mvce.rs:3:30
  |
3 | impl<const N: usize> Foo for BarType {}
  |                              ^^^^^^^ expected 1 generic argument
  |
note: struct defined here, with 1 generic parameter: `N`
 --> /tmp/icemaker_global_tempdir.Jl1slngaS2Bx/rustc_testrunner_tmpdir_reporting.3T320K0DEjES/mvce.rs:2:8
  |
2 | struct BarType<const N: usize>;
  |        ^^^^^^^ --------------
help: add missing generic argument
  |
3 | impl<const N: usize> Foo for BarType<N> {}
  |                                     +++

warning: trait objects without an explicit `dyn` are deprecated
 --> /tmp/icemaker_global_tempdir.Jl1slngaS2Bx/rustc_testrunner_tmpdir_reporting.3T320K0DEjES/mvce.rs:5:10
  |
5 | fn a(x: &Foo) {
  |          ^^^
  |
  = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
  = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2021/warnings-promoted-to-error.html>
  = note: `#[warn(bare_trait_objects)]` (part of `#[warn(rust_2021_compatibility)]`) on by default
help: if this is a dyn-compatible trait, use `dyn`
  |
5 | fn a(x: &dyn Foo) {
  |          +++


thread 'rustc' (1053593) panicked at /rustc-dev/b9ee8d8bcb1e4c7905a183a37e6d984c4b9a8468/compiler/rustc_infer/src/infer/context.rs:524:70:
called `Result::unwrap_err()` on an `Ok` value: {const error}
stack backtrace:
   0:     0x7f0c64e29516 - <<std[e7e31dd881df98bb]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[c88a0a802f7ea8d1]::fmt::Display>::fmt
   1:     0x7f0c6540d912 - core[c88a0a802f7ea8d1]::fmt::write
   2:     0x7f0c64e3e66c - <std[e7e31dd881df98bb]::sys::stdio::unix::Stderr as core[c88a0a802f7ea8d1]::io::write::Write>::write_fmt
   3:     0x7f0c64dfb14a - std[e7e31dd881df98bb]::panicking::default_hook::{closure#0}
   4:     0x7f0c64e1bbb3 - std[e7e31dd881df98bb]::panicking::default_hook
   5:     0x7f0c63da090b - std[e7e31dd881df98bb]::panicking::update_hook::<alloc[ec3dd5f39d329b4e]::boxed::Box<rustc_driver_impl[383cde084ff42635]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f0c64e1c052 - std[e7e31dd881df98bb]::panicking::panic_with_hook
   7:     0x7f0c64dfb202 - std[e7e31dd881df98bb]::panicking::panic_handler::{closure#0}
   8:     0x7f0c64df2df9 - std[e7e31dd881df98bb]::sys::backtrace::__rust_end_short_backtrace::<std[e7e31dd881df98bb]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f0c64dfcb5d - __rustc[db4185f8d0a68659]::rust_begin_unwind
  10:     0x7f0c6198dd0c - core[c88a0a802f7ea8d1]::panicking::panic_fmt
  11:     0x7f0c613a2de2 - core[c88a0a802f7ea8d1]::result::unwrap_failed
  12:     0x7f0c6673f745 - <rustc_infer[268a96dbfa1d9cb9]::infer::context::LowerUniverseFolder as rustc_type_ir[3f031e8ec3f876a6]::fold::TypeFolder<rustc_middle[c436af0429e6724f]::ty::context::TyCtxt>>::fold_const
  13:     0x7f0c659d20f0 - <&rustc_middle[c436af0429e6724f]::ty::list::RawList<(), rustc_middle[c436af0429e6724f]::ty::generic_args::GenericArg> as rustc_type_ir[3f031e8ec3f876a6]::fold::TypeFoldable<rustc_middle[c436af0429e6724f]::ty::context::TyCtxt>>::fold_with::<rustc_infer[268a96dbfa1d9cb9]::infer::context::LowerUniverseFolder>
  14:     0x7f0c659d242d - <rustc_middle[c436af0429e6724f]::ty::Ty as rustc_type_ir[3f031e8ec3f876a6]::fold::TypeSuperFoldable<rustc_middle[c436af0429e6724f]::ty::context::TyCtxt>>::super_fold_with::<rustc_infer[268a96dbfa1d9cb9]::infer::context::LowerUniverseFolder>
  15:     0x7f0c659cee15 - <rustc_infer[268a96dbfa1d9cb9]::infer::context::LowerUniverseFolder as rustc_type_ir[3f031e8ec3f876a6]::fold::TypeFolder<rustc_middle[c436af0429e6724f]::ty::context::TyCtxt>>::fold_ty
  16:     0x7f0c659cea66 - <rustc_infer[268a96dbfa1d9cb9]::infer::InferCtxt as rustc_type_ir[3f031e8ec3f876a6]::infer_ctxt::InferCtxtLike>::instantiate_ty_var_raw
  17:     0x7f0c659cf48c - <rustc_next_trait_solver[80bb1502c5d2ca88]::canonical::ResponseRelating<rustc_infer[268a96dbfa1d9cb9]::infer::InferCtxt, rustc_middle[c436af0429e6724f]::ty::context::TyCtxt> as rustc_type_ir[3f031e8ec3f876a6]::relate::TypeRelation<rustc_middle[c436af0429e6724f]::ty::context::TyCtxt>>::tys
  18:     0x7f0c659cf250 - rustc_next_trait_solver[80bb1502c5d2ca88]::canonical::unify_query_var_values::<rustc_trait_selection[ea9d61c240fbd158]::solve::delegate::SolverDelegate, rustc_middle[c436af0429e6724f]::ty::context::TyCtxt>
  19:     0x7f0c663194f5 - <rustc_trait_selection[ea9d61c240fbd158]::solve::inspect::analyse::InspectCandidate>::instantiate_nested_goals
  20:     0x7f0c65db0031 - <rustc_trait_selection[ea9d61c240fbd158]::solve::inspect::analyse::InspectCandidate>::visit_nested_no_probe::<rustc_hir_typeck[f17462d0efe89a72]::coercion::CoerceVisitor>
  21:     0x7f0c65dad0e0 - <rustc_hir_typeck[f17462d0efe89a72]::coercion::CoerceVisitor as rustc_trait_selection[ea9d61c240fbd158]::solve::inspect::analyse::ProofTreeVisitor>::visit_goal
  22:     0x7f0c6630c4e0 - <rustc_hir_typeck[f17462d0efe89a72]::coercion::Coerce>::coerce
  23:     0x7f0c640bba1a - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::may_coerce
  24:     0x7f0c6409037c - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::suggest_deref_or_ref
  25:     0x7f0c64094bb7 - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::suggest_deref_ref_or_into
  26:     0x7f0c6408849d - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::emit_type_mismatch_suggestions
  27:     0x7f0c64083a92 - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::emit_coerce_suggestions
  28:     0x7f0c640b27b4 - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::report_arg_errors
  29:     0x7f0c656534db - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::check_expr_call
  30:     0x7f0c656113fc - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  31:     0x7f0c65606d6b - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::check_expr_block
  32:     0x7f0c656113d8 - <rustc_hir_typeck[f17462d0efe89a72]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  33:     0x7f0c6562b8f5 - rustc_hir_typeck[f17462d0efe89a72]::check::check_fn
  34:     0x7f0c666df19f - rustc_hir_typeck[f17462d0efe89a72]::typeck_with_inspect::{closure#0}
  35:     0x7f0c666dd8b0 - rustc_query_impl[9a9b2dbab4aa42cf]::query_impl::typeck_root::invoke_provider_fn::__rust_begin_short_backtrace
  36:     0x7f0c65ab4629 - rustc_query_impl[9a9b2dbab4aa42cf]::execution::try_execute_query::<rustc_data_structures[b573e1ebd6629dca]::vec_cache::VecCache<rustc_span[5e0315be33f6440b]::def_id::LocalDefId, rustc_middle[c436af0429e6724f]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[c436af0429e6724f]::dep_graph::graph::DepNodeIndex>, false>
  37:     0x7f0c65ab4083 - rustc_query_impl[9a9b2dbab4aa42cf]::query_impl::typeck_root::execute_query_non_incr::__rust_end_short_backtrace
  38:     0x7f0c65ab6f7e - rustc_hir_analysis[18680bb4726cf8e]::check_crate
  39:     0x7f0c65ab0a4b - rustc_interface[1cdecb41dc13d5b0]::passes::analysis
  40:     0x7f0c6662c9c7 - rustc_query_impl[9a9b2dbab4aa42cf]::execution::try_execute_query::<rustc_middle[c436af0429e6724f]::query::caches::SingleCache<rustc_middle[c436af0429e6724f]::query::erase::ErasedData<[u8; 0usize]>>, false>
  41:     0x7f0c6662c6a9 - rustc_query_impl[9a9b2dbab4aa42cf]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  42:     0x7f0c66602ecd - rustc_interface[1cdecb41dc13d5b0]::interface::run_compiler::<(), rustc_driver_impl[383cde084ff42635]::run_compiler::{closure#0}>::{closure#2}
  43:     0x7f0c6663a8a4 - std[e7e31dd881df98bb]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[1cdecb41dc13d5b0]::util::run_in_thread_with_globals<rustc_interface[1cdecb41dc13d5b0]::util::run_in_thread_pool_with_globals<rustc_interface[1cdecb41dc13d5b0]::interface::run_compiler<(), rustc_driver_impl[383cde084ff42635]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  44:     0x7f0c6663a673 - <std[e7e31dd881df98bb]::thread::lifecycle::spawn_unchecked<rustc_interface[1cdecb41dc13d5b0]::util::run_in_thread_with_globals<rustc_interface[1cdecb41dc13d5b0]::util::run_in_thread_pool_with_globals<rustc_interface[1cdecb41dc13d5b0]::interface::run_compiler<(), rustc_driver_impl[383cde084ff42635]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[c88a0a802f7ea8d1]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  45:     0x7f0c6664547a - <std[e7e31dd881df98bb]::sys::thread::unix::Thread>::new::thread_start
  46:     0x7f0c5fc97739 - <unknown>
  47:     0x7f0c5fd1bedc - <unknown>
  48:                0x0 - <unknown>

error: 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: please make sure that you have updated to the latest nightly

note: rustc 1.99.0-nightly (b9ee8d8bc 2026-07-21) running on x86_64-unknown-linux-gnu

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

query stack during panic:
#0 [typeck_root] type-checking `a`
#1 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 2 previous errors; 1 warning emitted

Some errors have detailed explanations: E0107, E0601.
For more information about an error, try `rustc --explain E0107`.

@rustbot label +WG-trait-system-refactor