ICE: `expected type for ... but found ... when instantiating`

[cushionbadak]

Code

#![feature(min_generic_const_args)]
#![allow(incomplete_features)]

trait Trait1 {
    #[type_const]
    const ASSOC: usize;
}

trait Q {
    const ASSOC: usize;
}

impl<const N: usize> Q for [u8; N] {
    const ASSOC: usize = 1;
}

pub fn test<Z: Trait1>() -> [u8; <[u8; Z::ASSOC] as Q>::ASSOC] {}

fn main() {}

A mutant from tests/ui/const-generics/bad-subst-const-kind.rs

Meta

rustc --version --verbose:

rustc 1.89.0-nightly (1bbd62e54 2025-05-29)
binary: rustc
commit-hash: 1bbd62e547ba5cc08ccb44c27def3d33195d2dd5
commit-date: 2025-05-29
host: aarch64-apple-darwin
release: 1.89.0-nightly
LLVM version: 20.1.5

Error output

thread 'rustc' panicked at /rustc/1bbd62e547ba5cc08ccb44c27def3d33195d2dd5/compiler/rustc_type_ir/src/binder.rs:740:9:
expected type for `Z/#0` (Z/#0/0) but found Const(?0c) when instantiating, args=[?0c]

Backtrace

thread 'rustc' panicked at /rustc/1bbd62e547ba5cc08ccb44c27def3d33195d2dd5/compiler/rustc_type_ir/src/binder.rs:740:9:
expected type for `Z/#0` (Z/#0/0) but found Const(?0c) when instantiating, args=[?0c]
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt>>::type_param_expected
   3: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt> as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
   4: <&rustc_middle::ty::list::RawList<(), rustc_middle::ty::generic_args::GenericArg> as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::fold_with::<rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt>>
   5: <rustc_middle::ty::consts::Const as rustc_type_ir::fold::TypeSuperFoldable<rustc_middle::ty::context::TyCtxt>>::super_fold_with::<rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt>>
   6: <rustc_middle::ty::Ty as rustc_type_ir::fold::TypeSuperFoldable<rustc_middle::ty::context::TyCtxt>>::super_fold_with::<rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt>>
   7: <&rustc_middle::ty::list::RawList<(), rustc_middle::ty::generic_args::GenericArg> as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::fold_with::<rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt>>
   8: <rustc_middle::ty::consts::Const as rustc_type_ir::fold::TypeSuperFoldable<rustc_middle::ty::context::TyCtxt>>::super_fold_with::<rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt>>
   9: rustc_trait_selection::traits::project::opt_normalize_projection_term
  10: rustc_trait_selection::traits::project::normalize_projection_term
  11: <rustc_trait_selection::traits::normalize::AssocTypeNormalizer>::normalize_trait_projection::{closure#0}
  12: <rustc_trait_selection::traits::normalize::AssocTypeNormalizer as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_const
  13: <rustc_middle::ty::Ty as rustc_type_ir::fold::TypeSuperFoldable<rustc_middle::ty::context::TyCtxt>>::super_fold_with::<rustc_trait_selection::traits::normalize::AssocTypeNormalizer>
  14: <rustc_infer::infer::at::At as rustc_trait_selection::traits::normalize::NormalizeExt>::normalize::<rustc_middle::ty::Ty>
  15: <rustc_hir_analysis::check::wfcheck::WfCheckingCtxt>::normalize::<rustc_middle::ty::Ty>
  16: rustc_hir_analysis::check::wfcheck::check_fn_or_method
  17: rustc_hir_analysis::check::wfcheck::check_item_fn
  18: rustc_hir_analysis::check::wfcheck::check_item
  19: rustc_hir_analysis::check::wfcheck::check_well_formed
      [... omitted 1 frame ...]
  20: <rustc_middle::hir::ModuleItems>::par_opaques::<rustc_hir_analysis::check::wfcheck::check_mod_type_wf::{closure#4}>::{closure#0}
  21: rustc_hir_analysis::check::wfcheck::check_mod_type_wf
      [... omitted 1 frame ...]
  22: rustc_hir_analysis::check_crate
  23: rustc_interface::passes::run_required_analyses
  24: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  25: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
  26: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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: please attach the file at `/Users/jisukbyun/workspace/250203 scratch/rustc-ice-2025-05-30T16_11_31-96654.txt` to your bug report

query stack during panic:
#0 [check_well_formed] checking that `test` is well-formed
#1 [check_mod_type_wf] checking that types are well-formed in top-level module
#2 [analysis] running analysis passes on this crate
end of query stack

Bisects to

searched nightlies: from nightly-2025-01-01 to nightly-2025-05-30
regressed nightly: nightly-2025-05-24
searched commit range: 2eef478...3e674b0
regressed commit: 52bf0cf

bisected with cargo-bisect-rustc v0.6.9

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --end=2025-05-30 --regress ice --preserve --script rustc -- mgca_2.rs

Notes

• ICE location: compiler/rustc_type_ir/src/binder.rs Line-740
  

  rust/compiler/rustc_type_ir/src/binder.rs

  Lines 739 to 748 in 1bbd62e

  	fn type_param_expected(&self, p: I::ParamTy, ty: I::Ty, kind: ty::GenericArgKind<I>) -> ! {
  	panic!(
  	"expected type for `{:?}` ({:?}/{}) but found {:?} when instantiating, args={:?}",
  	p,
  	ty,
  	p.index(),
  	kind,
  	self.args,
  	)
  	}

@rustbot label +F-min_generic_const_args