panic: `cannot relate bound region: '^1_0.Named(FunctionId("foo")) <= '?0`

//@ check-pass

trait TraitA<'a> {
    type AsA;
}

trait TraitB<'a, 'b> {
    type AsB;
}

trait TraitC<'a, 'b, 'c> {}

struct X;

impl<'a, 'b, 'c> TraitC<'a, 'b, 'c> for X {}

struct Y;

impl<'a, 'b> TraitB<'a, 'b> for Y {
    type AsB = X;
}

struct Z;

impl<'a> TraitA<'a> for Z {
    type AsA = Y;
}

fn foo<T>()
where
    for<'a> T: TraitA<'a, AsA: for<'b> TraitB<'a, 'b, AsB: for<'c> TraitC<'a, 'b, 'c>>>,
{
}

fn main() {
    foo::<Z>();
}

this compiles with rustc
rust-analyzer analysis-stats . --run-all-ide-things

Database loaded:     560.07ms, 229minstr, 48mb (metadata 285.27ms, 1006kinstr, 150kb; build 29.71ms, 181kinstr, 1920b)
  item trees: 1
  dependency lines of code: 1_467_433, item trees: 2_948
  dependency item stats: traits: 848, impl: 9_686, mods: 3_059, macro calls: 5_580, macro rules: 1_054
Item Tree Collection: 4.11s, 23ginstr, 427mb
  Total Statistics:
    crates: 1, mods: 1, decls: 10, bodies: 2, adts: 3, consts: 0, signatures: 13, variants: 3
  Workspace:
    traits: 3, macro_rules macros: 0, proc_macros: 0
    lines of code: 37, item trees: 1
    usages: traits: 3, impl: 3, mods: 0, macro calls: 0, macro rules: 0
  Dependencies:
    lines of code: 1_467_433, item trees: 2_948
    declarations: traits: 848, impl: 9_686, mods: 3_059, macro calls: 5_580, macro rules: 1_054
Item Collection:     1.55s, 11ginstr, 179mb
Crate lang items:    233.21ms, 2474minstr, 31mb
Expression Store Lowering: 362.99µs, 953kinstr, 19kb
1/2 50% processing: ra::main
thread 'main' (1776853) panicked at src/tools/rust-analyzer/crates/hir-ty/src/next_solver/infer/region_constraints/mod.rs:448:17:
cannot relate bound region: '^1_0.Named(FunctionId("foo")) <= '?0
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <hir_ty::next_solver::infer::region_constraints::RegionConstraintCollector>::make_subregion
   3: <hir_ty::next_solver::infer::region_constraints::RegionConstraintCollector>::make_eqregion
   4: <ra_ap_rustc_type_ir::relate::solver_relating::SolverRelating<hir_ty::next_solver::infer::InferCtxt, hir_ty::next_solver::interner::DbInterner> as ra_ap_rustc_type_ir::relate::TypeRelation<hir_ty::next_solver::interner::DbInterner>>::relate::<hir_ty::next_solver::region::Region>
   5: <hir_ty::next_solver::generic_arg::GenericArg as ra_ap_rustc_type_ir::relate::Relate<hir_ty::next_solver::interner::DbInterner>>::relate::<ra_ap_rustc_type_ir::relate::solver_relating::SolverRelating<hir_ty::next_solver::infer::InferCtxt, hir_ty::next_solver::interner::DbInterner>>
   6: <ra_ap_rustc_type_ir::relate::solver_relating::SolverRelating<hir_ty::next_solver::infer::InferCtxt, hir_ty::next_solver::interner::DbInterner> as ra_ap_rustc_type_ir::relate::TypeRelation<hir_ty::next_solver::interner::DbInterner>>::relate_with_variance::<hir_ty::next_solver::generic_arg::GenericArg>
   7: <core::result::Result<hir_ty::next_solver::generic_arg::GenericArg, ra_ap_rustc_type_ir::error::TypeError<hir_ty::next_solver::interner::DbInterner>> as ra_ap_rustc_type_ir::interner::CollectAndApply<hir_ty::next_solver::generic_arg::GenericArg, hir_ty::next_solver::generic_arg::GenericArgs>>::collect_and_apply::<core::iter::adapters::map::Map<core::iter::adapters::zip::Zip<core::iter::adapters::copied::Copied<core::slice::iter::Iter<hir_ty::next_solver::generic_arg::GenericArg>>, core::iter::adapters::copied::Copied<core::slice::iter::Iter<hir_ty::next_solver::generic_arg::GenericArg>>>, ra_ap_rustc_type_ir::relate::relate_args_invariantly<hir_ty::next_solver::interner::DbInterner, ra_ap_rustc_type_ir::relate::solver_relating::SolverRelating<hir_ty::next_solver::infer::InferCtxt, hir_ty::next_solver::interner::DbInterner>>::{closure#0}>, <hir_ty::next_solver::generic_arg::GenericArgs>::new_from_iter<core::iter::adapters::map::Map<core::iter::adapters::zip::Zip<core::iter::adapters::copied::Copied<core::slice::iter::Iter<hir_ty::next_solver::generic_arg::GenericArg>>, core::iter::adapters::copied::Copied<core::slice::iter::Iter<hir_ty::next_solver::generic_arg::GenericArg>>>, ra_ap_rustc_type_ir::relate::relate_args_invariantly<hir_ty::next_solver::interner::DbInterner, ra_ap_rustc_type_ir::relate::solver_relating::SolverRelating<hir_ty::next_solver::infer::InferCtxt, hir_ty::next_solver::interner::DbInterner>>::{closure#0}>, core::result::Result<hir_ty::next_solver::generic_arg::GenericArg, ra_ap_rustc_type_ir::error::TypeError<hir_ty::next_solver::interner::DbInterner>>>::{closure#0}>
   8: <hir_ty::next_solver::infer::InferCtxt as ra_ap_rustc_type_ir::relate::solver_relating::RelateExt>::relate::<ra_ap_rustc_type_ir::predicate::TraitRef<hir_ty::next_solver::interner::DbInterner>>
   9: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::eq::<ra_ap_rustc_type_ir::predicate::TraitRef<hir_ty::next_solver::interner::DbInterner>>
  10: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::assemble_impl_candidates::<ra_ap_rustc_type_ir::predicate::TraitPredicate<hir_ty::next_solver::interner::DbInterner>>::{closure#0}
  11: <hir_ty::next_solver::interner::DbInterner as ra_ap_rustc_type_ir::interner::Interner>::for_each_relevant_impl::<<ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::assemble_impl_candidates<ra_ap_rustc_type_ir::predicate::TraitPredicate<hir_ty::next_solver::interner::DbInterner>>::{closure#0}>::{closure#0}::{closure#1}
  12: <hir_ty::method_resolution::TraitImpls>::for_each_crate_and_block_trait_and_type
  13: <hir_ty::next_solver::interner::DbInterner as ra_ap_rustc_type_ir::interner::Interner>::for_each_relevant_impl::<<ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::assemble_impl_candidates<ra_ap_rustc_type_ir::predicate::TraitPredicate<hir_ty::next_solver::interner::DbInterner>>::{closure#0}>::{closure#0}
  14: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::compute_trait_goal
  15: <ra_ap_rustc_type_ir::search_graph::SearchGraph<ra_ap_rustc_next_trait_solver::solve::search_graph::SearchGraphDelegate<hir_ty::next_solver::solver::SolverContext>, hir_ty::next_solver::interner::DbInterner>>::evaluate_goal_in_task
  16: <ra_ap_rustc_type_ir::search_graph::SearchGraph<ra_ap_rustc_next_trait_solver::solve::search_graph::SearchGraphDelegate<hir_ty::next_solver::solver::SolverContext>, hir_ty::next_solver::interner::DbInterner>>::evaluate_goal
  17: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::evaluate_goal_raw
  18: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::evaluate_goal
  19: <hir_ty::next_solver::solver::SolverContext as ra_ap_rustc_next_trait_solver::solve::eval_ctxt::SolverDelegateEvalExt>::evaluate_root_goal
  20: <hir_ty::next_solver::fulfill::FulfillmentCtxt>::try_evaluate_obligations
  21: <hir_ty::infer::InferenceContext>::check_call_arguments
  22: <hir_ty::infer::InferenceContext>::infer_call
  23: <hir_ty::infer::InferenceContext>::infer_expr_inner
  24: <hir_ty::infer::InferenceContext>::infer_expr_inner
  25: <hir_ty::infer::InferenceContext>::infer_return
  26: hir_ty::infer::infer_query_with_inspect
  27: <salsa::function::IngredientImpl<<hir_ty::infer::InferenceResult>::for_body::_::for_body__Configuration_>>::execute
  28: <salsa::attach::Attached>::attach::<dyn hir_ty::db::HirDatabase, &hir_ty::infer::InferenceResult, <hir_ty::infer::InferenceResult>::for_body::for_body_::{closure#0}>
  29: <rust_analyzer::cli::flags::AnalysisStats>::run_inference
  30: <rust_analyzer::cli::flags::AnalysisStats>::run::{closure#3}
  31: <rust_analyzer::cli::flags::AnalysisStats>::run
  32: rust_analyzer::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
infer panicked for ra::main: cannot relate bound region: '^1_0.Named(FunctionId("foo")) <= '?0
  exprs: 1, ??ty: 0 (0%), ?ty: 0 (0%), !ty: 0
  pats: 0, ??ty: 0 (100%), ?ty: 0 (100%), !ty: 0
  panics: 1
Inference:           197.48ms, 1408minstr, 40mb
2/2 100% mir lowering: ra::main2026-07-29T18:23:25.854038858+02:00  WARN Propagating panic for cycle head that panicked in an earlier execution in that revision

rust-analyzer 1.99.0-nightly (26ae60a 2026-07-28)

Start with the reproducer in the issue and run rust-analyzer analysis-stats . --run-all-ide-things. Read src/tools/rust-analyzer/crates/hir-ty/src/next_solver/infer/region_constraints/mod.rs around RegionConstraintCollector::make_subregion and follow the reported inference path. Done means the example completes without the bound-region panic.