panic: `0.Named(FunctionId("two_htrb_trait_param")) is a region but value is Type(?0t)`

trait Foo<'a> {
    type Assoc;
}

impl Foo<'_> for () {
    type Assoc = ();
}

trait Bar<'a> {
    type Assoc;
}

impl<'a> Bar<'a> for () {
    type Assoc = &'a ();
}

trait Qux<'a> {}

impl Qux<'_> for () {}

fn two_htrb_trait_param() -> impl for<'a> Foo<'a, Assoc = impl for<'b> Qux<'a>> {}


fn main() {}

rust-analyzer analysis-stats . --run-all-ide-things

Database loaded:     587.35ms, 229minstr, 48mb (metadata 298.51ms, 1006kinstr, 152kb; build 31.92ms, 182kinstr, 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: 4.21s, 23ginstr, 427mb
  Total Statistics:
    crates: 1, mods: 1, decls: 7, bodies: 2, adts: 0, consts: 0, signatures: 10, variants: 0
  Workspace:
    traits: 3, macro_rules macros: 0, proc_macros: 0
    lines of code: 24, 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:     2.47s, 11ginstr, 179mb
Crate lang items:    445.99ms, 2473minstr, 31mb
Expression Store Lowering: 461.30µs, 654kinstr, 12kb
0/2 0% processing: ra::two_htrb_trait_param
thread 'main' (1783588) panicked at src/tools/rust-analyzer/crates/hir-ty/src/next_solver/infer/canonical/instantiate.rs:95:22:
0.Named(FunctionId("two_htrb_trait_param")) is a region but value is Type(?0t)
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: hir_ty::next_solver::infer::canonical::instantiate::instantiate_value::<hir_ty::next_solver::infer::canonical::QueryResponse<hir_ty::next_solver::ty::Ty>>::{closure#0}
   3: <hir_ty::next_solver::generic_arg::GenericArg as ra_ap_rustc_type_ir::fold::TypeFoldable<hir_ty::next_solver::interner::DbInterner>>::fold_with::<hir_ty::next_solver::fold::BoundVarReplacer<hir_ty::next_solver::fold::FnMutDelegate>>
   4: <hir_ty::next_solver::generic_arg::GenericArgs as ra_ap_rustc_type_ir::fold::TypeFoldable<hir_ty::next_solver::interner::DbInterner>>::fold_with::<hir_ty::next_solver::fold::BoundVarReplacer<hir_ty::next_solver::fold::FnMutDelegate>>
   5: <ra_ap_rustc_type_ir::predicate_kind::ClauseKind<hir_ty::next_solver::interner::DbInterner> as ra_ap_rustc_type_ir::fold::TypeFoldable<hir_ty::next_solver::interner::DbInterner>>::fold_with::<hir_ty::next_solver::fold::BoundVarReplacer<hir_ty::next_solver::fold::FnMutDelegate>>
   6: <ra_ap_rustc_type_ir::solve::Goal<hir_ty::next_solver::interner::DbInterner, hir_ty::next_solver::predicate::Predicate> as ra_ap_rustc_type_ir::fold::TypeFoldable<hir_ty::next_solver::interner::DbInterner>>::fold_with::<hir_ty::next_solver::fold::BoundVarReplacer<hir_ty::next_solver::fold::FnMutDelegate>>
   7: <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
   8: <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
   9: <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
  10: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::evaluate_goal
  11: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::try_evaluate_added_goals
  12: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::evaluate_added_goals_and_make_canonical_response
  13: <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
  14: <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
  15: <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
  16: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::try_evaluate_added_goals
  17: <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
  18: <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
  19: <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
  20: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::evaluate_goal
  21: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::try_evaluate_added_goals
  22: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::evaluate_added_goals_and_make_canonical_response
  23: <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
  24: <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
  25: <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
  26: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::try_evaluate_added_goals
  27: <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
  28: <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
  29: <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
  30: <ra_ap_rustc_next_trait_solver::solve::eval_ctxt::EvalCtxt<hir_ty::next_solver::solver::SolverContext, hir_ty::next_solver::interner::DbInterner>>::evaluate_goal
  31: <hir_ty::next_solver::solver::SolverContext as ra_ap_rustc_next_trait_solver::solve::eval_ctxt::SolverDelegateEvalExt>::evaluate_root_goal
  32: <hir_ty::next_solver::fulfill::FulfillmentCtxt>::try_evaluate_obligations
  33: <hir_ty::next_solver::infer::at::At>::structurally_normalize_term
  34: <hir_ty::infer::unify::InferenceTable>::try_structurally_resolve_type
  35: hir_ty::infer::infer_query_with_inspect
  36: <salsa::function::IngredientImpl<<hir_ty::infer::InferenceResult>::for_body::_::for_body__Configuration_>>::execute
  37: <salsa::attach::Attached>::attach::<dyn hir_ty::db::HirDatabase, &hir_ty::infer::InferenceResult, <hir_ty::infer::InferenceResult>::for_body::for_body_::{closure#0}>
  38: <rust_analyzer::cli::flags::AnalysisStats>::run_inference
  39: <rust_analyzer::cli::flags::AnalysisStats>::run::{closure#3}
  40: <rust_analyzer::cli::flags::AnalysisStats>::run
  41: rust_analyzer::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
infer panicked for ra::two_htrb_trait_param: 0.Named(FunctionId("two_htrb_trait_param")) is a region but value is Type(?0t)
  exprs: 1, ??ty: 0 (0%), ?ty: 0 (0%), !ty: 0
  pats: 0, ??ty: 0 (100%), ?ty: 0 (100%), !ty: 0
  panics: 1
Inference:           24.23ms, 63minstr, 7mb
1/2 50% mir lowering: ra::two_htrb_trait_param2026-07-29T18:30:14.728357633+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 by reproducing the panic with rust-analyzer analysis-stats . --run-all-ide-things using the Rust example in the issue. Then inspect src/tools/rust-analyzer/crates/hir-ty/src/next_solver/infer/canonical/instantiate.rs around line 95 and trace the canonical instantiation path. Done means the example no longer panics during analysis.