panic: `could not replace AliasTerm `

trait Other<T> {}

trait Super<T> {
    type Assoc;
}

trait Foo<A, B>: Super<<A as Mirror>::Assoc, Assoc = A> {
    type FooAssoc: Other<<Self as Super<<A as Super>::Assoc>>::Assoc>;
}

fn is_foo<F: Foo + ?Sized, T, U>() {}

fn main() {
    is_foo::<dyn Foo<i32, u32, FooAssoc = i32>, _, _>();
}

RUST_BACKTRACE=full RUSTFLAGS="" ~/.rustup/toolchains/master/bin/rust-analyzer highlight < code.rs

thread 'main' (1166083) panicked at /rust/deps/ra-ap-rustc_next_trait_solver-0.165.0/src/solve/assembly/structural_traits.rs:1005:13:
could not replace AliasTerm { args: [dyn [Binder { value: Trait(ExistentialTraitRef("Foo"[[i32, u32]])), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [?0t], term: i32, .. }), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("FooAssoc"), args: [i32, u32], term: i32, .. }), bound_vars: [] }] + '!1, Alias(AliasTy { args: [i32, {type error}], kind: Projection { def_id: TypeAliasId("Assoc") }, .. })], kind: ProjectionTy { def_id: TypeAliasId("Assoc") }, .. } with term from from dyn [Binder { value: Trait(ExistentialTraitRef("Foo"[[i32, u32]])), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [?0t], term: i32, .. }), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("FooAssoc"), args: [i32, u32], term: i32, .. }), bound_vars: [] }] + '!1
stack backtrace:
   0:     0x7f9c7684bed6 - <<std[cf0678d25afc9ccf]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[9662e8fc1527f23c]::fmt::Display>::fmt
   1:     0x7f9c76e0f0c8 - core[9662e8fc1527f23c]::fmt::write
   2:     0x7f9c7686113c - <std[cf0678d25afc9ccf]::sys::stdio::unix::Stderr as core[9662e8fc1527f23c]::io::write::Write>::write_fmt
   3:     0x7f9c7681e04a - std[cf0678d25afc9ccf]::panicking::default_hook::{closure#0}
   4:     0x7f9c7683e5f3 - std[cf0678d25afc9ccf]::panicking::default_hook
   5:     0x7f9c7683ea5b - std[cf0678d25afc9ccf]::panicking::panic_with_hook
   6:     0x7f9c7681e102 - std[cf0678d25afc9ccf]::panicking::panic_handler::{closure#0}
   7:     0x7f9c76816a89 - std[cf0678d25afc9ccf]::sys::backtrace::__rust_end_short_backtrace::<std[cf0678d25afc9ccf]::panicking::panic_handler::{closure#0}, !>
   8:     0x7f9c7681fa5d - __rustc[c7d2cd00dee8f709]::rust_begin_unwind
   9:     0x7f9c73560bac - core[9662e8fc1527f23c]::panicking::panic_fmt
  10:     0x55bac9748ddc - <ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::structural_traits::ReplaceProjectionWith<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner, hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext>>::try_eagerly_replace_alias
  11:     0x55bac9683d0b - <hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArg as ra_ap_rustc_type_ir[4e6de8744a899384]::fold::TypeFoldable<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::try_fold_with::<ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::structural_traits::ReplaceProjectionWith<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner, hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext>>
  12:     0x55bac96793c3 - <core[9662e8fc1527f23c]::result::Result<hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArg, ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::structural_traits::Ambiguous> as ra_ap_rustc_type_ir[4e6de8744a899384]::interner::CollectAndApply<hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArg, hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArgs>>::collect_and_apply::<core[9662e8fc1527f23c]::iter::adapters::map::Map<core[9662e8fc1527f23c]::iter::adapters::copied::Copied<core[9662e8fc1527f23c]::slice::iter::Iter<hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArg>>, <hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArgs as ra_ap_rustc_type_ir[4e6de8744a899384]::fold::TypeFoldable<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::try_fold_with<ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::structural_traits::ReplaceProjectionWith<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner, hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext>>::{closure#0}>, <hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArgs>::new_from_iter<core[9662e8fc1527f23c]::iter::adapters::map::Map<core[9662e8fc1527f23c]::iter::adapters::copied::Copied<core[9662e8fc1527f23c]::slice::iter::Iter<hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArg>>, <hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArgs as ra_ap_rustc_type_ir[4e6de8744a899384]::fold::TypeFoldable<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::try_fold_with<ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::structural_traits::ReplaceProjectionWith<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner, hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext>>::{closure#0}>, core[9662e8fc1527f23c]::result::Result<hir_ty[cdb9a6d3e6bba482]::next_solver::generic_arg::GenericArg, ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::structural_traits::Ambiguous>>::{closure#0}>
  13:     0x55bac95cf781 - ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::structural_traits::predicates_for_object_candidate::<hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext, hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>
  14:     0x55bac968902d - <ra_ap_rustc_type_ir[4e6de8744a899384]::predicate::TraitPredicate<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner> as ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::GoalKind<hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext, hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::probe_and_match_goal_against_assumption::<<ra_ap_rustc_type_ir[4e6de8744a899384]::predicate::TraitPredicate<hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner> as ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::assembly::GoalKind<hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext, hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::probe_and_consider_object_bound_candidate::{closure#0}>
  15:     0x55bac976b333 - <ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::eval_ctxt::EvalCtxt<hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext, hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::compute_trait_goal
  16:     0x55bac97e1e61 - <ra_ap_rustc_type_ir[4e6de8744a899384]::search_graph::SearchGraph<ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::search_graph::SearchGraphDelegate<hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext>, hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::evaluate_goal_in_task
  17:     0x55bac97dabc8 - <ra_ap_rustc_type_ir[4e6de8744a899384]::search_graph::SearchGraph<ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::search_graph::SearchGraphDelegate<hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext>, hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::evaluate_goal
  18:     0x55bac9770294 - <ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::eval_ctxt::EvalCtxt<hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext, hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::evaluate_goal_raw
  19:     0x55bac976f7e9 - <ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::eval_ctxt::EvalCtxt<hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext, hir_ty[cdb9a6d3e6bba482]::next_solver::interner::DbInterner>>::evaluate_goal
  20:     0x55bac9986aa7 - <hir_ty[cdb9a6d3e6bba482]::next_solver::solver::SolverContext as ra_ap_rustc_next_trait_solver[8b18a8e3020f8ede]::solve::eval_ctxt::SolverDelegateEvalExt>::evaluate_root_goal
  21:     0x55bac9783425 - <hir_ty[cdb9a6d3e6bba482]::next_solver::fulfill::FulfillmentCtxt>::try_evaluate_obligations
  22:     0x55bac9823664 - <hir_ty[cdb9a6d3e6bba482]::infer::unify::InferenceTable>::select_obligations_where_possible
  23:     0x55bac9823530 - <hir_ty[cdb9a6d3e6bba482]::infer::unify::InferenceTable>::try_structurally_resolve_type
  24:     0x55bac9730a5d - <hir_ty[cdb9a6d3e6bba482]::infer::InferenceContext>::infer_path
  25:     0x55bac97231e1 - <hir_ty[cdb9a6d3e6bba482]::infer::InferenceContext>::infer_expr_inner
  26:     0x55bac9731b7e - <hir_ty[cdb9a6d3e6bba482]::infer::InferenceContext>::infer_call
  27:     0x55bac9724c37 - <hir_ty[cdb9a6d3e6bba482]::infer::InferenceContext>::infer_expr_inner
  28:     0x55bac97243d7 - <hir_ty[cdb9a6d3e6bba482]::infer::InferenceContext>::infer_expr_inner
  29:     0x55bac9722407 - <hir_ty[cdb9a6d3e6bba482]::infer::InferenceContext>::infer_return
  30:     0x55bac98448ae - hir_ty[cdb9a6d3e6bba482]::infer::infer_query_with_inspect
  31:     0x55bac96ff164 - <salsa[65a99894a6a39c25]::function::IngredientImpl<<hir_ty[cdb9a6d3e6bba482]::infer::InferenceResult>::for_body::_::for_body__Configuration_>>::execute
  32:     0x55bac94fef27 - <salsa[65a99894a6a39c25]::attach::Attached>::attach::<dyn hir_ty[cdb9a6d3e6bba482]::db::HirDatabase, &hir_ty[cdb9a6d3e6bba482]::infer::InferenceResult, <hir_ty[cdb9a6d3e6bba482]::infer::InferenceResult>::for_body::for_body_::{closure#0}>
  33:     0x55bac915ebcc - <hir[c107f094967a75ab]::semantics::SemanticsImpl>::get_unsafe_ops
  34:     0x55bac9ac92b3 - ide[37737ece80204ca3]::syntax_highlighting::highlight
  35:     0x55bac9b02e6e - ide[37737ece80204ca3]::syntax_highlighting::html::highlight_as_html_with_config
  36:     0x55bac9a69dec - <ide[37737ece80204ca3]::Analysis>::highlight_as_html
  37:     0x55baca4b8872 - <rust_analyzer[c9a7b5a74bd4f6b9]::cli::flags::Highlight>::run
  38:     0x55baca758034 - rust_analyzer[e3769bb504499c95]::main
  39:     0x55baca6ee713 - std[cf0678d25afc9ccf]::sys::backtrace::__rust_begin_short_backtrace::<fn() -> core[9662e8fc1527f23c]::result::Result<std[cf0678d25afc9ccf]::process::ExitCode, anyhow[6279a09abd95b935]::Error>, core[9662e8fc1527f23c]::result::Result<std[cf0678d25afc9ccf]::process::ExitCode, anyhow[6279a09abd95b935]::Error>>
  40:     0x55baca751082 - std[cf0678d25afc9ccf]::rt::lang_start::<core[9662e8fc1527f23c]::result::Result<std[cf0678d25afc9ccf]::process::ExitCode, anyhow[6279a09abd95b935]::Error>>::{closure#0}
  41:     0x7f9c77f6af99 - std[cf0678d25afc9ccf]::rt::lang_start_internal
  42:     0x55baca771148 - main
  43:     0x7f9c71827741 - <unknown>
  44:     0x7f9c71827879 - __libc_start_main
  45:     0x55bac90626e9 - <unknown>
  46:                0x0 - <unknown>

Start with the provided Rust reproducer and run the rust-analyzer highlight command to confirm the panic. Trace the stack to structural_traits.rs, especially try_eagerly_replace_alias, and inspect how the next trait solver handles the AliasTerm shown in the panic. Done means highlighting the reproducer completes without panicking.