panic: `ExistentialMismatch`

trait Sup<T> {
    type Assoc;
}

impl<T> Sup<T> for () {
    type Assoc = T;
}

trait Trait<A, B>: Sup<A, Assoc = A> + Sup<B, Assoc = B> {}

impl<T, U> Trait<T, U> for () {}

fn main() {
    let x: &dyn Trait<(), _> = &();
    let y: &dyn Trait<_, ()> = x;
}

code compiles with rustc

RUST_BACKTRACE=full rust-analyzer analysis-stats . --run-all-ide-things

Database loaded:     612.27ms, 224minstr, 48mb (metadata 319.50ms, 1011kinstr, 152kb; build 34.67ms, 179kinstr, 1744b)
  item trees: 1
  dependency lines of code: 1_462_633, item trees: 2_900
  dependency item stats: traits: 851, impl: 9_701, mods: 3_012, macro calls: 5_501, macro rules: 1_053
Item Tree Collection: 4.15s, 22ginstr, 425mb
  Total Statistics:
    crates: 1, mods: 1, decls: 4, bodies: 1, adts: 0, consts: 0, signatures: 6, variants: 0
  Workspace:
    traits: 2, macro_rules macros: 0, proc_macros: 0
    lines of code: 16, item trees: 1
    usages: traits: 2, impl: 2, mods: 0, macro calls: 0, macro rules: 0
  Dependencies:
    lines of code: 1_462_633, item trees: 2_900
    declarations: traits: 851, impl: 9_701, mods: 3_012, macro calls: 5_501, macro rules: 1_053
Item Collection:     2.47s, 11ginstr, 177mb
Crate lang items:    464.00ms, 2471minstr, 31mb
Expression Store Lowering: 396.40µs, 488kinstr, 9kb
0/1 0% processing: dummy::main
thread 'main' (2354605) panicked at /rust/deps/ra-ap-rustc_next_trait_solver-0.165.0/src/canonical/mod.rs:263:88:
called `Result::unwrap()` on an `Err` value: ExistentialMismatch(ExpectedFound { expected: [Binder { value: Trait(ExistentialTraitRef("Trait"[[(), ()]])), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [()], term: (), .. }), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [()], term: (), .. }), bound_vars: [] }], found: [Binder { value: Trait(ExistentialTraitRef("Trait"[[(), ?19t]])), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [?19t], term: ?19t, .. }), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [()], term: (), .. }), bound_vars: [] }] })
stack backtrace:
   0:     0x7ff8eac43416 - <<std[5264d755c47072bd]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[435d43ddfce3abfa]::fmt::Display>::fmt
   1:     0x7ff8eb20ec88 - core[435d43ddfce3abfa]::fmt::write
   2:     0x7ff8eac5867c - <std[5264d755c47072bd]::sys::stdio::unix::Stderr as core[435d43ddfce3abfa]::io::write::Write>::write_fmt
   3:     0x7ff8eac1558a - std[5264d755c47072bd]::panicking::default_hook::{closure#0}
   4:     0x7ff8eac35b33 - std[5264d755c47072bd]::panicking::default_hook
   5:     0x7ff8eac35f9b - std[5264d755c47072bd]::panicking::panic_with_hook
   6:     0x7ff8eac15642 - std[5264d755c47072bd]::panicking::panic_handler::{closure#0}
   7:     0x7ff8eac0dfc9 - std[5264d755c47072bd]::sys::backtrace::__rust_end_short_backtrace::<std[5264d755c47072bd]::panicking::panic_handler::{closure#0}, !>
   8:     0x7ff8eac16f9d - __rustc[efc60ea9c7e5e02b]::rust_begin_unwind
   9:     0x7ff8e793c8ec - core[435d43ddfce3abfa]::panicking::panic_fmt
  10:     0x7ff8e733aac2 - core[435d43ddfce3abfa]::result::unwrap_failed
  11:     0x558b8782ed6d - ra_ap_rustc_next_trait_solver[1e1bbc9a2ad92865]::canonical::unify_query_var_values::<hir_ty[a11d2f9205227a88]::next_solver::solver::SolverContext, hir_ty[a11d2f9205227a88]::next_solver::interner::DbInterner>
  12:     0x558b87a3f9af - <hir_ty[a11d2f9205227a88]::next_solver::inspect::InspectCandidate>::instantiate_nested_goals
  13:     0x558b877bb469 - <hir_ty[a11d2f9205227a88]::next_solver::inspect::InspectCandidate>::visit_nested_no_probe::<hir_ty[a11d2f9205227a88]::infer::coerce::CoerceVisitor<hir_ty[a11d2f9205227a88]::infer::coerce::InferenceCoercionDelegate>>
  14:     0x558b87bcaf45 - <hir_ty[a11d2f9205227a88]::infer::coerce::CoerceVisitor<hir_ty[a11d2f9205227a88]::infer::coerce::InferenceCoercionDelegate> as hir_ty[a11d2f9205227a88]::next_solver::inspect::ProofTreeVisitor>::visit_goal
  15:     0x558b877c4a99 - <hir_ty[a11d2f9205227a88]::next_solver::infer::InferCtxt>::visit_proof_tree::<hir_ty[a11d2f9205227a88]::infer::coerce::CoerceVisitor<hir_ty[a11d2f9205227a88]::infer::coerce::InferenceCoercionDelegate>>
  16:     0x558b87996749 - <hir_ty[a11d2f9205227a88]::infer::coerce::Coerce<hir_ty[a11d2f9205227a88]::infer::coerce::InferenceCoercionDelegate>>::coerce
  17:     0x558b87a2600d - <hir_ty[a11d2f9205227a88]::infer::InferenceContext>::coerce
  18:     0x558b8798230b - <hir_ty[a11d2f9205227a88]::infer::InferenceContext>::infer_expr_inner
  19:     0x558b87980047 - <hir_ty[a11d2f9205227a88]::infer::InferenceContext>::infer_return
  20:     0x558b87ade5ce - hir_ty[a11d2f9205227a88]::infer::infer_query_with_inspect
  21:     0x558b879ce114 - <salsa[bfa5eed162dfe307]::function::IngredientImpl<<hir_ty[a11d2f9205227a88]::infer::InferenceResult>::for_body::_::for_body__Configuration_>>::execute
  22:     0x558b87798fe7 - <salsa[bfa5eed162dfe307]::attach::Attached>::attach::<dyn hir_ty[a11d2f9205227a88]::db::HirDatabase, &hir_ty[a11d2f9205227a88]::infer::InferenceResult, <hir_ty[a11d2f9205227a88]::infer::InferenceResult>::for_body::for_body_::{closure#0}>
  23:     0x558b8873bc6f - <rust_analyzer[54a442221c605e5]::cli::flags::AnalysisStats>::run_inference
  24:     0x558b886e8cd4 - <rust_analyzer[54a442221c605e5]::cli::flags::AnalysisStats>::run::{closure#3}
  25:     0x558b88747e69 - <rust_analyzer[54a442221c605e5]::cli::flags::AnalysisStats>::run
  26:     0x558b889f1272 - rust_analyzer[25166d87b799c29f]::main
  27:     0x558b88987aa3 - std[5264d755c47072bd]::sys::backtrace::__rust_begin_short_backtrace::<fn() -> core[435d43ddfce3abfa]::result::Result<std[5264d755c47072bd]::process::ExitCode, anyhow[f9c252109e1c84fa]::Error>, core[435d43ddfce3abfa]::result::Result<std[5264d755c47072bd]::process::ExitCode, anyhow[f9c252109e1c84fa]::Error>>
  28:     0x558b889ea412 - std[5264d755c47072bd]::rt::lang_start::<core[435d43ddfce3abfa]::result::Result<std[5264d755c47072bd]::process::ExitCode, anyhow[f9c252109e1c84fa]::Error>>::{closure#0}
  29:     0x7ff8ec414e99 - std[5264d755c47072bd]::rt::lang_start_internal
  30:     0x558b88a0a4f8 - main
  31:     0x7ff8e5c27741 - <unknown>
  32:     0x7ff8e5c27879 - __libc_start_main
  33:     0x558b872fc369 - <unknown>
  34:                0x0 - <unknown>
infer panicked for dummy::main: called `Result::unwrap()` on an `Err` value: ExistentialMismatch(ExpectedFound { expected: [Binder { value: Trait(ExistentialTraitRef("Trait"[[(), ()]])), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [()], term: (), .. }), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [()], term: (), .. }), bound_vars: [] }], found: [Binder { value: Trait(ExistentialTraitRef("Trait"[[(), ?19t]])), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [?19t], term: ?19t, .. }), bound_vars: [] }, Binder { value: Projection(ExistentialProjection { def_id: TypeAliasId("Assoc"), args: [()], term: (), .. }), bound_vars: [] }] })
  exprs: 0, ??ty: 0 (100%), ?ty: 0 (100%), !ty: 0
  pats: 0, ??ty: 0 (100%), ?ty: 0 (100%), !ty: 0
  panics: 1
Inference:           369.55ms, 1306minstr, 40mb
1/1 100% mir lowering: dummy::main2026-07-16T07:25:08.715840537+02:00  WARN Propagating panic for cycle head that panicked in an earlier execution in that revision

Reproduce the panic with the supplied Rust snippet and RUST_BACKTRACE=full rust-analyzer analysis-stats . --run-all-ide-things. Start at canonical::unify_query_var_values and the infer::coerce path shown in the backtrace. Done means inference handles this existential mismatch without panicking, with regression coverage for the example.