"unexpected ambiguity: `CanonicalQueryInput`" with conflicting associated type specifiers

[Jules-Bertholet]

Code

pub trait Trait {
    type Assoc: Copy + 'static;
}

const fn conjure<T>() -> T {
    panic!()
}

const fn get_assoc<T: Trait>() -> impl Copy {
    conjure::<<T as Trait>::Assoc>()
}

pub fn foo<T: Trait<Assoc = i32> + Trait<Assoc = i64>>() {
    const {
        get_assoc::<T>();
    }
}

(slightly adapted from @theemathas's example here)

Meta

cargo bisect-rustc:

(Edit: #146371 (comment))

Regression in nightly-2024-11-27

• commit[0] 2024-11-25: Auto merge of Reduce integer Display implementation size #133247 - GuillaumeGomez:reduce-integer-display-impl, r=workingjubilee
• commit[1] 2024-11-26: Auto merge of Update cargo and books #133465 - ehuss:update-cargo, r=ehuss
• commit[2] 2024-11-26: Auto merge of Refactor where predicates, and reserve for attributes support #132894 - frank-king:feature/where-refactor, r=cjgillot
• commit[3] 2024-11-26: Auto merge of Rollup of 13 pull requests #133500 - GuillaumeGomez:rollup-4vcthwo, r=GuillaumeGomez~~

Error output

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: unexpected ambiguity: CanonicalQueryInput { canonical: Canonical { value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [Binder { value: ProjectionPredicate(AliasTerm { args: [T/#0], def_id: DefId(0:4 ~ playground[81cc]::Trait::Assoc), .. }, Term::Ty(i64)), bound_vars: [] }, Binder { value: ProjectionPredicate(AliasTerm { args: [T/#0], def_id: DefId(0:4 ~ playground[81cc]::Trait::Assoc), .. }, Term::Ty(i32)), bound_vars: [] }, Binder { value: TraitPredicate(<T as Trait>, polarity:Positive), bound_vars: [] }, Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }] }, value: AliasTy { args: [T/#0], def_id: DefId(0:4 ~ playground[81cc]::Trait::Assoc), .. } }, max_universe: U0, variables: [] }, typing_mode: PostAnalysis } Canonical { value: QueryResponse { var_values: CanonicalVarValues { var_values: [] }, region_constraints: QueryRegionConstraints { outlives: [] }, certainty: Ambiguous, opaque_types: [], value: NormalizationResult { normalized_ty: ^0 } }, max_universe: U0, variables: [Ty(General(U0))] }
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/query/normalize.rs:288:30 - disabled backtrace

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: rustc 1.89.0 (29483883e 2025-08-04) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack

@rustbot label A-impossible-bounds T-types regression-from-stable-to-stable

[fmease]

Regression in nightly-2024-11-27

This can't be. I've just tried nightly-2024-11-01 (to see what the output used to look like) and it also reproduces.
Maybe it's flip-flopping?

---

Edit: As a matter of fact, the regression didn't occur in nightly-2024-11-27; nightly-2024-11-26, nightly-2024-11-27, nightly-2024-11-28 all ICE for me (in check or build). I'm not sure what your bisect-rustc invocation looked like.

[fmease]

Smaller (no const blocks, etc.):

trait Trait { type Assoc; }

fn conjure<T>() -> T { loop {} }

fn get_assoc<T: Trait>() -> impl Sized { conjure::<T::Assoc>() }

fn foo<T: Trait<Assoc = i32> + Trait<Assoc = i64>>() { get_assoc::<T>(); }

Without conjure but with assoc fn:

trait Trait { type Assoc; fn make() -> Self::Assoc; }

fn get_assoc<T: Trait>() -> impl Sized { T::make() }

fn foo<T: Trait<Assoc = i32> + Trait<Assoc = i64>>() { get_assoc::<T>(); }