Change:
Code gives the same error as , giving (dyn Xyz + Abc + 'static)XyzCode passes coherence, but wfcheck fails because of the confusion:

Discussion:
// This is something like impl Trait1 for Trait2. Illegal // if Trait1 is a supertrait of Trait2 or Trait2 is not object safe. <del> if let Some(principal_def_id) = data.principal_def_id() { if !tcx.is_object_safe(principal_def_id) { </del> <ins> let component_def_ids = data.iter().flat_map(|predicate| { match predicate.skip_binder() { ty::ExistentialPredicate::Trait(tr) => Some(tr.def_id), ty::ExistentialPredicate::AutoTrait(def_id) => Some(*def_id), // An associated type projection necessarily comes with // an additional `Trait` requirement. ty::ExistentialPredicate::Projection(..) => None, } }); for component_def_id in component_def_ids { if !tcx.is_object_safe(component_def_id) { </ins> // This is an error, but it will be reported by wfcheck.  Ignore it here. // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`. } else { let mut supertrait_def_ids = <del> traits::supertrait_def_ids(tcx, principal_def_id); </del> <ins> traits::supertrait_def_ids(tcx, component_def_id); </ins> if supertrait_def_ids.any(|d| d == trait_def_id) { let sp = tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap()); struct_span_err!(tcx.sess,

} } } <del> // FIXME: also check auto-trait def-ids? (e.g. `impl Sync for Foo+Sync`)? </del> } }

<ins> #![feature(optin_builtin_traits)] // Test for issue #56934 - that it is impossible to redundantly // implement an auto-trait for a trait object type that contains it. // Negative impl variant. auto trait Marker1 {} auto trait Marker2 {} trait Object: Marker1 {} // A supertrait marker is illegal... impl !Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371 // ...and also a direct component. impl !Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371 // But implementing a marker if it is not present is OK. impl !Marker2 for dyn Object {} // OK // A non-principal trait-object type is orphan even in its crate. impl !Send for dyn Marker2 {} //~ ERROR E0117 // And impl'ing a remote marker for a local trait object is forbidden // by one of these special orphan-like rules. impl !Send for dyn Object {} //~ ERROR E0321 impl !Send for dyn Object + Marker2 {} //~ ERROR E0321 fn main() { } </ins>

<ins> error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1` --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:14:1 | LL | impl !Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1` error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2` --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:16:1 | LL | impl !Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2` error[E0117]: only traits defined in the current crate can be implemented for arbitrary types --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:22:1 | LL | impl !Send for dyn Marker2 {} //~ ERROR E0117 | ^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use types inside crate | = note: the impl does not reference only types defined in this crate = note: define and implement a trait or new type instead error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)` --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:26:1 | LL | impl !Send for dyn Object {} //~ ERROR E0321 | ^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)` --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:27:1 | LL | impl !Send for dyn Object + Marker2 {} //~ ERROR E0321 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type error: aborting due to 5 previous errors Some errors occurred: E0117, E0321, E0371. For more information about an error, try `rustc --explain E0117`. </ins>

<ins> #![feature(optin_builtin_traits)] // Test for issue #56934 - that it is impossible to redundantly // implement an auto-trait for a trait object type that contains it. // Positive impl variant. auto trait Marker1 {} auto trait Marker2 {} trait Object: Marker1 {} // A supertrait marker is illegal... impl Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371 // ...and also a direct component. impl Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371 // But implementing a marker if it is not present is OK. impl Marker2 for dyn Object {} // OK // A non-principal trait-object type is orphan even in its crate. unsafe impl Send for dyn Marker2 {} //~ ERROR E0117 // And impl'ing a remote marker for a local trait object is forbidden // by one of these special orphan-like rules. unsafe impl Send for dyn Object {} //~ ERROR E0321 unsafe impl Send for dyn Object + Marker2 {} //~ ERROR E0321 fn main() { } </ins>

<ins> error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1` --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:14:1 | LL | impl Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1` error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2` --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:16:1 | LL | impl Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2` error[E0117]: only traits defined in the current crate can be implemented for arbitrary types --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:22:1 | LL | unsafe impl Send for dyn Marker2 {} //~ ERROR E0117 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use types inside crate | = note: the impl does not reference only types defined in this crate = note: define and implement a trait or new type instead error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)` --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:26:1 | LL | unsafe impl Send for dyn Object {} //~ ERROR E0321 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)` --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:27:1 | LL | unsafe impl Send for dyn Object + Marker2 {} //~ ERROR E0321 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type error: aborting due to 5 previous errors Some errors occurred: E0117, E0321, E0371. For more information about an error, try `rustc --explain E0117`. </ins>