Rollup merge of #121863 - lukas-code:silence-mismatched-super-projections, r=lcnr

silence mismatched types errors for implied projections

Currently, if a trait bound is not satisfied, then we suppress any errors for the trait's supertraits not being satisfied, but still report errors for super projections not being satisfied.

For example:
```rust
trait Super {
    type Assoc;
}
trait Sub: Super<Assoc = ()> {}
```
Before this PR, if `T: Sub` is not satisfied, then errors for `T: Super` are suppressed, but errors for `<T as Super>::Assoc == ()` are still shown. This PR makes it so that errors about super projections not being satisfied are also suppressed.

The errors are only suppressed if the span of the trait obligation matches the span of the super predicate obligation to avoid silencing error that are not related. This PR removes some differences between the spans of supertraits and super projections to make the suppression work correctly.

This PR fixes the majority of the diagnostics fallout when making `Thin` a supertrait of `Sized` (in a future PR).
cc https://github.com/rust-lang/rust/pull/120354#issuecomment-1930585382
cc `@lcnr`

10 files changed, 310 insertions, 102 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index 8d457da89e8..3b02b3c255e 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -298,31 +298,31 @@ fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) -> Result<()
         hir::ItemKind::Const(ty, ..) => {
             check_item_type(tcx, def_id, ty.span, UnsizedHandling::Forbid)
         }
-        hir::ItemKind::Struct(_, ast_generics) => {
+        hir::ItemKind::Struct(_, hir_generics) => {
             let res = check_type_defn(tcx, item, false);
-            check_variances_for_type_defn(tcx, item, ast_generics);
+            check_variances_for_type_defn(tcx, item, hir_generics);
             res
         }
-        hir::ItemKind::Union(_, ast_generics) => {
+        hir::ItemKind::Union(_, hir_generics) => {
             let res = check_type_defn(tcx, item, true);
-            check_variances_for_type_defn(tcx, item, ast_generics);
+            check_variances_for_type_defn(tcx, item, hir_generics);
             res
         }
-        hir::ItemKind::Enum(_, ast_generics) => {
+        hir::ItemKind::Enum(_, hir_generics) => {
             let res = check_type_defn(tcx, item, true);
-            check_variances_for_type_defn(tcx, item, ast_generics);
+            check_variances_for_type_defn(tcx, item, hir_generics);
             res
         }
         hir::ItemKind::Trait(..) => check_trait(tcx, item),
         hir::ItemKind::TraitAlias(..) => check_trait(tcx, item),
         // `ForeignItem`s are handled separately.
         hir::ItemKind::ForeignMod { .. } => Ok(()),
-        hir::ItemKind::TyAlias(hir_ty, ast_generics) => {
+        hir::ItemKind::TyAlias(hir_ty, hir_generics) => {
             if tcx.type_alias_is_lazy(item.owner_id) {
                 // Bounds of lazy type aliases and of eager ones that contain opaque types are respected.
                 // E.g: `type X = impl Trait;`, `type X = (impl Trait, Y);`.
                 let res = check_item_type(tcx, def_id, hir_ty.span, UnsizedHandling::Allow);
-                check_variances_for_type_defn(tcx, item, ast_generics);
+                check_variances_for_type_defn(tcx, item, hir_generics);
                 res
             } else {
                 Ok(())
@@ -1277,16 +1277,16 @@ fn check_item_type(
     })
 }
 
-#[instrument(level = "debug", skip(tcx, ast_self_ty, ast_trait_ref))]
+#[instrument(level = "debug", skip(tcx, hir_self_ty, hir_trait_ref))]
 fn check_impl<'tcx>(
     tcx: TyCtxt<'tcx>,
     item: &'tcx hir::Item<'tcx>,
-    ast_self_ty: &hir::Ty<'_>,
-    ast_trait_ref: &Option<hir::TraitRef<'_>>,
+    hir_self_ty: &hir::Ty<'_>,
+    hir_trait_ref: &Option<hir::TraitRef<'_>>,
 ) -> Result<(), ErrorGuaranteed> {
     enter_wf_checking_ctxt(tcx, item.span, item.owner_id.def_id, |wfcx| {
-        match ast_trait_ref {
-            Some(ast_trait_ref) => {
+        match hir_trait_ref {
+            Some(hir_trait_ref) => {
                 // `#[rustc_reservation_impl]` impls are not real impls and
                 // therefore don't need to be WF (the trait's `Self: Trait` predicate
                 // won't hold).
@@ -1294,8 +1294,9 @@ fn check_impl<'tcx>(
                 // Avoid bogus "type annotations needed `Foo: Bar`" errors on `impl Bar for Foo` in case
                 // other `Foo` impls are incoherent.
                 tcx.ensure().coherent_trait(trait_ref.def_id)?;
+                let trait_span = hir_trait_ref.path.span;
                 let trait_ref = wfcx.normalize(
-                    ast_trait_ref.path.span,
+                    trait_span,
                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
                     trait_ref,
                 );
@@ -1306,14 +1307,23 @@ fn check_impl<'tcx>(
                     wfcx.param_env,
                     wfcx.body_def_id,
                     trait_pred,
-                    ast_trait_ref.path.span,
+                    trait_span,
                     item,
                 );
                 for obligation in &mut obligations {
+                    if obligation.cause.span != trait_span {
+                        // We already have a better span.
+                        continue;
+                    }
                     if let Some(pred) = obligation.predicate.to_opt_poly_trait_pred()
-                        && pred.self_ty().skip_binder() == trait_ref.self_ty()
+                        && pred.skip_binder().self_ty() == trait_ref.self_ty()
+                    {
+                        obligation.cause.span = hir_self_ty.span;
+                    }
+                    if let Some(pred) = obligation.predicate.to_opt_poly_projection_pred()
+                        && pred.skip_binder().self_ty() == trait_ref.self_ty()
                     {
-                        obligation.cause.span = ast_self_ty.span;
+                        obligation.cause.span = hir_self_ty.span;
                     }
                 }
                 debug!(?obligations);
@@ -1327,7 +1337,7 @@ fn check_impl<'tcx>(
                     self_ty,
                 );
                 wfcx.register_wf_obligation(
-                    ast_self_ty.span,
+                    hir_self_ty.span,
                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
                     self_ty.into(),
                 );
diff --git a/compiler/rustc_infer/src/infer/relate/higher_ranked.rs b/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
index f30e366c198..cc0f00254ff 100644
--- a/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
+++ b/compiler/rustc_infer/src/infer/relate/higher_ranked.rs
@@ -77,7 +77,7 @@ impl<'tcx> InferCtxt<'tcx> {
         // that name placeholders created in this function. Nested goals from type relations can
         // also contain placeholders created by this function.
         let value = self.enter_forall_and_leak_universe(forall);
-        debug!("?value");
+        debug!(?value);
         f(value)
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index 7a930937255..2f9075b1370 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -1431,45 +1431,64 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
 #[extension(pub(super) trait InferCtxtPrivExt<'tcx>)]
 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
+    fn can_match_trait(
+        &self,
+        goal: ty::TraitPredicate<'tcx>,
+        assumption: ty::PolyTraitPredicate<'tcx>,
+    ) -> bool {
+        if goal.polarity != assumption.polarity() {
+            return false;
+        }
+
+        let trait_goal = goal.trait_ref;
+        let trait_assumption = self.instantiate_binder_with_fresh_vars(
+            DUMMY_SP,
+            infer::BoundRegionConversionTime::HigherRankedType,
+            assumption.to_poly_trait_ref(),
+        );
+
+        self.can_eq(ty::ParamEnv::empty(), trait_goal, trait_assumption)
+    }
+
+    fn can_match_projection(
+        &self,
+        goal: ty::ProjectionPredicate<'tcx>,
+        assumption: ty::PolyProjectionPredicate<'tcx>,
+    ) -> bool {
+        let assumption = self.instantiate_binder_with_fresh_vars(
+            DUMMY_SP,
+            infer::BoundRegionConversionTime::HigherRankedType,
+            assumption,
+        );
+
+        let param_env = ty::ParamEnv::empty();
+        self.can_eq(param_env, goal.projection_ty, assumption.projection_ty)
+            && self.can_eq(param_env, goal.term, assumption.term)
+    }
+
     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
     // `error` occurring implies that `cond` occurs.
+    #[instrument(level = "debug", skip(self), ret)]
     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
         if cond == error {
             return true;
         }
 
-        // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
-        let bound_error = error.kind();
-        let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
-            (
-                ty::PredicateKind::Clause(ty::ClauseKind::Trait(..)),
-                ty::PredicateKind::Clause(ty::ClauseKind::Trait(error)),
-            ) => (cond, bound_error.rebind(error)),
-            _ => {
-                // FIXME: make this work in other cases too.
-                return false;
-            }
-        };
-
-        for pred in elaborate(self.tcx, std::iter::once(cond)) {
-            let bound_predicate = pred.kind();
-            if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(implication)) =
-                bound_predicate.skip_binder()
-            {
-                let error = error.to_poly_trait_ref();
-                let implication = bound_predicate.rebind(implication.trait_ref);
-                // FIXME: I'm just not taking associated types at all here.
-                // Eventually I'll need to implement param-env-aware
-                // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
-                let param_env = ty::ParamEnv::empty();
-                if self.can_sub(param_env, error, implication) {
-                    debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
-                    return true;
-                }
-            }
+        if let Some(error) = error.to_opt_poly_trait_pred() {
+            self.enter_forall(error, |error| {
+                elaborate(self.tcx, std::iter::once(cond))
+                    .filter_map(|implied| implied.to_opt_poly_trait_pred())
+                    .any(|implied| self.can_match_trait(error, implied))
+            })
+        } else if let Some(error) = error.to_opt_poly_projection_pred() {
+            self.enter_forall(error, |error| {
+                elaborate(self.tcx, std::iter::once(cond))
+                    .filter_map(|implied| implied.to_opt_poly_projection_pred())
+                    .any(|implied| self.can_match_projection(error, implied))
+            })
+        } else {
+            false
         }
-
-        false
     }
 
     #[instrument(skip(self), level = "debug")]
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index 15059bc6613..b09a803e856 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -223,60 +223,87 @@ enum Elaborate {
     None,
 }
 
+/// Points the cause span of a super predicate at the relevant associated type.
+///
+/// Given a trait impl item:
+///
+/// ```ignore (incomplete)
+/// impl TargetTrait for TargetType {
+///    type Assoc = SomeType;
+/// }
+/// ```
+///
+/// And a super predicate of `TargetTrait` that has any of the following forms:
+///
+/// 1. `<OtherType as OtherTrait>::Assoc == <TargetType as TargetTrait>::Assoc`
+/// 2. `<<TargetType as TargetTrait>::Assoc as OtherTrait>::Assoc == OtherType`
+/// 3. `<TargetType as TargetTrait>::Assoc: OtherTrait`
+///
+/// Replace the span of the cause with the span of the associated item:
+///
+/// ```ignore (incomplete)
+/// impl TargetTrait for TargetType {
+///     type Assoc = SomeType;
+/// //               ^^^^^^^^ this span
+/// }
+/// ```
+///
+/// Note that bounds that can be expressed as associated item bounds are **not**
+/// super predicates. This means that form 2 and 3 from above are only relevant if
+/// the [`GenericArgsRef`] of the projection type are not its identity arguments.
 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
     tcx: TyCtxt<'tcx>,
-    trait_ref: ty::TraitRef<'tcx>,
     item: Option<&hir::Item<'tcx>>,
     cause: &mut traits::ObligationCause<'tcx>,
     pred: ty::Predicate<'tcx>,
 ) {
-    debug!(
-        "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
-        trait_ref, item, cause, pred
-    );
+    debug!(?item, ?cause, ?pred, "extended_cause_with_original_assoc_item_obligation");
     let (items, impl_def_id) = match item {
         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), owner_id, .. }) => {
             (impl_.items, *owner_id)
         }
         _ => return,
     };
-    let fix_span =
-        |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
-            hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::Type(ty) => ty.span,
-            _ => impl_item_ref.span,
-        };
+
+    let ty_to_impl_span = |ty: Ty<'_>| {
+        if let ty::Alias(ty::Projection, projection_ty) = ty.kind()
+            && let Some(&impl_item_id) =
+                tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.def_id)
+            && let Some(impl_item) =
+                items.iter().find(|item| item.id.owner_id.to_def_id() == impl_item_id)
+        {
+            Some(tcx.hir().impl_item(impl_item.id).expect_type().span)
+        } else {
+            None
+        }
+    };
 
     // It is fine to skip the binder as we don't care about regions here.
     match pred.kind().skip_binder() {
         ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj)) => {
-            // The obligation comes not from the current `impl` nor the `trait` being implemented,
-            // but rather from a "second order" obligation, where an associated type has a
-            // projection coming from another associated type. See
-            // `tests/ui/associated-types/point-at-type-on-obligation-failure.rs` and
-            // `traits-assoc-type-in-supertrait-bad.rs`.
-            if let Some(ty::Alias(ty::Projection, projection_ty)) =
-                proj.term.ty().map(|ty| ty.kind())
-                && let Some(&impl_item_id) =
-                    tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.def_id)
-                && let Some(impl_item_span) = items
-                    .iter()
-                    .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
-                    .map(fix_span)
+            // Form 1: The obligation comes not from the current `impl` nor the `trait` being
+            // implemented, but rather from a "second order" obligation, where an associated
+            // type has a projection coming from another associated type.
+            // See `tests/ui/traits/assoc-type-in-superbad.rs` for an example.
+            if let Some(term_ty) = proj.term.ty()
+                && let Some(impl_item_span) = ty_to_impl_span(term_ty)
             {
                 cause.span = impl_item_span;
             }
+
+            // Form 2: A projection obligation for an associated item failed to be met.
+            // We overwrite the span from above to ensure that a bound like
+            // `Self::Assoc1: Trait<OtherAssoc = Self::Assoc2>` gets the same
+            // span for both obligations that it is lowered to.
+            if let Some(impl_item_span) = ty_to_impl_span(proj.self_ty()) {
+                cause.span = impl_item_span;
+            }
         }
+
         ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred)) => {
-            // An associated item obligation born out of the `trait` failed to be met. An example
-            // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
+            // Form 3: A trait obligation for an associated item failed to be met.
             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
-            if let ty::Alias(ty::Projection, ty::AliasTy { def_id, .. }) = *pred.self_ty().kind()
-                && let Some(&impl_item_id) = tcx.impl_item_implementor_ids(impl_def_id).get(&def_id)
-                && let Some(impl_item_span) = items
-                    .iter()
-                    .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
-                    .map(fix_span)
-            {
+            if let Some(impl_item_span) = ty_to_impl_span(pred.self_ty()) {
                 cause.span = impl_item_span;
             }
         }
@@ -355,9 +382,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                     traits::ObligationCauseCode::DerivedObligation,
                 );
             }
-            extend_cause_with_original_assoc_item_obligation(
-                tcx, trait_ref, item, &mut cause, predicate,
-            );
+            extend_cause_with_original_assoc_item_obligation(tcx, item, &mut cause, predicate);
             traits::Obligation::with_depth(tcx, cause, depth, param_env, predicate)
         };
 
diff --git a/tests/ui/associated-types/hr-associated-type-projection-1.rs b/tests/ui/associated-types/hr-associated-type-projection-1.rs
index 3df3f68ab1e..d7fc5d122c3 100644
--- a/tests/ui/associated-types/hr-associated-type-projection-1.rs
+++ b/tests/ui/associated-types/hr-associated-type-projection-1.rs
@@ -11,8 +11,8 @@ where
 }
 
 impl<T: Copy + std::ops::Deref> UnsafeCopy<'_, T> for T {
-    //~^ type mismatch resolving `<T as Deref>::Target == T`
     type Item = T;
+    //~^ type mismatch resolving `<T as Deref>::Target == T`
 }
 
 pub fn main() {
diff --git a/tests/ui/associated-types/hr-associated-type-projection-1.stderr b/tests/ui/associated-types/hr-associated-type-projection-1.stderr
index 65221718ee6..b871bb51ae3 100644
--- a/tests/ui/associated-types/hr-associated-type-projection-1.stderr
+++ b/tests/ui/associated-types/hr-associated-type-projection-1.stderr
@@ -1,10 +1,10 @@
 error[E0271]: type mismatch resolving `<T as Deref>::Target == T`
-  --> $DIR/hr-associated-type-projection-1.rs:13:33
+  --> $DIR/hr-associated-type-projection-1.rs:14:17
    |
 LL | impl<T: Copy + std::ops::Deref> UnsafeCopy<'_, T> for T {
-   |      -                          ^^^^^^^^^^^^^^^^^ expected type parameter `T`, found associated type
-   |      |
-   |      expected this type parameter
+   |      - expected this type parameter
+LL |     type Item = T;
+   |                 ^ expected type parameter `T`, found associated type
    |
    = note: expected type parameter `T`
              found associated type `<T as Deref>::Target`
diff --git a/tests/ui/issues/issue-33941.rs b/tests/ui/issues/issue-33941.rs
index 0ad7cbe8efc..7b5be30834b 100644
--- a/tests/ui/issues/issue-33941.rs
+++ b/tests/ui/issues/issue-33941.rs
@@ -5,5 +5,4 @@ use std::collections::HashMap;
 fn main() {
     for _ in HashMap::new().iter().cloned() {} //~ ERROR expected `Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
     //~^ ERROR expected `Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
-    //~| ERROR expected `Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
 }
diff --git a/tests/ui/issues/issue-33941.stderr b/tests/ui/issues/issue-33941.stderr
index e7f4a4fa004..f1b6b6ba17e 100644
--- a/tests/ui/issues/issue-33941.stderr
+++ b/tests/ui/issues/issue-33941.stderr
@@ -27,16 +27,6 @@ LL |     for _ in HashMap::new().iter().cloned() {}
    = note: required for `Cloned<std::collections::hash_map::Iter<'_, _, _>>` to implement `Iterator`
    = note: required for `Cloned<std::collections::hash_map::Iter<'_, _, _>>` to implement `IntoIterator`
 
-error[E0271]: expected `Iter<'_, _, _>` to be an iterator that yields `&_`, but it yields `(&_, &_)`
-  --> $DIR/issue-33941.rs:6:14
-   |
-LL |     for _ in HashMap::new().iter().cloned() {}
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `(&_, &_)`, found `&_`
-   |
-   = note:  expected tuple `(&_, &_)`
-           found reference `&_`
-   = note: required for `Cloned<std::collections::hash_map::Iter<'_, _, _>>` to implement `Iterator`
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0271`.
diff --git a/tests/ui/trait-bounds/super-assoc-mismatch.rs b/tests/ui/trait-bounds/super-assoc-mismatch.rs
new file mode 100644
index 00000000000..97dfec80e31
--- /dev/null
+++ b/tests/ui/trait-bounds/super-assoc-mismatch.rs
@@ -0,0 +1,60 @@
+trait Super {
+    type Assoc;
+}
+impl Super for () {
+    type Assoc = u8;
+}
+trait Sub: Super<Assoc = u16> {}
+
+trait BoundOnSelf: Sub {}
+impl BoundOnSelf for () {}
+//~^ ERROR the trait bound `(): Sub` is not satisfied
+
+trait BoundOnParam<T: Sub> {}
+impl BoundOnParam<()> for () {}
+//~^ ERROR the trait bound `(): Sub` is not satisfied
+
+trait BoundOnAssoc {
+    type Assoc: Sub;
+}
+impl BoundOnAssoc for () {
+    type Assoc = ();
+    //~^ ERROR the trait bound `(): Sub` is not satisfied
+}
+
+trait BoundOnGat where Self::Assoc<u8>: Sub {
+    type Assoc<T>;
+}
+impl BoundOnGat for u8 {
+    type Assoc<T> = ();
+    //~^ ERROR the trait bound `(): Sub` is not satisfied
+}
+
+fn trivial_bound() where (): Sub {}
+//~^ ERROR the trait bound `(): Sub` is not satisfied
+
+// The following is an edge case where the unsatisfied projection predicate
+// `<<u8 as MultiAssoc>::Assoc1<()> as SuperGeneric<u16>>::Assoc == <u8 as MultiAssoc>::Assoc2`
+// contains both associated types of `MultiAssoc`. To suppress the error about the unsatisfied
+// super projection, the error's span must be equal to the span of the unsatisfied trait error.
+trait SuperGeneric<T> {
+    type Assoc;
+}
+trait SubGeneric<T>: SuperGeneric<T, Assoc = T> {}
+trait MultiAssoc
+where
+    Self::Assoc1<()>: SubGeneric<Self::Assoc2>
+{
+    type Assoc1<T>;
+    type Assoc2;
+}
+impl SuperGeneric<u16> for () {
+    type Assoc = u8;
+}
+impl MultiAssoc for u8 {
+    type Assoc1<T> = ();
+    //~^ ERROR the trait bound `(): SubGeneric<u16>` is not satisfied
+    type Assoc2 = u16;
+}
+
+fn main() {}
diff --git a/tests/ui/trait-bounds/super-assoc-mismatch.stderr b/tests/ui/trait-bounds/super-assoc-mismatch.stderr
new file mode 100644
index 00000000000..47535776348
--- /dev/null
+++ b/tests/ui/trait-bounds/super-assoc-mismatch.stderr
@@ -0,0 +1,105 @@
+error[E0277]: the trait bound `(): Sub` is not satisfied
+  --> $DIR/super-assoc-mismatch.rs:10:22
+   |
+LL | impl BoundOnSelf for () {}
+   |                      ^^ the trait `Sub` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/super-assoc-mismatch.rs:7:1
+   |
+LL | trait Sub: Super<Assoc = u16> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `BoundOnSelf`
+  --> $DIR/super-assoc-mismatch.rs:9:20
+   |
+LL | trait BoundOnSelf: Sub {}
+   |                    ^^^ required by this bound in `BoundOnSelf`
+
+error[E0277]: the trait bound `(): Sub` is not satisfied
+  --> $DIR/super-assoc-mismatch.rs:14:27
+   |
+LL | impl BoundOnParam<()> for () {}
+   |                           ^^ the trait `Sub` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/super-assoc-mismatch.rs:7:1
+   |
+LL | trait Sub: Super<Assoc = u16> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `BoundOnParam`
+  --> $DIR/super-assoc-mismatch.rs:13:23
+   |
+LL | trait BoundOnParam<T: Sub> {}
+   |                       ^^^ required by this bound in `BoundOnParam`
+
+error[E0277]: the trait bound `(): Sub` is not satisfied
+  --> $DIR/super-assoc-mismatch.rs:21:18
+   |
+LL |     type Assoc = ();
+   |                  ^^ the trait `Sub` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/super-assoc-mismatch.rs:7:1
+   |
+LL | trait Sub: Super<Assoc = u16> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `BoundOnAssoc::Assoc`
+  --> $DIR/super-assoc-mismatch.rs:18:17
+   |
+LL |     type Assoc: Sub;
+   |                 ^^^ required by this bound in `BoundOnAssoc::Assoc`
+
+error[E0277]: the trait bound `(): Sub` is not satisfied
+  --> $DIR/super-assoc-mismatch.rs:29:21
+   |
+LL |     type Assoc<T> = ();
+   |                     ^^ the trait `Sub` is not implemented for `()`, which is required by `<u8 as BoundOnGat>::Assoc<u8>: Sub`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/super-assoc-mismatch.rs:7:1
+   |
+LL | trait Sub: Super<Assoc = u16> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `BoundOnGat`
+  --> $DIR/super-assoc-mismatch.rs:25:41
+   |
+LL | trait BoundOnGat where Self::Assoc<u8>: Sub {
+   |                                         ^^^ required by this bound in `BoundOnGat`
+
+error[E0277]: the trait bound `(): Sub` is not satisfied
+  --> $DIR/super-assoc-mismatch.rs:33:26
+   |
+LL | fn trivial_bound() where (): Sub {}
+   |                          ^^^^^^^ the trait `Sub` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/super-assoc-mismatch.rs:7:1
+   |
+LL | trait Sub: Super<Assoc = u16> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: see issue #48214
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
+
+error[E0277]: the trait bound `(): SubGeneric<u16>` is not satisfied
+  --> $DIR/super-assoc-mismatch.rs:55:22
+   |
+LL |     type Assoc1<T> = ();
+   |                      ^^ the trait `SubGeneric<u16>` is not implemented for `()`, which is required by `<u8 as MultiAssoc>::Assoc1<()>: SubGeneric<<u8 as MultiAssoc>::Assoc2>`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/super-assoc-mismatch.rs:43:1
+   |
+LL | trait SubGeneric<T>: SuperGeneric<T, Assoc = T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `MultiAssoc`
+  --> $DIR/super-assoc-mismatch.rs:46:23
+   |
+LL | trait MultiAssoc
+   |       ---------- required by a bound in this trait
+LL | where
+LL |     Self::Assoc1<()>: SubGeneric<Self::Assoc2>
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `MultiAssoc`
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0277`.