Auto merge of #107507 - BoxyUwU:deferred_projection_equality, r=lcnr

Implement `deferred_projection_equality` for erica solver

Somewhat of a revival of #96912. When relating projections now emit an `AliasEq` obligation instead of attempting to determine equality of projections that may not be as normalized as possible (i.e. because of lazy norm, or just containing inference variables that prevent us from resolving an impl). Only do this when the new solver is enabled

46 files changed, 585 insertions, 163 deletions

diff --git a/compiler/rustc_borrowck/src/type_check/relate_tys.rs b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
index b2702eafd33..8dd06187877 100644
--- a/compiler/rustc_borrowck/src/type_check/relate_tys.rs
+++ b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
@@ -1,4 +1,4 @@
-use rustc_infer::infer::nll_relate::{NormalizationStrategy, TypeRelating, TypeRelatingDelegate};
+use rustc_infer::infer::nll_relate::{TypeRelating, TypeRelatingDelegate};
 use rustc_infer::infer::NllRegionVariableOrigin;
 use rustc_infer::traits::PredicateObligations;
 use rustc_middle::mir::ConstraintCategory;
@@ -140,10 +140,6 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx>
         );
     }
 
-    fn normalization() -> NormalizationStrategy {
-        NormalizationStrategy::Eager
-    }
-
     fn forbid_inference_vars() -> bool {
         true
     }
diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index f121979be71..75cc7cbaa60 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -1320,6 +1320,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     ty::Clause::RegionOutlives(_) => bug!(),
                 },
                 ty::PredicateKind::WellFormed(_)
+                | ty::PredicateKind::AliasEq(..)
                 | ty::PredicateKind::ObjectSafe(_)
                 | ty::PredicateKind::ClosureKind(_, _, _)
                 | ty::PredicateKind::Subtype(_)
diff --git a/compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs b/compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs
index a5dcfab9be8..02f77f9d6af 100644
--- a/compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs
+++ b/compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs
@@ -517,6 +517,7 @@ fn trait_predicate_kind<'tcx>(
         ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_))
         | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_))
         | ty::PredicateKind::Clause(ty::Clause::Projection(_))
+        | ty::PredicateKind::AliasEq(..)
         | ty::PredicateKind::WellFormed(_)
         | ty::PredicateKind::Subtype(_)
         | ty::PredicateKind::Coerce(_)
diff --git a/compiler/rustc_hir_analysis/src/outlives/explicit.rs b/compiler/rustc_hir_analysis/src/outlives/explicit.rs
index 663f1c49db7..ecd6849426d 100644
--- a/compiler/rustc_hir_analysis/src/outlives/explicit.rs
+++ b/compiler/rustc_hir_analysis/src/outlives/explicit.rs
@@ -55,6 +55,7 @@ impl<'tcx> ExplicitPredicatesMap<'tcx> {
                     ty::PredicateKind::Clause(ty::Clause::Trait(..))
                     | ty::PredicateKind::Clause(ty::Clause::Projection(..))
                     | ty::PredicateKind::WellFormed(..)
+                    | ty::PredicateKind::AliasEq(..)
                     | ty::PredicateKind::ObjectSafe(..)
                     | ty::PredicateKind::ClosureKind(..)
                     | ty::PredicateKind::Subtype(..)
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
index e84b3de124c..52c2dabee29 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
@@ -669,6 +669,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
                 | ty::PredicateKind::WellFormed(..)
                 | ty::PredicateKind::ObjectSafe(..)
+                | ty::PredicateKind::AliasEq(..)
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
                 // N.B., this predicate is created by breaking down a
diff --git a/compiler/rustc_hir_typeck/src/method/probe.rs b/compiler/rustc_hir_typeck/src/method/probe.rs
index 0cf58179ec2..16b0d48002e 100644
--- a/compiler/rustc_hir_typeck/src/method/probe.rs
+++ b/compiler/rustc_hir_typeck/src/method/probe.rs
@@ -837,6 +837,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
                 | ty::PredicateKind::Ambiguous
+                | ty::PredicateKind::AliasEq(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
             }
         });
diff --git a/compiler/rustc_infer/src/infer/canonical/query_response.rs b/compiler/rustc_infer/src/infer/canonical/query_response.rs
index 7cc9e49b1b6..0c97217bd6a 100644
--- a/compiler/rustc_infer/src/infer/canonical/query_response.rs
+++ b/compiler/rustc_infer/src/infer/canonical/query_response.rs
@@ -12,7 +12,7 @@ use crate::infer::canonical::{
     Canonical, CanonicalQueryResponse, CanonicalVarValues, Certainty, OriginalQueryValues,
     QueryOutlivesConstraint, QueryRegionConstraints, QueryResponse,
 };
-use crate::infer::nll_relate::{NormalizationStrategy, TypeRelating, TypeRelatingDelegate};
+use crate::infer::nll_relate::{TypeRelating, TypeRelatingDelegate};
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::{InferCtxt, InferOk, InferResult, NllRegionVariableOrigin};
 use crate::traits::query::{Fallible, NoSolution};
@@ -717,10 +717,6 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for QueryTypeRelatingDelegate<'_, 'tcx> {
         });
     }
 
-    fn normalization() -> NormalizationStrategy {
-        NormalizationStrategy::Eager
-    }
-
     fn forbid_inference_vars() -> bool {
         true
     }
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index 1360044fe75..4da2a674144 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -38,8 +38,8 @@ use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{
-    self, FallibleTypeFolder, InferConst, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable,
-    TypeVisitable,
+    self, AliasKind, FallibleTypeFolder, InferConst, ToPredicate, Ty, TyCtxt, TypeFoldable,
+    TypeSuperFoldable, TypeVisitable,
 };
 use rustc_middle::ty::{IntType, UintType};
 use rustc_span::{Span, DUMMY_SP};
@@ -74,7 +74,7 @@ impl<'tcx> InferCtxt<'tcx> {
         b: Ty<'tcx>,
     ) -> RelateResult<'tcx, Ty<'tcx>>
     where
-        R: TypeRelation<'tcx>,
+        R: ObligationEmittingRelation<'tcx>,
     {
         let a_is_expected = relation.a_is_expected();
 
@@ -122,6 +122,15 @@ impl<'tcx> InferCtxt<'tcx> {
                 Err(TypeError::Sorts(ty::relate::expected_found(relation, a, b)))
             }
 
+            (ty::Alias(AliasKind::Projection, _), _) if self.tcx.trait_solver_next() => {
+                relation.register_type_equate_obligation(a.into(), b.into());
+                Ok(b)
+            }
+            (_, ty::Alias(AliasKind::Projection, _)) if self.tcx.trait_solver_next() => {
+                relation.register_type_equate_obligation(b.into(), a.into());
+                Ok(a)
+            }
+
             _ => ty::relate::super_relate_tys(relation, a, b),
         }
     }
@@ -133,7 +142,7 @@ impl<'tcx> InferCtxt<'tcx> {
         b: ty::Const<'tcx>,
     ) -> RelateResult<'tcx, ty::Const<'tcx>>
     where
-        R: ConstEquateRelation<'tcx>,
+        R: ObligationEmittingRelation<'tcx>,
     {
         debug!("{}.consts({:?}, {:?})", relation.tag(), a, b);
         if a == b {
@@ -169,7 +178,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 // FIXME(#59490): Need to remove the leak check to accommodate
                 // escaping bound variables here.
                 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
-                    relation.const_equate_obligation(a, b);
+                    relation.register_const_equate_obligation(a, b);
                 }
                 return Ok(b);
             }
@@ -177,7 +186,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 // FIXME(#59490): Need to remove the leak check to accommodate
                 // escaping bound variables here.
                 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
-                    relation.const_equate_obligation(a, b);
+                    relation.register_const_equate_obligation(a, b);
                 }
                 return Ok(a);
             }
@@ -435,32 +444,21 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         Ok(Generalization { ty, needs_wf })
     }
 
-    pub fn add_const_equate_obligation(
+    pub fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
+        self.obligations.extend(obligations.into_iter());
+    }
+
+    pub fn register_predicates(
         &mut self,
-        a_is_expected: bool,
-        a: ty::Const<'tcx>,
-        b: ty::Const<'tcx>,
+        obligations: impl IntoIterator<Item = impl ToPredicate<'tcx>>,
     ) {
-        let predicate = if a_is_expected {
-            ty::PredicateKind::ConstEquate(a, b)
-        } else {
-            ty::PredicateKind::ConstEquate(b, a)
-        };
-        self.obligations.push(Obligation::new(
-            self.tcx(),
-            self.trace.cause.clone(),
-            self.param_env,
-            ty::Binder::dummy(predicate),
-        ));
+        self.obligations.extend(obligations.into_iter().map(|to_pred| {
+            Obligation::new(self.infcx.tcx, self.trace.cause.clone(), self.param_env, to_pred)
+        }))
     }
 
     pub fn mark_ambiguous(&mut self) {
-        self.obligations.push(Obligation::new(
-            self.tcx(),
-            self.trace.cause.clone(),
-            self.param_env,
-            ty::Binder::dummy(ty::PredicateKind::Ambiguous),
-        ));
+        self.register_predicates([ty::Binder::dummy(ty::PredicateKind::Ambiguous)]);
     }
 }
 
@@ -779,11 +777,42 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
     }
 }
 
-pub trait ConstEquateRelation<'tcx>: TypeRelation<'tcx> {
+pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
+    /// Register obligations that must hold in order for this relation to hold
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>);
+
+    /// Register predicates that must hold in order for this relation to hold. Uses
+    /// a default obligation cause, [`ObligationEmittingRelation::register_obligations`] should
+    /// be used if control over the obligaton causes is required.
+    fn register_predicates(
+        &mut self,
+        obligations: impl IntoIterator<Item = impl ToPredicate<'tcx>>,
+    );
+
     /// Register an obligation that both constants must be equal to each other.
     ///
     /// If they aren't equal then the relation doesn't hold.
-    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
+    fn register_const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
+        let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
+
+        self.register_predicates([ty::Binder::dummy(if self.tcx().trait_solver_next() {
+            ty::PredicateKind::AliasEq(a.into(), b.into())
+        } else {
+            ty::PredicateKind::ConstEquate(a, b)
+        })]);
+    }
+
+    /// Register an obligation that both types must be equal to each other.
+    ///
+    /// If they aren't equal then the relation doesn't hold.
+    fn register_type_equate_obligation(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) {
+        let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
+
+        self.register_predicates([ty::Binder::dummy(ty::PredicateKind::AliasEq(
+            a.into(),
+            b.into(),
+        ))]);
+    }
 }
 
 fn int_unification_error<'tcx>(
diff --git a/compiler/rustc_infer/src/infer/equate.rs b/compiler/rustc_infer/src/infer/equate.rs
index 7db4d92a177..742c01efff6 100644
--- a/compiler/rustc_infer/src/infer/equate.rs
+++ b/compiler/rustc_infer/src/infer/equate.rs
@@ -1,4 +1,6 @@
-use super::combine::{CombineFields, ConstEquateRelation, RelationDir};
+use crate::traits::PredicateObligations;
+
+use super::combine::{CombineFields, ObligationEmittingRelation, RelationDir};
 use super::Subtype;
 
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
@@ -198,8 +200,15 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
     }
 }
 
-impl<'tcx> ConstEquateRelation<'tcx> for Equate<'_, '_, 'tcx> {
-    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
-        self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
+impl<'tcx> ObligationEmittingRelation<'tcx> for Equate<'_, '_, 'tcx> {
+    fn register_predicates(
+        &mut self,
+        obligations: impl IntoIterator<Item = impl ty::ToPredicate<'tcx>>,
+    ) {
+        self.fields.register_predicates(obligations);
+    }
+
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
+        self.fields.register_obligations(obligations);
     }
 }
diff --git a/compiler/rustc_infer/src/infer/glb.rs b/compiler/rustc_infer/src/infer/glb.rs
index b92b162a978..74abca7bbea 100644
--- a/compiler/rustc_infer/src/infer/glb.rs
+++ b/compiler/rustc_infer/src/infer/glb.rs
@@ -1,12 +1,11 @@
 //! Greatest lower bound. See [`lattice`].
 
-use super::combine::CombineFields;
+use super::combine::{CombineFields, ObligationEmittingRelation};
 use super::lattice::{self, LatticeDir};
 use super::InferCtxt;
 use super::Subtype;
 
-use crate::infer::combine::ConstEquateRelation;
-use crate::traits::{ObligationCause, PredicateObligation};
+use crate::traits::{ObligationCause, PredicateObligations};
 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
@@ -136,10 +135,6 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Glb<'combine, 'infcx,
         &self.fields.trace.cause
     }
 
-    fn add_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>) {
-        self.fields.obligations.extend(obligations)
-    }
-
     fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
         let mut sub = self.fields.sub(self.a_is_expected);
         sub.relate(v, a)?;
@@ -152,8 +147,15 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Glb<'combine, 'infcx,
     }
 }
 
-impl<'tcx> ConstEquateRelation<'tcx> for Glb<'_, '_, 'tcx> {
-    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
-        self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
+impl<'tcx> ObligationEmittingRelation<'tcx> for Glb<'_, '_, 'tcx> {
+    fn register_predicates(
+        &mut self,
+        obligations: impl IntoIterator<Item = impl ty::ToPredicate<'tcx>>,
+    ) {
+        self.fields.register_predicates(obligations);
+    }
+
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
+        self.fields.register_obligations(obligations);
     }
 }
diff --git a/compiler/rustc_infer/src/infer/lattice.rs b/compiler/rustc_infer/src/infer/lattice.rs
index 4dbb4b4d7b4..f377ac1d19e 100644
--- a/compiler/rustc_infer/src/infer/lattice.rs
+++ b/compiler/rustc_infer/src/infer/lattice.rs
@@ -17,11 +17,12 @@
 //!
 //! [lattices]: https://en.wikipedia.org/wiki/Lattice_(order)
 
+use super::combine::ObligationEmittingRelation;
 use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use super::InferCtxt;
 
-use crate::traits::{ObligationCause, PredicateObligation};
-use rustc_middle::ty::relate::{RelateResult, TypeRelation};
+use crate::traits::ObligationCause;
+use rustc_middle::ty::relate::RelateResult;
 use rustc_middle::ty::TyVar;
 use rustc_middle::ty::{self, Ty};
 
@@ -30,13 +31,11 @@ use rustc_middle::ty::{self, Ty};
 ///
 /// GLB moves "down" the lattice (to smaller values); LUB moves
 /// "up" the lattice (to bigger values).
-pub trait LatticeDir<'f, 'tcx>: TypeRelation<'tcx> {
+pub trait LatticeDir<'f, 'tcx>: ObligationEmittingRelation<'tcx> {
     fn infcx(&self) -> &'f InferCtxt<'tcx>;
 
     fn cause(&self) -> &ObligationCause<'tcx>;
 
-    fn add_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>);
-
     fn define_opaque_types(&self) -> bool;
 
     // Relates the type `v` to `a` and `b` such that `v` represents
@@ -113,7 +112,7 @@ where
         | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
             if this.define_opaque_types() && def_id.is_local() =>
         {
-            this.add_obligations(
+            this.register_obligations(
                 infcx
                     .handle_opaque_type(a, b, this.a_is_expected(), this.cause(), this.param_env())?
                     .obligations,
diff --git a/compiler/rustc_infer/src/infer/lub.rs b/compiler/rustc_infer/src/infer/lub.rs
index f6e0554fd1f..f997171b97f 100644
--- a/compiler/rustc_infer/src/infer/lub.rs
+++ b/compiler/rustc_infer/src/infer/lub.rs
@@ -1,12 +1,11 @@
 //! Least upper bound. See [`lattice`].
 
-use super::combine::CombineFields;
+use super::combine::{CombineFields, ObligationEmittingRelation};
 use super::lattice::{self, LatticeDir};
 use super::InferCtxt;
 use super::Subtype;
 
-use crate::infer::combine::ConstEquateRelation;
-use crate::traits::{ObligationCause, PredicateObligation};
+use crate::traits::{ObligationCause, PredicateObligations};
 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
@@ -127,12 +126,6 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
     }
 }
 
-impl<'tcx> ConstEquateRelation<'tcx> for Lub<'_, '_, 'tcx> {
-    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
-        self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
-    }
-}
-
 impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Lub<'combine, 'infcx, 'tcx> {
     fn infcx(&self) -> &'infcx InferCtxt<'tcx> {
         self.fields.infcx
@@ -142,10 +135,6 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Lub<'combine, 'infcx,
         &self.fields.trace.cause
     }
 
-    fn add_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>) {
-        self.fields.obligations.extend(obligations)
-    }
-
     fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
         let mut sub = self.fields.sub(self.a_is_expected);
         sub.relate(a, v)?;
@@ -157,3 +146,16 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Lub<'combine, 'infcx,
         self.fields.define_opaque_types
     }
 }
+
+impl<'tcx> ObligationEmittingRelation<'tcx> for Lub<'_, '_, 'tcx> {
+    fn register_predicates(
+        &mut self,
+        obligations: impl IntoIterator<Item = impl ty::ToPredicate<'tcx>>,
+    ) {
+        self.fields.register_predicates(obligations);
+    }
+
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
+        self.fields.register_obligations(obligations)
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index 35918b8bae1..e77924900a0 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -4,6 +4,7 @@ pub use self::LateBoundRegionConversionTime::*;
 pub use self::RegionVariableOrigin::*;
 pub use self::SubregionOrigin::*;
 pub use self::ValuePairs::*;
+pub use combine::ObligationEmittingRelation;
 
 use self::opaque_types::OpaqueTypeStorage;
 pub(crate) use self::undo_log::{InferCtxtUndoLogs, Snapshot, UndoLog};
diff --git a/compiler/rustc_infer/src/infer/nll_relate/mod.rs b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
index a2cfe8d8881..1dd5062acaf 100644
--- a/compiler/rustc_infer/src/infer/nll_relate/mod.rs
+++ b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
@@ -21,11 +21,10 @@
 //!   thing we relate in chalk are basically domain goals and their
 //!   constituents)
 
-use crate::infer::combine::ConstEquateRelation;
 use crate::infer::InferCtxt;
 use crate::infer::{ConstVarValue, ConstVariableValue};
 use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
-use crate::traits::{Obligation, PredicateObligation};
+use crate::traits::{Obligation, PredicateObligations};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::error::TypeError;
@@ -36,11 +35,7 @@ use rustc_span::Span;
 use std::fmt::Debug;
 use std::ops::ControlFlow;
 
-#[derive(PartialEq)]
-pub enum NormalizationStrategy {
-    Lazy,
-    Eager,
-}
+use super::combine::ObligationEmittingRelation;
 
 pub struct TypeRelating<'me, 'tcx, D>
 where
@@ -92,7 +87,7 @@ pub trait TypeRelatingDelegate<'tcx> {
         info: ty::VarianceDiagInfo<'tcx>,
     );
 
-    fn register_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>);
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>);
 
     /// Creates a new universe index. Used when instantiating placeholders.
     fn create_next_universe(&mut self) -> ty::UniverseIndex;
@@ -125,9 +120,6 @@ pub trait TypeRelatingDelegate<'tcx> {
     /// relation stating that `'?0: 'a`).
     fn generalize_existential(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx>;
 
-    /// Define the normalization strategy to use, eager or lazy.
-    fn normalization() -> NormalizationStrategy;
-
     /// Enables some optimizations if we do not expect inference variables
     /// in the RHS of the relation.
     fn forbid_inference_vars() -> bool;
@@ -265,38 +257,6 @@ where
         self.delegate.push_outlives(sup, sub, info);
     }
 
-    /// Relate a projection type and some value type lazily. This will always
-    /// succeed, but we push an additional `ProjectionEq` goal depending
-    /// on the value type:
-    /// - if the value type is any type `T` which is not a projection, we push
-    ///   `ProjectionEq(projection = T)`.
-    /// - if the value type is another projection `other_projection`, we create
-    ///   a new inference variable `?U` and push the two goals
-    ///   `ProjectionEq(projection = ?U)`, `ProjectionEq(other_projection = ?U)`.
-    fn relate_projection_ty(
-        &mut self,
-        projection_ty: ty::AliasTy<'tcx>,
-        value_ty: Ty<'tcx>,
-    ) -> Ty<'tcx> {
-        use rustc_span::DUMMY_SP;
-
-        match *value_ty.kind() {
-            ty::Alias(ty::Projection, other_projection_ty) => {
-                let var = self.infcx.next_ty_var(TypeVariableOrigin {
-                    kind: TypeVariableOriginKind::MiscVariable,
-                    span: DUMMY_SP,
-                });
-                // FIXME(lazy-normalization): This will always ICE, because the recursive
-                // call will end up in the _ arm below.
-                self.relate_projection_ty(projection_ty, var);
-                self.relate_projection_ty(other_projection_ty, var);
-                var
-            }
-
-            _ => bug!("should never be invoked with eager normalization"),
-        }
-    }
-
     /// Relate a type inference variable with a value type. This works
     /// by creating a "generalization" G of the value where all the
     /// lifetimes are replaced with fresh inference values. This
@@ -335,12 +295,6 @@ where
                 return Ok(value_ty);
             }
 
-            ty::Alias(ty::Projection, projection_ty)
-                if D::normalization() == NormalizationStrategy::Lazy =>
-            {
-                return Ok(self.relate_projection_ty(projection_ty, self.infcx.tcx.mk_ty_var(vid)));
-            }
-
             _ => (),
         }
 
@@ -627,18 +581,6 @@ where
                 self.relate_opaques(a, b)
             }
 
-            (&ty::Alias(ty::Projection, projection_ty), _)
-                if D::normalization() == NormalizationStrategy::Lazy =>
-            {
-                Ok(self.relate_projection_ty(projection_ty, b))
-            }
-
-            (_, &ty::Alias(ty::Projection, projection_ty))
-                if D::normalization() == NormalizationStrategy::Lazy =>
-            {
-                Ok(self.relate_projection_ty(projection_ty, a))
-            }
-
             _ => {
                 debug!(?a, ?b, ?self.ambient_variance);
 
@@ -813,17 +755,26 @@ where
     }
 }
 
-impl<'tcx, D> ConstEquateRelation<'tcx> for TypeRelating<'_, 'tcx, D>
+impl<'tcx, D> ObligationEmittingRelation<'tcx> for TypeRelating<'_, 'tcx, D>
 where
     D: TypeRelatingDelegate<'tcx>,
 {
-    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
-        self.delegate.register_obligations(vec![Obligation::new(
-            self.tcx(),
-            ObligationCause::dummy(),
-            self.param_env(),
-            ty::Binder::dummy(ty::PredicateKind::ConstEquate(a, b)),
-        )]);
+    fn register_predicates(
+        &mut self,
+        obligations: impl IntoIterator<Item = impl ty::ToPredicate<'tcx>>,
+    ) {
+        self.delegate.register_obligations(
+            obligations
+                .into_iter()
+                .map(|to_pred| {
+                    Obligation::new(self.tcx(), ObligationCause::dummy(), self.param_env(), to_pred)
+                })
+                .collect(),
+        );
+    }
+
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
+        self.delegate.register_obligations(obligations);
     }
 }
 
diff --git a/compiler/rustc_infer/src/infer/outlives/mod.rs b/compiler/rustc_infer/src/infer/outlives/mod.rs
index 4daa257672c..a8e668d81ea 100644
--- a/compiler/rustc_infer/src/infer/outlives/mod.rs
+++ b/compiler/rustc_infer/src/infer/outlives/mod.rs
@@ -21,6 +21,7 @@ pub fn explicit_outlives_bounds<'tcx>(
         .filter_map(move |kind| match kind {
             ty::PredicateKind::Clause(ty::Clause::Projection(..))
             | ty::PredicateKind::Clause(ty::Clause::Trait(..))
+            | ty::PredicateKind::AliasEq(..)
             | ty::PredicateKind::Coerce(..)
             | ty::PredicateKind::Subtype(..)
             | ty::PredicateKind::WellFormed(..)
diff --git a/compiler/rustc_infer/src/infer/projection.rs b/compiler/rustc_infer/src/infer/projection.rs
index 4667d99ff00..f795047709e 100644
--- a/compiler/rustc_infer/src/infer/projection.rs
+++ b/compiler/rustc_infer/src/infer/projection.rs
@@ -21,16 +21,28 @@ impl<'tcx> InferCtxt<'tcx> {
         recursion_depth: usize,
         obligations: &mut Vec<PredicateObligation<'tcx>>,
     ) -> Ty<'tcx> {
-        let def_id = projection_ty.def_id;
-        let ty_var = self.next_ty_var(TypeVariableOrigin {
-            kind: TypeVariableOriginKind::NormalizeProjectionType,
-            span: self.tcx.def_span(def_id),
-        });
-        let projection =
-            ty::Binder::dummy(ty::ProjectionPredicate { projection_ty, term: ty_var.into() });
-        let obligation =
-            Obligation::with_depth(self.tcx, cause, recursion_depth, param_env, projection);
-        obligations.push(obligation);
-        ty_var
+        if self.tcx.trait_solver_next() {
+            // FIXME(-Ztrait-solver=next): Instead of branching here,
+            // completely change the normalization routine with the new solver.
+            //
+            // The new solver correctly handles projection equality so this hack
+            // is not necessary. if re-enabled it should emit `PredicateKind::AliasEq`
+            // not `PredicateKind::Clause(Clause::Projection(..))` as in the new solver
+            // `Projection` is used as `normalizes-to` which will fail for `<T as Trait>::Assoc eq ?0`.
+            return projection_ty.to_ty(self.tcx);
+        } else {
+            let def_id = projection_ty.def_id;
+            let ty_var = self.next_ty_var(TypeVariableOrigin {
+                kind: TypeVariableOriginKind::NormalizeProjectionType,
+                span: self.tcx.def_span(def_id),
+            });
+            let projection = ty::Binder::dummy(ty::PredicateKind::Clause(ty::Clause::Projection(
+                ty::ProjectionPredicate { projection_ty, term: ty_var.into() },
+            )));
+            let obligation =
+                Obligation::with_depth(self.tcx, cause, recursion_depth, param_env, projection);
+            obligations.push(obligation);
+            ty_var
+        }
     }
 }
diff --git a/compiler/rustc_infer/src/infer/sub.rs b/compiler/rustc_infer/src/infer/sub.rs
index 532fbd0ffe4..bf1b3441547 100644
--- a/compiler/rustc_infer/src/infer/sub.rs
+++ b/compiler/rustc_infer/src/infer/sub.rs
@@ -1,8 +1,7 @@
 use super::combine::{CombineFields, RelationDir};
-use super::SubregionOrigin;
+use super::{ObligationEmittingRelation, SubregionOrigin};
 
-use crate::infer::combine::ConstEquateRelation;
-use crate::traits::Obligation;
+use crate::traits::{Obligation, PredicateObligations};
 use rustc_middle::ty::relate::{Cause, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::visit::TypeVisitable;
 use rustc_middle::ty::TyVar;
@@ -228,8 +227,15 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
     }
 }
 
-impl<'tcx> ConstEquateRelation<'tcx> for Sub<'_, '_, 'tcx> {
-    fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>) {
-        self.fields.add_const_equate_obligation(self.a_is_expected, a, b);
+impl<'tcx> ObligationEmittingRelation<'tcx> for Sub<'_, '_, 'tcx> {
+    fn register_predicates(
+        &mut self,
+        obligations: impl IntoIterator<Item = impl ty::ToPredicate<'tcx>>,
+    ) {
+        self.fields.register_predicates(obligations);
+    }
+
+    fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
+        self.fields.register_obligations(obligations);
     }
 }
diff --git a/compiler/rustc_infer/src/traits/util.rs b/compiler/rustc_infer/src/traits/util.rs
index 18a966449aa..e617eb68d47 100644
--- a/compiler/rustc_infer/src/traits/util.rs
+++ b/compiler/rustc_infer/src/traits/util.rs
@@ -294,6 +294,9 @@ impl<'tcx> Elaborator<'tcx> {
                 // Nothing to elaborate
             }
             ty::PredicateKind::Ambiguous => {}
+            ty::PredicateKind::AliasEq(..) => {
+                // No
+            }
         }
     }
 }
diff --git a/compiler/rustc_lint/src/builtin.rs b/compiler/rustc_lint/src/builtin.rs
index 5d85cfe330a..7a50b6aec87 100644
--- a/compiler/rustc_lint/src/builtin.rs
+++ b/compiler/rustc_lint/src/builtin.rs
@@ -1594,12 +1594,14 @@ impl<'tcx> LateLintPass<'tcx> for TrivialConstraints {
                     // Ignore projections, as they can only be global
                     // if the trait bound is global
                     Clause(Clause::Projection(..)) |
+                    AliasEq(..) |
                     // Ignore bounds that a user can't type
                     WellFormed(..) |
                     ObjectSafe(..) |
                     ClosureKind(..) |
                     Subtype(..) |
                     Coerce(..) |
+                    // FIXME(generic_const_exprs): `ConstEvaluatable` can be written
                     ConstEvaluatable(..) |
                     ConstEquate(..) |
                     Ambiguous |
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 01f18e8158a..04537ad635a 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -2254,6 +2254,10 @@ impl<'tcx> TyCtxt<'tcx> {
             })
         )
     }
+
+    pub fn trait_solver_next(self) -> bool {
+        self.sess.opts.unstable_opts.trait_solver == rustc_session::config::TraitSolver::Next
+    }
 }
 
 impl<'tcx> TyCtxtAt<'tcx> {
diff --git a/compiler/rustc_middle/src/ty/flags.rs b/compiler/rustc_middle/src/ty/flags.rs
index dc6f5851b7d..258bc9c3e41 100644
--- a/compiler/rustc_middle/src/ty/flags.rs
+++ b/compiler/rustc_middle/src/ty/flags.rs
@@ -264,10 +264,7 @@ impl FlagComputation {
                 term,
             })) => {
                 self.add_projection_ty(projection_ty);
-                match term.unpack() {
-                    ty::TermKind::Ty(ty) => self.add_ty(ty),
-                    ty::TermKind::Const(c) => self.add_const(c),
-                }
+                self.add_term(term);
             }
             ty::PredicateKind::WellFormed(arg) => {
                 self.add_substs(slice::from_ref(&arg));
@@ -287,6 +284,10 @@ impl FlagComputation {
                 self.add_ty(ty);
             }
             ty::PredicateKind::Ambiguous => {}
+            ty::PredicateKind::AliasEq(t1, t2) => {
+                self.add_term(t1);
+                self.add_term(t2);
+            }
         }
     }
 
@@ -380,4 +381,11 @@ impl FlagComputation {
             }
         }
     }
+
+    fn add_term(&mut self, term: ty::Term<'_>) {
+        match term.unpack() {
+            ty::TermKind::Ty(ty) => self.add_ty(ty),
+            ty::TermKind::Const(ct) => self.add_const(ct),
+        }
+    }
 }
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 09c3d5b736c..0e86b2666b2 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -545,6 +545,7 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::Clause(Clause::RegionOutlives(_))
             | PredicateKind::Clause(Clause::TypeOutlives(_))
             | PredicateKind::Clause(Clause::Projection(_))
+            | PredicateKind::AliasEq(..)
             | PredicateKind::ObjectSafe(_)
             | PredicateKind::ClosureKind(_, _, _)
             | PredicateKind::Subtype(_)
@@ -632,6 +633,12 @@ pub enum PredicateKind<'tcx> {
     /// A marker predicate that is always ambiguous.
     /// Used for coherence to mark opaque types as possibly equal to each other but ambiguous.
     Ambiguous,
+
+    /// Separate from `Clause::Projection` which is used for normalization in new solver.
+    /// This predicate requires two terms to be equal to eachother.
+    ///
+    /// Only used for new solver
+    AliasEq(Term<'tcx>, Term<'tcx>),
 }
 
 /// The crate outlives map is computed during typeck and contains the
@@ -963,6 +970,33 @@ impl<'tcx> Term<'tcx> {
             TermKind::Const(c) => c.into(),
         }
     }
+
+    /// This function returns `None` for `AliasKind::Opaque`.
+    ///
+    /// FIXME: rename `AliasTy` to `AliasTerm` and make sure we correctly
+    /// deal with constants.
+    pub fn to_alias_term_no_opaque(&self, tcx: TyCtxt<'tcx>) -> Option<AliasTy<'tcx>> {
+        match self.unpack() {
+            TermKind::Ty(ty) => match ty.kind() {
+                ty::Alias(kind, alias_ty) => match kind {
+                    AliasKind::Projection => Some(*alias_ty),
+                    AliasKind::Opaque => None,
+                },
+                _ => None,
+            },
+            TermKind::Const(ct) => match ct.kind() {
+                ConstKind::Unevaluated(uv) => Some(tcx.mk_alias_ty(uv.def.did, uv.substs)),
+                _ => None,
+            },
+        }
+    }
+
+    pub fn is_infer(&self) -> bool {
+        match self.unpack() {
+            TermKind::Ty(ty) => ty.is_ty_or_numeric_infer(),
+            TermKind::Const(ct) => ct.is_ct_infer(),
+        }
+    }
 }
 
 const TAG_MASK: usize = 0b11;
@@ -1152,6 +1186,7 @@ impl<'tcx> Predicate<'tcx> {
         match predicate.skip_binder() {
             PredicateKind::Clause(Clause::Trait(t)) => Some(predicate.rebind(t)),
             PredicateKind::Clause(Clause::Projection(..))
+            | PredicateKind::AliasEq(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)
             | PredicateKind::Clause(Clause::RegionOutlives(..))
@@ -1171,6 +1206,7 @@ impl<'tcx> Predicate<'tcx> {
         match predicate.skip_binder() {
             PredicateKind::Clause(Clause::Projection(t)) => Some(predicate.rebind(t)),
             PredicateKind::Clause(Clause::Trait(..))
+            | PredicateKind::AliasEq(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)
             | PredicateKind::Clause(Clause::RegionOutlives(..))
@@ -1191,6 +1227,7 @@ impl<'tcx> Predicate<'tcx> {
             PredicateKind::Clause(Clause::TypeOutlives(data)) => Some(predicate.rebind(data)),
             PredicateKind::Clause(Clause::Trait(..))
             | PredicateKind::Clause(Clause::Projection(..))
+            | PredicateKind::AliasEq(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)
             | PredicateKind::Clause(Clause::RegionOutlives(..))
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index a8b23e64e82..7ba531fba78 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -2842,6 +2842,7 @@ define_print_and_forward_display! {
                 p!("the type `", print(ty), "` is found in the environment")
             }
             ty::PredicateKind::Ambiguous => p!("ambiguous"),
+            ty::PredicateKind::AliasEq(t1, t2) => p!(print(t1), " == ", print(t2)),
         }
     }
 
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
index 8df639750c7..1ef66b01ea0 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -177,6 +177,7 @@ impl<'tcx> fmt::Debug for ty::PredicateKind<'tcx> {
                 write!(f, "TypeWellFormedFromEnv({:?})", ty)
             }
             ty::PredicateKind::Ambiguous => write!(f, "Ambiguous"),
+            ty::PredicateKind::AliasEq(t1, t2) => write!(f, "AliasEq({t1:?}, {t2:?})"),
         }
     }
 }
diff --git a/compiler/rustc_trait_selection/src/solve/fulfill.rs b/compiler/rustc_trait_selection/src/solve/fulfill.rs
index c1936b7dbe4..a55b984fd63 100644
--- a/compiler/rustc_trait_selection/src/solve/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/solve/fulfill.rs
@@ -73,6 +73,11 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                         MismatchedProjectionTypes { err: TypeError::Mismatch },
                                     )
                                 }
+                                ty::PredicateKind::AliasEq(_, _) => {
+                                    FulfillmentErrorCode::CodeProjectionError(
+                                        MismatchedProjectionTypes { err: TypeError::Mismatch },
+                                    )
+                                }
                                 ty::PredicateKind::Subtype(pred) => {
                                     let (a, b) = infcx.instantiate_binder_with_placeholders(
                                         goal.predicate.kind().rebind((pred.a, pred.b)),
diff --git a/compiler/rustc_trait_selection/src/solve/mod.rs b/compiler/rustc_trait_selection/src/solve/mod.rs
index 9f092b6018f..e56588c58bd 100644
--- a/compiler/rustc_trait_selection/src/solve/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/mod.rs
@@ -42,6 +42,8 @@ mod trait_goals;
 
 pub use fulfill::FulfillmentCtxt;
 
+use self::infcx_ext::InferCtxtExt;
+
 /// A goal is a statement, i.e. `predicate`, we want to prove
 /// given some assumptions, i.e. `param_env`.
 ///
@@ -81,6 +83,21 @@ pub struct Response<'tcx> {
     pub certainty: Certainty,
 }
 
+trait CanonicalResponseExt {
+    fn has_no_inference_or_external_constraints(&self) -> bool;
+}
+
+impl<'tcx> CanonicalResponseExt for Canonical<'tcx, Response<'tcx>> {
+    fn has_no_inference_or_external_constraints(&self) -> bool {
+        // so that we get a compile error when regions are supported
+        // so this code can be checked for being correct
+        let _: () = self.value.external_constraints.regions;
+
+        self.value.var_values.is_identity()
+            && self.value.external_constraints.opaque_types.is_empty()
+    }
+}
+
 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
 pub enum Certainty {
     Yes,
@@ -302,6 +319,9 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
+                ty::PredicateKind::AliasEq(lhs, rhs) => {
+                    self.compute_alias_eq_goal(Goal { param_env, predicate: (lhs, rhs) })
+                }
             }
         } else {
             let kind = self.infcx.instantiate_binder_with_placeholders(kind);
@@ -398,6 +418,63 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             None => self.make_canonical_response(Certainty::AMBIGUOUS),
         }
     }
+
+    #[instrument(level = "debug", skip(self), ret)]
+    fn compute_alias_eq_goal(
+        &mut self,
+        goal: Goal<'tcx, (ty::Term<'tcx>, ty::Term<'tcx>)>,
+    ) -> QueryResult<'tcx> {
+        let tcx = self.tcx();
+
+        let evaluate_normalizes_to = |ecx: &mut EvalCtxt<'_, 'tcx>, alias, other| {
+            debug!("evaluate_normalizes_to(alias={:?}, other={:?})", alias, other);
+            let r = ecx.infcx.probe(|_| {
+                let (_, certainty) = ecx.evaluate_goal(goal.with(
+                    tcx,
+                    ty::Binder::dummy(ty::ProjectionPredicate {
+                        projection_ty: alias,
+                        term: other,
+                    }),
+                ))?;
+                ecx.make_canonical_response(certainty)
+            });
+            debug!("evaluate_normalizes_to(..) -> {:?}", r);
+            r
+        };
+
+        if goal.predicate.0.is_infer() || goal.predicate.1.is_infer() {
+            bug!(
+                "`AliasEq` goal with an infer var on lhs or rhs which should have been instantiated"
+            );
+        }
+
+        match (
+            goal.predicate.0.to_alias_term_no_opaque(tcx),
+            goal.predicate.1.to_alias_term_no_opaque(tcx),
+        ) {
+            (None, None) => bug!("`AliasEq` goal without an alias on either lhs or rhs"),
+            (Some(alias), None) => evaluate_normalizes_to(self, alias, goal.predicate.1),
+            (None, Some(alias)) => evaluate_normalizes_to(self, alias, goal.predicate.0),
+            (Some(alias_lhs), Some(alias_rhs)) => {
+                debug!("compute_alias_eq_goal: both sides are aliases");
+
+                let mut candidates = Vec::with_capacity(3);
+
+                // Evaluate all 3 potential candidates for the alias' being equal
+                candidates.push(evaluate_normalizes_to(self, alias_lhs, goal.predicate.1));
+                candidates.push(evaluate_normalizes_to(self, alias_rhs, goal.predicate.0));
+                candidates.push(self.infcx.probe(|_| {
+                    debug!("compute_alias_eq_goal: alias defids are equal, equating substs");
+                    let nested_goals = self.infcx.eq(goal.param_env, alias_lhs, alias_rhs)?;
+                    self.evaluate_all_and_make_canonical_response(nested_goals)
+                }));
+
+                debug!(?candidates);
+
+                self.try_merge_responses(candidates.into_iter())
+            }
+        }
+    }
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
@@ -449,6 +526,43 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     ) -> QueryResult<'tcx> {
         self.evaluate_all(goals).and_then(|certainty| self.make_canonical_response(certainty))
     }
+
+    fn try_merge_responses(
+        &mut self,
+        responses: impl Iterator<Item = QueryResult<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        let candidates = responses.into_iter().flatten().collect::<Box<[_]>>();
+
+        if candidates.is_empty() {
+            return Err(NoSolution);
+        }
+
+        // FIXME(-Ztreat-solver=next): We should instead try to find a `Certainty::Yes` response with
+        // a subset of the constraints that all the other responses have.
+        let one = candidates[0];
+        if candidates[1..].iter().all(|resp| resp == &one) {
+            return Ok(one);
+        }
+
+        if let Some(response) = candidates.iter().find(|response| {
+            response.value.certainty == Certainty::Yes
+                && response.has_no_inference_or_external_constraints()
+        }) {
+            return Ok(response.clone());
+        }
+
+        let certainty = candidates.iter().fold(Certainty::AMBIGUOUS, |certainty, response| {
+            certainty.unify_and(response.value.certainty)
+        });
+        // FIXME(-Ztrait-solver=next): We should take the intersection of the constraints on all the
+        // responses and use that for the constraints of this ambiguous response.
+        let response = self.make_canonical_response(certainty);
+        if let Ok(response) = &response {
+            assert!(response.has_no_inference_or_external_constraints());
+        }
+
+        response
+    }
 }
 
 #[instrument(level = "debug", skip(infcx), ret)]
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index 948632ccc6c..6a840704e86 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -823,14 +823,17 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                         _ => return false,
                     }
                 }
+
                 // There's not really much we can do with these predicates -
                 // we start out with a `ParamEnv` with no inference variables,
                 // and these don't correspond to adding any new bounds to
                 // the `ParamEnv`.
                 ty::PredicateKind::WellFormed(..)
+                | ty::PredicateKind::AliasEq(..)
                 | ty::PredicateKind::ObjectSafe(..)
                 | ty::PredicateKind::ClosureKind(..)
                 | ty::PredicateKind::Subtype(..)
+                // FIXME(generic_const_exprs): you can absolutely add this as a where clauses
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::Coerce(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
index ba9ee57d409..9474c70cb53 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
@@ -1,5 +1,7 @@
 use crate::infer::InferCtxt;
 
+use rustc_infer::infer::ObligationEmittingRelation;
+use rustc_infer::traits::PredicateObligations;
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -88,3 +90,16 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
     }
 }
+
+impl<'tcx> ObligationEmittingRelation<'tcx> for CollectAllMismatches<'_, 'tcx> {
+    fn register_obligations(&mut self, _obligations: PredicateObligations<'tcx>) {
+        // FIXME(deferred_projection_equality)
+    }
+
+    fn register_predicates(
+        &mut self,
+        _obligations: impl IntoIterator<Item = impl ty::ToPredicate<'tcx>>,
+    ) {
+        // FIXME(deferred_projection_equality)
+    }
+}
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index cf1e05ada47..4867855c2ae 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1278,6 +1278,11 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         span,
                         "TypeWellFormedFromEnv predicate should only exist in the environment"
                     ),
+
+                    ty::PredicateKind::AliasEq(..) => span_bug!(
+                        span,
+                        "AliasEq predicate should never be the predicate cause of a SelectionError"
+                    ),
                 }
             }
 
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 3adc1e62e0d..19d47d33f67 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -328,6 +328,9 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
+                ty::PredicateKind::AliasEq(..) => {
+                    bug!("AliasEq is only used for new solver")
+                }
             },
             Some(pred) => match pred {
                 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
@@ -594,6 +597,9 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
+                ty::PredicateKind::AliasEq(..) => {
+                    bug!("AliasEq is only used for new solver")
+                }
             },
         }
     }
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index bafa2981a87..977446894e7 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -327,6 +327,8 @@ fn predicate_references_self<'tcx>(
             // possible alternatives.
             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
         }
+        ty::PredicateKind::AliasEq(..) => bug!("`AliasEq` not allowed as assumption"),
+
         ty::PredicateKind::WellFormed(..)
         | ty::PredicateKind::ObjectSafe(..)
         | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
@@ -334,6 +336,7 @@ fn predicate_references_self<'tcx>(
         | ty::PredicateKind::ClosureKind(..)
         | ty::PredicateKind::Subtype(..)
         | ty::PredicateKind::Coerce(..)
+        // FIXME(generic_const_exprs): this can mention `Self`
         | ty::PredicateKind::ConstEvaluatable(..)
         | ty::PredicateKind::ConstEquate(..)
         | ty::PredicateKind::Ambiguous
@@ -368,6 +371,7 @@ fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
             | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::ConstEquate(..)
+            | ty::PredicateKind::AliasEq(..)
             | ty::PredicateKind::Ambiguous
             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
         }
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 984d6fde268..45c4811321a 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -991,6 +991,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for chalk")
                 }
+                ty::PredicateKind::AliasEq(..) => {
+                    bug!("AliasEq is only used for new solver")
+                }
                 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
             }
         })
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index 7c5e147a950..1136b70a0b9 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -187,6 +187,9 @@ pub fn predicate_obligations<'tcx>(
         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
             bug!("TypeWellFormedFromEnv is only used for Chalk")
         }
+        ty::PredicateKind::AliasEq(..) => {
+            bug!("We should only wf check where clauses and `AliasEq` is not a `Clause`")
+        }
     }
 
     wf.normalize(infcx)
@@ -928,6 +931,7 @@ pub(crate) fn required_region_bounds<'tcx>(
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
                 | ty::PredicateKind::Ambiguous
+                | ty::PredicateKind::AliasEq(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
                     ref t,
diff --git a/compiler/rustc_traits/src/chalk/lowering.rs b/compiler/rustc_traits/src/chalk/lowering.rs
index 65ed3105d10..7c9a014e2ab 100644
--- a/compiler/rustc_traits/src/chalk/lowering.rs
+++ b/compiler/rustc_traits/src/chalk/lowering.rs
@@ -116,6 +116,7 @@ impl<'tcx> LowerInto<'tcx, chalk_ir::InEnvironment<chalk_ir::Goal<RustInterner<'
                     )),
                 },
                 ty::PredicateKind::ObjectSafe(..)
+                | ty::PredicateKind::AliasEq(..)
                 | ty::PredicateKind::ClosureKind(..)
                 | ty::PredicateKind::Subtype(..)
                 | ty::PredicateKind::Coerce(..)
@@ -210,6 +211,7 @@ impl<'tcx> LowerInto<'tcx, chalk_ir::GoalData<RustInterner<'tcx>>> for ty::Predi
             // We can defer this, but ultimately we'll want to express
             // some of these in terms of chalk operations.
             ty::PredicateKind::ClosureKind(..)
+            | ty::PredicateKind::AliasEq(..)
             | ty::PredicateKind::Coerce(..)
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::Ambiguous
@@ -645,6 +647,7 @@ impl<'tcx> LowerInto<'tcx, Option<chalk_ir::QuantifiedWhereClause<RustInterner<'
             ty::PredicateKind::WellFormed(_ty) => None,
 
             ty::PredicateKind::ObjectSafe(..)
+            | ty::PredicateKind::AliasEq(..)
             | ty::PredicateKind::ClosureKind(..)
             | ty::PredicateKind::Subtype(..)
             | ty::PredicateKind::Coerce(..)
@@ -778,6 +781,7 @@ impl<'tcx> LowerInto<'tcx, Option<chalk_solve::rust_ir::QuantifiedInlineBound<Ru
             ty::PredicateKind::WellFormed(_ty) => None,
 
             ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
+            | ty::PredicateKind::AliasEq(..)
             | ty::PredicateKind::ObjectSafe(..)
             | ty::PredicateKind::ClosureKind(..)
             | ty::PredicateKind::Subtype(..)
diff --git a/compiler/rustc_traits/src/implied_outlives_bounds.rs b/compiler/rustc_traits/src/implied_outlives_bounds.rs
index 2c6c77072e6..93f9b66e0f8 100644
--- a/compiler/rustc_traits/src/implied_outlives_bounds.rs
+++ b/compiler/rustc_traits/src/implied_outlives_bounds.rs
@@ -85,7 +85,8 @@ fn compute_implied_outlives_bounds<'tcx>(
             // learn anything new from those.
             if obligation.predicate.has_non_region_infer() {
                 match obligation.predicate.kind().skip_binder() {
-                    ty::PredicateKind::Clause(ty::Clause::Projection(..)) => {
+                    ty::PredicateKind::Clause(ty::Clause::Projection(..))
+                    | ty::PredicateKind::AliasEq(..) => {
                         ocx.register_obligation(obligation.clone());
                     }
                     _ => {}
@@ -106,6 +107,7 @@ fn compute_implied_outlives_bounds<'tcx>(
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
                 | ty::PredicateKind::Ambiguous
+                | ty::PredicateKind::AliasEq(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
 
                 // We need to search through *all* WellFormed predicates
diff --git a/compiler/rustc_traits/src/normalize_erasing_regions.rs b/compiler/rustc_traits/src/normalize_erasing_regions.rs
index 5cad2c2ccb0..07e716cda42 100644
--- a/compiler/rustc_traits/src/normalize_erasing_regions.rs
+++ b/compiler/rustc_traits/src/normalize_erasing_regions.rs
@@ -60,6 +60,7 @@ fn not_outlives_predicate(p: ty::Predicate<'_>) -> bool {
         | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..)) => false,
         ty::PredicateKind::Clause(ty::Clause::Trait(..))
         | ty::PredicateKind::Clause(ty::Clause::Projection(..))
+        | ty::PredicateKind::AliasEq(..)
         | ty::PredicateKind::WellFormed(..)
         | ty::PredicateKind::ObjectSafe(..)
         | ty::PredicateKind::ClosureKind(..)
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index 5bda3620dd0..d497f7ea679 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -311,10 +311,12 @@ pub(crate) fn clean_predicate<'tcx>(
         ty::PredicateKind::Clause(ty::Clause::Projection(pred)) => {
             Some(clean_projection_predicate(bound_predicate.rebind(pred), cx))
         }
+        // FIXME(generic_const_exprs): should this do something?
         ty::PredicateKind::ConstEvaluatable(..) => None,
         ty::PredicateKind::WellFormed(..) => None,
 
         ty::PredicateKind::Subtype(..)
+        | ty::PredicateKind::AliasEq(..)
         | ty::PredicateKind::Coerce(..)
         | ty::PredicateKind::ObjectSafe(..)
         | ty::PredicateKind::ClosureKind(..)
diff --git a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
index 72705878075..26b1d019749 100644
--- a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
+++ b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
@@ -36,6 +36,7 @@ pub fn is_min_const_fn<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, msrv: &Msrv)
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => continue,
+                ty::PredicateKind::AliasEq(..) => panic!("alias eq predicate on function: {predicate:#?}"),
                 ty::PredicateKind::ObjectSafe(_) => panic!("object safe predicate on function: {predicate:#?}"),
                 ty::PredicateKind::ClosureKind(..) => panic!("closure kind predicate on function: {predicate:#?}"),
                 ty::PredicateKind::Subtype(_) => panic!("subtype predicate on function: {predicate:#?}"),
diff --git a/tests/ui/traits/new-solver/alias_eq_cant_be_furthur_normalized.rs b/tests/ui/traits/new-solver/alias_eq_cant_be_furthur_normalized.rs
new file mode 100644
index 00000000000..dc726ba51f9
--- /dev/null
+++ b/tests/ui/traits/new-solver/alias_eq_cant_be_furthur_normalized.rs
@@ -0,0 +1,29 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+
+// check that a goal such as `alias-eq(<T as TraitB>::Assoc<bool>, <T as TraitB>::Assoc<?0>)`
+// succeeds with a constraint that `?0 = bool`
+
+// FIXME(deferred_projection_equality): add a test that this is true during coherence
+
+trait TraitA {}
+
+trait TraitB {
+    type Assoc<T: ?Sized>;
+}
+
+impl<T: TraitB> TraitA for (T, T::Assoc<bool>) {}
+
+impl TraitB for i32 {
+    type Assoc<T: ?Sized> = u32;
+}
+
+fn needs_a<T: TraitA>() {}
+
+fn bar<T: TraitB>() {
+    needs_a::<(T, <T as TraitB>::Assoc<_>)>();
+}
+
+fn main() {
+    bar::<i32>();
+}
diff --git a/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.rs b/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.rs
new file mode 100644
index 00000000000..fd5d0e3b194
--- /dev/null
+++ b/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.rs
@@ -0,0 +1,45 @@
+// compile-flags: -Ztrait-solver=next
+
+// check that when computing `alias-eq(<() as Foo<u16, T>>::Assoc, <() as Foo<?0, T>>::Assoc)`
+//  we do not infer `?0 = u8` via the `for<STOP> (): Foo<u8, STOP>` impl or `?0 = u16` by
+// relating substs as either could be a valid solution.
+
+trait Foo<T, STOP> {
+    type Assoc;
+}
+
+impl<STOP> Foo<u8, STOP> for ()
+where
+    (): Foo<u16, STOP>,
+{
+    type Assoc = <() as Foo<u16, STOP>>::Assoc;
+}
+
+impl Foo<u16, i8> for () {
+    type Assoc = u8;
+}
+
+impl Foo<u16, i16> for () {
+    type Assoc = u16;
+}
+
+fn output<T, U>() -> <() as Foo<T, U>>::Assoc
+where
+    (): Foo<T, U>,
+{
+    todo!()
+}
+
+fn incomplete<T>()
+where
+    (): Foo<u16, T>,
+{
+    // `<() as Foo<u16, STOP>>::Assoc == <() as Foo<_, STOP>>::Assoc`
+    let _: <() as Foo<u16, T>>::Assoc = output::<_, T>();
+    //~^ error: type annotations needed
+
+    // let _: <() as Foo<u16, T>>::Assoc = output::<u8, T>(); // OK
+    // let _: <() as Foo<u16, T>>::Assoc = output::<u16, T>(); // OK
+}
+
+fn main() {}
diff --git a/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.stderr b/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.stderr
new file mode 100644
index 00000000000..a6712332c37
--- /dev/null
+++ b/tests/ui/traits/new-solver/alias_eq_dont_use_normalizes_to_if_substs_eq.stderr
@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/alias_eq_dont_use_normalizes_to_if_substs_eq.rs:38:41
+   |
+LL |     let _: <() as Foo<u16, T>>::Assoc = output::<_, T>();
+   |                                         ^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `output`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.
diff --git a/tests/ui/traits/new-solver/alias_eq_simple.rs b/tests/ui/traits/new-solver/alias_eq_simple.rs
new file mode 100644
index 00000000000..6792cf3ce35
--- /dev/null
+++ b/tests/ui/traits/new-solver/alias_eq_simple.rs
@@ -0,0 +1,22 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+
+// test that the new solver can handle `alias-eq(<i32 as TraitB>::Assoc, u32)`
+
+trait TraitA {}
+
+trait TraitB {
+    type Assoc;
+}
+
+impl<T: TraitB> TraitA for (T, T::Assoc) {}
+
+impl TraitB for i32 {
+    type Assoc = u32;
+}
+
+fn needs_a<T: TraitA>() {}
+
+fn main() {
+    needs_a::<(i32, u32)>();
+}
diff --git a/tests/ui/traits/new-solver/alias_eq_substs_eq_not_intercrate.rs b/tests/ui/traits/new-solver/alias_eq_substs_eq_not_intercrate.rs
new file mode 100644
index 00000000000..d4cc380fa21
--- /dev/null
+++ b/tests/ui/traits/new-solver/alias_eq_substs_eq_not_intercrate.rs
@@ -0,0 +1,20 @@
+// compile-flags: -Ztrait-solver=next
+
+// check that a `alias-eq(<?0 as TraitB>::Assoc, <T as TraitB>::Assoc)` goal fails.
+
+// FIXME(deferred_projection_equality): add a test that this is true during coherence
+
+trait TraitB {
+    type Assoc;
+}
+
+fn needs_a<T: TraitB>() -> T::Assoc {
+    unimplemented!()
+}
+
+fn bar<T: TraitB>() {
+    let _: <_ as TraitB>::Assoc = needs_a::<T>();
+    //~^ error: type annotations needed
+}
+
+fn main() {}
diff --git a/tests/ui/traits/new-solver/alias_eq_substs_eq_not_intercrate.stderr b/tests/ui/traits/new-solver/alias_eq_substs_eq_not_intercrate.stderr
new file mode 100644
index 00000000000..d063d8fce11
--- /dev/null
+++ b/tests/ui/traits/new-solver/alias_eq_substs_eq_not_intercrate.stderr
@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/alias_eq_substs_eq_not_intercrate.rs:16:12
+   |
+LL |     let _: <_ as TraitB>::Assoc = needs_a::<T>();
+   |            ^^^^^^^^^^^^^^^^^^^^ cannot infer type for associated type `<_ as TraitB>::Assoc`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.
diff --git a/tests/ui/traits/new-solver/normalizes_to_ignores_unnormalizable_candidate.rs b/tests/ui/traits/new-solver/normalizes_to_ignores_unnormalizable_candidate.rs
new file mode 100644
index 00000000000..46343241b45
--- /dev/null
+++ b/tests/ui/traits/new-solver/normalizes_to_ignores_unnormalizable_candidate.rs
@@ -0,0 +1,40 @@
+// [no_self_infer] check-pass
+// compile-flags: -Ztrait-solver=next
+// revisions: self_infer no_self_infer
+
+// checks that the new solver is smart enough to infer `?0 = U` when solving:
+// `normalizes-to(<Vec<?0> as Trait>::Assoc, u8)`
+// with `normalizes-to(<Vec<U> as Trait>::Assoc, u8)` in the paramenv even when
+// there is a separate `Vec<T>: Trait` bound  in the paramenv.
+//
+// FIXME(-Ztrait-solver=next)
+// This could also compile for `normalizes-to(<?0 as Trait>::Assoc, u8)` but
+// we currently immediately consider a goal ambiguous if the self type is an
+// inference variable.
+
+trait Trait {
+    type Assoc;
+}
+
+fn foo<T: Trait<Assoc = u8>>(x: T) {}
+
+#[cfg(self_infer)]
+fn unconstrained<T>() -> T {
+    todo!()
+}
+
+#[cfg(no_self_infer)]
+fn unconstrained<T>() -> Vec<T> {
+    todo!()
+}
+
+fn bar<T, U>()
+where
+    Vec<T>: Trait,
+    Vec<U>: Trait<Assoc = u8>,
+{
+    foo(unconstrained())
+    //[self_infer]~^ ERROR type annotations needed
+}
+
+fn main() {}
diff --git a/tests/ui/traits/new-solver/normalizes_to_ignores_unnormalizable_candidate.self_infer.stderr b/tests/ui/traits/new-solver/normalizes_to_ignores_unnormalizable_candidate.self_infer.stderr
new file mode 100644
index 00000000000..06283201261
--- /dev/null
+++ b/tests/ui/traits/new-solver/normalizes_to_ignores_unnormalizable_candidate.self_infer.stderr
@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed
+  --> $DIR/normalizes_to_ignores_unnormalizable_candidate.rs:36:5
+   |
+LL |     foo(unconstrained())
+   |     ^^^ cannot infer type of the type parameter `T` declared on the function `foo`
+   |
+help: consider specifying the generic argument
+   |
+LL |     foo::<T>(unconstrained())
+   |        +++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.