diff --git a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
index 6a07d2988fdfc..fa497cc21e40d 100644
--- a/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
+++ b/compiler/rustc_hir_analysis/src/check/compare_impl_item.rs
@@ -236,7 +236,26 @@ fn compare_method_predicate_entailment<'tcx>(
 
     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_def_id);
     let param_env = ty::ParamEnv::new(tcx.mk_clauses(&hybrid_preds));
-    let param_env = traits::normalize_param_env_or_error(tcx, param_env, normalize_cause);
+    // FIXME(-Zhigher-ranked-assumptions): The `hybrid_preds`
+    // should be well-formed. However, using them may result in
+    // region errors as we currently don't track placeholder
+    // assumptions.
+    //
+    // To avoid being backwards incompatible with the old solver,
+    // we also eagerly normalize the where-bounds in the new solver
+    // here while ignoring region constraints. This means we can then
+    // use where-bounds whose normalization results in placeholder
+    // errors further down without getting any errors.
+    //
+    // It should be sound to do so as the only region errors here
+    // should be due to missing implied bounds.
+    //
+    // cc trait-system-refactor-initiative/issues/166.
+    let param_env = if tcx.next_trait_solver_globally() {
+        traits::deeply_normalize_param_env_ignoring_regions(tcx, param_env, normalize_cause)
+    } else {
+        traits::normalize_param_env_or_error(tcx, param_env, normalize_cause)
+    };
     debug!(caller_bounds=?param_env.caller_bounds());
 
     let infcx = &tcx.infer_ctxt().build(TypingMode::non_body_analysis());
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 6032bcacec6a4..c9f7237b123fb 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -477,6 +477,69 @@ pub fn normalize_param_env_or_error<'tcx>(
     ty::ParamEnv::new(tcx.mk_clauses(&predicates))
 }
 
+/// Deeply normalize the param env using the next solver ignoring
+/// region errors.
+///
+/// FIXME(-Zhigher-ranked-assumptions): this is a hack to work around
+/// the fact that we don't support placeholder assumptions right now
+/// and is necessary for `compare_method_predicate_entailment`, see the
+/// use of this function for more info. We should remove this once we
+/// have proper support for implied bounds on binders.
+#[instrument(level = "debug", skip(tcx))]
+pub fn deeply_normalize_param_env_ignoring_regions<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    unnormalized_env: ty::ParamEnv<'tcx>,
+    cause: ObligationCause<'tcx>,
+) -> ty::ParamEnv<'tcx> {
+    let predicates: Vec<_> =
+        util::elaborate(tcx, unnormalized_env.caller_bounds().into_iter()).collect();
+
+    debug!("normalize_param_env_or_error: elaborated-predicates={:?}", predicates);
+
+    let elaborated_env = ty::ParamEnv::new(tcx.mk_clauses(&predicates));
+    if !elaborated_env.has_aliases() {
+        return elaborated_env;
+    }
+
+    let span = cause.span;
+    let infcx = tcx
+        .infer_ctxt()
+        .with_next_trait_solver(true)
+        .ignoring_regions()
+        .build(TypingMode::non_body_analysis());
+    let predicates = match crate::solve::deeply_normalize::<_, FulfillmentError<'tcx>>(
+        infcx.at(&cause, elaborated_env),
+        predicates,
+    ) {
+        Ok(predicates) => predicates,
+        Err(errors) => {
+            infcx.err_ctxt().report_fulfillment_errors(errors);
+            // An unnormalized env is better than nothing.
+            debug!("normalize_param_env_or_error: errored resolving predicates");
+            return elaborated_env;
+        }
+    };
+
+    debug!("do_normalize_predicates: normalized predicates = {:?}", predicates);
+    // FIXME(-Zhigher-ranked-assumptions): We're ignoring region errors for now.
+    // There're placeholder constraints `leaking` out.
+    // See the fixme in the enclosing function's docs for more.
+    let _errors = infcx.resolve_regions(cause.body_id, elaborated_env, []);
+
+    let predicates = match infcx.fully_resolve(predicates) {
+        Ok(predicates) => predicates,
+        Err(fixup_err) => {
+            span_bug!(
+                span,
+                "inference variables in normalized parameter environment: {}",
+                fixup_err
+            )
+        }
+    };
+    debug!("normalize_param_env_or_error: final predicates={:?}", predicates);
+    ty::ParamEnv::new(tcx.mk_clauses(&predicates))
+}
+
 #[derive(Debug)]
 pub enum EvaluateConstErr {
     /// The constant being evaluated was either a generic parameter or inference variable, *or*,
diff --git a/tests/ui/higher-ranked/trait-bounds/issue-100689.rs b/tests/ui/higher-ranked/trait-bounds/issue-100689.rs
index f405abfb2a2ef..6bfad56b93d29 100644
--- a/tests/ui/higher-ranked/trait-bounds/issue-100689.rs
+++ b/tests/ui/higher-ranked/trait-bounds/issue-100689.rs
@@ -1,4 +1,6 @@
 //@ check-pass
+//@ revisions: old next
+//@[next] compile-flags: -Znext-solver
 
 struct Foo<'a> {
     foo: &'a mut usize,
diff --git a/tests/ui/higher-ranked/trait-bounds/issue-102899.rs b/tests/ui/higher-ranked/trait-bounds/issue-102899.rs
index b4ef75319e5c1..77d4d0179b1ba 100644
--- a/tests/ui/higher-ranked/trait-bounds/issue-102899.rs
+++ b/tests/ui/higher-ranked/trait-bounds/issue-102899.rs
@@ -1,4 +1,6 @@
 //@ check-pass
+//@ revisions: old next
+//@[next] compile-flags: -Znext-solver
 
 pub trait BufferTrait<'buffer> {
     type Subset<'channel>
diff --git a/tests/ui/traits/next-solver/normalize/deeply-normalize-env-in-compare-impl-item.rs b/tests/ui/traits/next-solver/normalize/deeply-normalize-env-in-compare-impl-item.rs
new file mode 100644
index 0000000000000..e68f82da88af8
--- /dev/null
+++ b/tests/ui/traits/next-solver/normalize/deeply-normalize-env-in-compare-impl-item.rs
@@ -0,0 +1,38 @@
+//@ check-pass
+//@ compile-flags: -Znext-solver
+
+// See trait-system-refactor-initiative/issues/166.
+// The old solver doesn't check normalization constraints in `compare_impl_item`.
+// The new solver performs lazy normalization so those region constraints may get postponed to
+// an infcx that considers regions.
+trait Trait {
+    type Assoc<'a>
+    where
+        Self: 'a;
+}
+impl<'b> Trait for &'b u32 {
+    type Assoc<'a> = &'a u32
+    where
+        Self: 'a;
+}
+
+trait Bound<T> {}
+trait Entailment<T: Trait> {
+    fn method()
+    where
+        Self: for<'a> Bound<<T as Trait>::Assoc<'a>>;
+}
+
+impl<'b, T> Entailment<&'b u32> for T {
+    // Instantiates trait where-clauses with `&'b u32` and then normalizes
+    // `T: for<'a> Bound<<&'b u32 as Trait>::Assoc<'a>>` in a separate infcx
+    // without checking region constraints.
+    //
+    // It normalizes to `T: Bound<&'a u32>`, dropping the `&'b u32: 'a` constraint.
+    fn method()
+    where
+        Self: for<'a> Bound<&'a u32>
+    {}
+}
+
+fn main() {}
diff --git a/tests/ui/traits/next-solver/normalize/normalize-param-env-2.stderr b/tests/ui/traits/next-solver/normalize/normalize-param-env-2.stderr
index d179c80596238..82a5f33628e61 100644
--- a/tests/ui/traits/next-solver/normalize/normalize-param-env-2.stderr
+++ b/tests/ui/traits/next-solver/normalize/normalize-param-env-2.stderr
@@ -1,3 +1,11 @@
+error[E0275]: overflow evaluating the requirement `<() as A<T>>::Assoc == _`
+  --> $DIR/normalize-param-env-2.rs:22:5
+   |
+LL | /     fn f()
+LL | |     where
+LL | |         Self::Assoc: A<T>,
+   | |__________________________^
+
 error[E0275]: overflow evaluating the requirement `<() as A<T>>::Assoc: A<T>`
   --> $DIR/normalize-param-env-2.rs:24:22
    |
@@ -46,6 +54,6 @@ LL |     where
 LL |         Self::Assoc: A<T>,
    |                      ^^^^ required by this bound in `A::f`
 
-error: aborting due to 5 previous errors
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0275`.