diff --git a/compiler/rustc_borrowck/messages.ftl b/compiler/rustc_borrowck/messages.ftl
index 587536e1f9a3b..9f248cb30682c 100644
--- a/compiler/rustc_borrowck/messages.ftl
+++ b/compiler/rustc_borrowck/messages.ftl
@@ -132,6 +132,10 @@ borrowck_moved_due_to_usage_in_operator =
         *[false] operator
     }
 
+borrowck_must_define_opaque =
+    method must define opaque type
+    .label = because it shows up in the args of this method
+
 borrowck_opaque_type_lifetime_mismatch =
     opaque type used twice with different lifetimes
     .label = lifetime `{$arg}` used here
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index d5875a226fe02..3596c8c0fb7aa 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -16,8 +16,9 @@ use rustc_span::Span;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
 use rustc_trait_selection::traits::ObligationCtxt;
 
-use crate::session_diagnostics::LifetimeMismatchOpaqueParam;
-use crate::session_diagnostics::NonGenericOpaqueTypeParam;
+use crate::session_diagnostics::{
+    LifetimeMismatchOpaqueParam, MustDefineOpaque, NonGenericOpaqueTypeParam,
+};
 
 use super::RegionInferenceContext;
 
@@ -42,12 +43,15 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// Check that all opaque types have the same region parameters if they have the same
     /// non-region parameters. This is necessary because within the new solver we perform various query operations
     /// modulo regions, and thus could unsoundly select some impls that don't hold.
-    fn check_unique(
+    fn check_unique_and_defined(
         &self,
         infcx: &InferCtxt<'tcx>,
+        anchor: LocalDefId,
         opaque_ty_decls: &FxIndexMap<OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>>,
     ) {
+        let mut seen = FxIndexSet::default();
         for (i, (a, a_ty)) in opaque_ty_decls.iter().enumerate() {
+            seen.insert(a.def_id);
             for (b, b_ty) in opaque_ty_decls.iter().skip(i + 1) {
                 if a.def_id != b.def_id {
                     continue;
@@ -78,6 +82,24 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 }
             }
         }
+
+        if !infcx.tcx.is_typeck_child(anchor.to_def_id()) {
+            // Check that all the in-scope ATPITs are defined by the anchor
+            for def_id in infcx.tcx.opaque_types_defined_by(anchor) {
+                if seen.contains(&def_id) {
+                    continue;
+                }
+                match infcx.tcx.opaque_type_origin(def_id) {
+                    OpaqueTyOrigin::TyAlias { in_assoc_ty: true, .. } => {
+                        infcx.dcx().emit_err(MustDefineOpaque {
+                            span: infcx.tcx.def_span(def_id),
+                            item_span: infcx.tcx.def_span(anchor),
+                        });
+                    }
+                    _ => {}
+                }
+            }
+        }
     }
 
     /// Resolve any opaque types that were encountered while borrow checking
@@ -125,7 +147,11 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         infcx: &InferCtxt<'tcx>,
         opaque_ty_decls: FxIndexMap<OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>>,
     ) -> FxIndexMap<LocalDefId, OpaqueHiddenType<'tcx>> {
-        self.check_unique(infcx, &opaque_ty_decls);
+        self.check_unique_and_defined(
+            infcx,
+            self.universal_regions().defining_ty.def_id().expect_local(),
+            &opaque_ty_decls,
+        );
 
         let mut result: FxIndexMap<LocalDefId, OpaqueHiddenType<'tcx>> = FxIndexMap::default();
 
diff --git a/compiler/rustc_borrowck/src/session_diagnostics.rs b/compiler/rustc_borrowck/src/session_diagnostics.rs
index 40c2ef1c91e14..09059c32941ed 100644
--- a/compiler/rustc_borrowck/src/session_diagnostics.rs
+++ b/compiler/rustc_borrowck/src/session_diagnostics.rs
@@ -304,6 +304,15 @@ pub(crate) struct NonGenericOpaqueTypeParam<'a, 'tcx> {
     pub param_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(borrowck_must_define_opaque)]
+pub(crate) struct MustDefineOpaque {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub item_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(borrowck_opaque_type_lifetime_mismatch)]
 pub(crate) struct LifetimeMismatchOpaqueParam<'tcx> {
diff --git a/tests/ui/generic-associated-types/issue-87258_a.stderr b/tests/ui/generic-associated-types/issue-87258_a.stderr
index 01f2a92f94a6b..d79754ad6a561 100644
--- a/tests/ui/generic-associated-types/issue-87258_a.stderr
+++ b/tests/ui/generic-associated-types/issue-87258_a.stderr
@@ -1,3 +1,12 @@
+error: method must define opaque type
+  --> $DIR/issue-87258_a.rs:17:26
+   |
+LL |     type FooFuture<'a> = impl Trait1;
+   |                          ^^^^^^^^^^^
+LL |
+LL |     fn foo<'a>() -> Self::FooFuture<'a> {
+   |     ----------------------------------- because it shows up in the args of this method
+
 error: unconstrained opaque type
   --> $DIR/issue-87258_a.rs:17:26
    |
@@ -6,5 +15,5 @@ LL |     type FooFuture<'a> = impl Trait1;
    |
    = note: `FooFuture` must be used in combination with a concrete type within the same impl
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
diff --git a/tests/ui/type-alias-impl-trait/in-assoc-ty-early-bound2.stderr b/tests/ui/type-alias-impl-trait/in-assoc-ty-early-bound2.stderr
index 1274a8b60dea0..a02dff11c99ce 100644
--- a/tests/ui/type-alias-impl-trait/in-assoc-ty-early-bound2.stderr
+++ b/tests/ui/type-alias-impl-trait/in-assoc-ty-early-bound2.stderr
@@ -9,6 +9,16 @@ LL |     fn bar<'a: 'a>()
 LL |             let _: Self::Assoc<'a> = x;
    |                    ^^^^^^^^^^^^^^^
 
+error: method must define opaque type
+  --> $DIR/in-assoc-ty-early-bound2.rs:9:22
+   |
+LL |       type Assoc<'a> = impl Sized;
+   |                        ^^^^^^^^^^
+LL | /     fn bar<'a: 'a>()
+LL | |     where
+LL | |         Self::Assoc<'a>:,
+   | |_________________________- because it shows up in the args of this method
+
 error: unconstrained opaque type
   --> $DIR/in-assoc-ty-early-bound2.rs:9:22
    |
@@ -17,6 +27,6 @@ LL |     type Assoc<'a> = impl Sized;
    |
    = note: `Assoc` must be used in combination with a concrete type within the same impl
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0700`.