diff --git a/compiler/rustc_typeck/src/coherence/orphan.rs b/compiler/rustc_typeck/src/coherence/orphan.rs
index 05932427bcf7a..4bff51adeeb9b 100644
--- a/compiler/rustc_typeck/src/coherence/orphan.rs
+++ b/compiler/rustc_typeck/src/coherence/orphan.rs
@@ -5,7 +5,7 @@ use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::itemlikevisit::ItemLikeVisitor;
 use rustc_infer::infer::TyCtxtInferExt;
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, fold::BottomUpFolder, TyCtxt, TypeFoldable};
 use rustc_trait_selection::traits;
 
 pub fn check(tcx: TyCtxt<'_>) {
@@ -233,13 +233,26 @@ impl ItemLikeVisitor<'v> for OrphanChecker<'tcx> {
                 }
             }
 
-            if let ty::Opaque(def_id, _) = *trait_ref.self_ty().kind() {
-                self.tcx
-                    .sess
-                    .struct_span_err(sp, "cannot implement trait on type alias impl trait")
-                    .span_note(self.tcx.def_span(def_id), "type alias impl trait defined here")
-                    .emit();
-            }
+            // Ensure no opaque types are present in this impl header. See issues #76202 and #86411 for examples,
+            // and #84660 where it would otherwise allow unsoundness.
+            trait_ref.substs.fold_with(&mut BottomUpFolder {
+                tcx: self.tcx,
+                ty_op: |ty| {
+                    if let ty::Opaque(def_id, _) = *ty.kind() {
+                        self.tcx
+                            .sess
+                            .struct_span_err(sp, "cannot implement trait on type alias impl trait")
+                            .span_note(
+                                self.tcx.def_span(def_id),
+                                "type alias impl trait defined here",
+                            )
+                            .emit();
+                    }
+                    ty
+                },
+                lt_op: |lt| lt,
+                ct_op: |ct| ct,
+            });
         }
     }
 
diff --git a/src/test/ui/impl-trait/auto-trait.full_tait.stderr b/src/test/ui/impl-trait/auto-trait.full_tait.stderr
index 314617748b0ba..5eac112c65431 100644
--- a/src/test/ui/impl-trait/auto-trait.full_tait.stderr
+++ b/src/test/ui/impl-trait/auto-trait.full_tait.stderr
@@ -16,6 +16,18 @@ LL | impl<T: Send> AnotherTrait for T {}
 LL | impl AnotherTrait for D<OpaqueType> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<impl OpaqueTrait>`
 
-error: aborting due to previous error; 1 warning emitted
+error: cannot implement trait on type alias impl trait
+  --> $DIR/auto-trait.rs:24:1
+   |
+LL | impl AnotherTrait for D<OpaqueType> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/auto-trait.rs:10:19
+   |
+LL | type OpaqueType = impl OpaqueTrait;
+   |                   ^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/auto-trait.min_tait.stderr b/src/test/ui/impl-trait/auto-trait.min_tait.stderr
index 75a5b0cb87db1..a1958cca14947 100644
--- a/src/test/ui/impl-trait/auto-trait.min_tait.stderr
+++ b/src/test/ui/impl-trait/auto-trait.min_tait.stderr
@@ -7,6 +7,18 @@ LL | impl<T: Send> AnotherTrait for T {}
 LL | impl AnotherTrait for D<OpaqueType> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<impl OpaqueTrait>`
 
-error: aborting due to previous error
+error: cannot implement trait on type alias impl trait
+  --> $DIR/auto-trait.rs:24:1
+   |
+LL | impl AnotherTrait for D<OpaqueType> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/auto-trait.rs:10:19
+   |
+LL | type OpaqueType = impl OpaqueTrait;
+   |                   ^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/auto-trait.rs b/src/test/ui/impl-trait/auto-trait.rs
index c965a34c12b32..a094c34049ef8 100644
--- a/src/test/ui/impl-trait/auto-trait.rs
+++ b/src/test/ui/impl-trait/auto-trait.rs
@@ -21,7 +21,7 @@ impl<T: Send> AnotherTrait for T {}
 // This is in error, because we cannot assume that `OpaqueType: !Send`.
 // (We treat opaque types as "foreign types" that could grow more impls
 // in the future.)
-impl AnotherTrait for D<OpaqueType> {
+impl AnotherTrait for D<OpaqueType> { //~ ERROR cannot implement trait
     //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<impl OpaqueTrait>`
 }
 
diff --git a/src/test/ui/impl-trait/negative-reasoning.full_tait.stderr b/src/test/ui/impl-trait/negative-reasoning.full_tait.stderr
index bccbc8cb36b15..2cfba380222ba 100644
--- a/src/test/ui/impl-trait/negative-reasoning.full_tait.stderr
+++ b/src/test/ui/impl-trait/negative-reasoning.full_tait.stderr
@@ -18,6 +18,18 @@ LL | impl AnotherTrait for D<OpaqueType> {
    |
    = note: upstream crates may add a new impl of trait `std::fmt::Debug` for type `impl OpaqueTrait` in future versions
 
-error: aborting due to previous error; 1 warning emitted
+error: cannot implement trait on type alias impl trait
+  --> $DIR/negative-reasoning.rs:22:1
+   |
+LL | impl AnotherTrait for D<OpaqueType> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/negative-reasoning.rs:10:19
+   |
+LL | type OpaqueType = impl OpaqueTrait;
+   |                   ^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/negative-reasoning.min_tait.stderr b/src/test/ui/impl-trait/negative-reasoning.min_tait.stderr
index 5727a372ddbe8..ed778b99c813c 100644
--- a/src/test/ui/impl-trait/negative-reasoning.min_tait.stderr
+++ b/src/test/ui/impl-trait/negative-reasoning.min_tait.stderr
@@ -9,6 +9,18 @@ LL | impl AnotherTrait for D<OpaqueType> {
    |
    = note: upstream crates may add a new impl of trait `std::fmt::Debug` for type `impl OpaqueTrait` in future versions
 
-error: aborting due to previous error
+error: cannot implement trait on type alias impl trait
+  --> $DIR/negative-reasoning.rs:22:1
+   |
+LL | impl AnotherTrait for D<OpaqueType> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/negative-reasoning.rs:10:19
+   |
+LL | type OpaqueType = impl OpaqueTrait;
+   |                   ^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/impl-trait/negative-reasoning.rs b/src/test/ui/impl-trait/negative-reasoning.rs
index 7f608cc99db15..21c2b212a3676 100644
--- a/src/test/ui/impl-trait/negative-reasoning.rs
+++ b/src/test/ui/impl-trait/negative-reasoning.rs
@@ -19,7 +19,7 @@ trait AnotherTrait {}
 impl<T: std::fmt::Debug> AnotherTrait for T {}
 
 // This is in error, because we cannot assume that `OpaqueType: !Debug`
-impl AnotherTrait for D<OpaqueType> {
+impl AnotherTrait for D<OpaqueType> { //~ ERROR cannot implement trait
     //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<impl OpaqueTrait>`
 }
 
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs
new file mode 100644
index 0000000000000..1a0acca7881d1
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs
@@ -0,0 +1,23 @@
+// Regression test for issues #84660 and #86411: both are variations on #76202.
+// Tests that we don't ICE when we have an opaque type appearing anywhere in an impl header.
+
+#![feature(min_type_alias_impl_trait)]
+
+trait Foo {}
+impl Foo for () {}
+type Bar = impl Foo;
+fn _defining_use() -> Bar {}
+
+trait TraitArg<T> {
+    fn f();
+}
+
+impl TraitArg<Bar> for () { //~ ERROR cannot implement trait
+    fn f() {
+        println!("ho");
+    }
+}
+
+fn main() {
+    <() as TraitArg<Bar>>::f();
+}
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr
new file mode 100644
index 0000000000000..1b8eee407178c
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr
@@ -0,0 +1,14 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/issue-84660-trait-impl-for-tait.rs:15:1
+   |
+LL | impl TraitArg<Bar> for () {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/issue-84660-trait-impl-for-tait.rs:8:12
+   |
+LL | type Bar = impl Foo;
+   |            ^^^^^^^^
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs
new file mode 100644
index 0000000000000..18213dfbe5bf7
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs
@@ -0,0 +1,41 @@
+// Another example from issue #84660, this time weaponized as a safe transmut: an opaque type in an
+// impl header being accepted was used to create unsoundness.
+
+#![feature(min_type_alias_impl_trait)]
+
+trait Foo {}
+impl Foo for () {}
+type Bar = impl Foo;
+fn _defining_use() -> Bar {}
+
+trait Trait<T, In> {
+    type Out;
+    fn convert(i: In) -> Self::Out;
+}
+
+impl<In, Out> Trait<Bar, In> for Out { //~ ERROR cannot implement trait
+    type Out = Out;
+    fn convert(_i: In) -> Self::Out {
+        unreachable!();
+    }
+}
+
+impl<In, Out> Trait<(), In> for Out {
+    type Out = In;
+    fn convert(i: In) -> Self::Out {
+        i
+    }
+}
+
+fn transmute<In, Out>(i: In) -> Out {
+    <Out as Trait<Bar, In>>::convert(i)
+}
+
+fn main() {
+    let d;
+    {
+        let x = "Hello World".to_string();
+        d = transmute::<&String, &String>(&x);
+    }
+    println!("{}", d);
+}
diff --git a/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr
new file mode 100644
index 0000000000000..b1128d830f967
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr
@@ -0,0 +1,14 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/issue-84660-unsoundness.rs:16:1
+   |
+LL | impl<In, Out> Trait<Bar, In> for Out {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/issue-84660-unsoundness.rs:8:12
+   |
+LL | type Bar = impl Foo;
+   |            ^^^^^^^^
+
+error: aborting due to previous error
+