Check for opaque types behind projections

compiler/rustc_typeck/src/coherence/orphan.rs

@@ -52,12 +52,14 @@ fn do_orphan_check_impl<'tcx>(
 
     // 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.
-    if trait_ref.has_opaque_types() {
+    if trait_ref.has_opaque_types() || trait_ref.has_projections() {
+        let param_env = tcx.param_env(def_id);
         trace!("{:#?}", item);
         // First we find the opaque type in question.
         for ty in trait_ref.substs {
             for ty in ty.walk() {
                 let ty::subst::GenericArgKind::Type(ty) = ty.unpack() else { continue };
+                let ty = tcx.try_normalize_erasing_regions(param_env, ty).unwrap_or(ty);
                 let ty::Opaque(def_id, _) = *ty.kind() else { continue };
                 trace!(?def_id);
 
@@ -99,7 +101,6 @@ fn do_orphan_check_impl<'tcx>(
                 return Err(reported);
             }
         }
-        span_bug!(sp, "opaque type not found, but `has_opaque_types` is set")
     }
 
     match traits::orphan_check(tcx, item.def_id.to_def_id()) {

src/test/ui/type-alias-impl-trait/impl_trait_for_tait.rs

@@ -0,0 +1,21 @@
+// compile-flags: --crate-type=lib
+
+#![feature(type_alias_impl_trait)]
+type Alias = impl Sized;
+
+fn constrain() -> Alias {
+    1i32
+}
+
+trait HideIt {
+    type Assoc;
+}
+
+impl HideIt for () {
+    type Assoc = Alias;
+}
+
+pub trait Yay {}
+
+impl Yay for <() as HideIt>::Assoc {}
+//~^ ERROR: cannot implement trait on type alias impl trait

src/test/ui/type-alias-impl-trait/impl_trait_for_tait.stderr

@@ -0,0 +1,14 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/impl_trait_for_tait.rs:20:1
+   |
+LL | impl Yay for <() as HideIt>::Assoc {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/impl_trait_for_tait.rs:4:14
+   |
+LL | type Alias = impl Sized;
+   |              ^^^^^^^^^^
+
+error: aborting due to previous error
+