diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 6cc439e9113ea..3802be1c9e143 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -4365,10 +4365,7 @@ ERROR(distributed_actor_protocol_illegal_inheritance,none,
       "non-distributed actor type %0 cannot conform to the 'DistributedActor' protocol",
       (DeclName))
 
-// FIXME: This diagnostic was temporarily downgraded from an error because
-// it spuriously triggers when building the Foundation module from its textual
-// swiftinterface. (rdar://78932296)
-WARNING(unowned_executor_outside_actor,none,
+ERROR(unowned_executor_outside_actor,none,
       "'unownedExecutor' can only be implemented within the main "
       "definition of an actor", ())
 ERROR(override_implicit_unowned_executor,none,
diff --git a/lib/Sema/TypeCheckProtocol.cpp b/lib/Sema/TypeCheckProtocol.cpp
index 3475ed6710089..7270985fe8d25 100644
--- a/lib/Sema/TypeCheckProtocol.cpp
+++ b/lib/Sema/TypeCheckProtocol.cpp
@@ -4657,10 +4657,7 @@ void ConformanceChecker::resolveValueWitnesses() {
           Adoptee->getClassOrBoundGenericClass() &&
           Adoptee->getClassOrBoundGenericClass()->isActor()) {
         witness->diagnose(diag::unowned_executor_outside_actor);
-        // FIXME: This diagnostic was temporarily downgraded from an error to a
-        // warning because it spuriously triggers when building the Foundation
-        // module from its textual swiftinterface. (rdar://78932296)
-        //return;
+        return;
       }
 
       // Objective-C checking for @objc requirements.