unique_ptr: Resolve RobotLocomotion/drake#8160

Author: EricCousineau-TRI

include/pybind11/cast.h

@@ -1721,6 +1721,24 @@ struct move_only_holder_caster : type_caster_base<type> {
     }
 
     holder_type holder;
+
+    // N.B. We have to use this weird `holder_transaction` struct because
+    // trying to introduce a destructor in `move_only_holder_caster` confuses
+    // the compiler, and makes it think it's copying the caster...
+    struct holder_transaction {
+        holder_type& holder;
+        holder_transaction() = default;
+        ~holder_transaction() {
+            if (holder) {
+                // If the argument was loaded into C++, but not transferred out,
+                // then this was most likely part of a failed overload in
+                // `argument_loader`. Transfer ownership back to Python.
+                move_only_holder_caster::cast(
+                    std::move(holder), return_value_policy{}, handle{});
+            }
+        }
+    };
+    holder_transaction transaction{holder};
 };
 
 template <typename type, typename deleter>