address ambiguous auto variable. (#2481)

Signed-off-by: Tomoya Fujita <[email protected]>
Signed-off-by: Steve Nogar <[email protected]>

rclcpp/include/rclcpp/client.hpp

@@ -864,7 +864,7 @@ class Client : public ClientBase
         "Received invalid sequence number. Ignoring...");
       return std::nullopt;
     }
-    auto value = std::move(it->second.second);
+    std::optional<CallbackInfoVariant> value = std::move(it->second.second);
     this->pending_requests_.erase(request_number);
     return value;
   }