MessagingMethodInvokerHelper Error Message Improvement

The following:

@Router(inputChannel = "routingChannel")
public String route(List<?> payload) {
	if (payload.size() == 0) {
		return null;
	}
	if (payload.get(0) instanceof Integer) {
		return "integerChannel";
	}
	else {
		return "stringChannel";
	}
}

Results in:

Exception in thread "main" org.springframework.messaging.MessageHandlingException: nested exception is org.springframework.expression.AccessException: Unable to access property 'messages' through getter method
...
Caused by: java.lang.IllegalStateException: Invalid method parameter for messages: was expecting a single payload.

public String route(List<Object> payload) {
    ...
}

Works fine.

The issue is that List<?> matches dummyMessages and is interpreted to be a collection of Message<?>, so the invocation expression is #target.route(messages).

If possible, we should attempt to determine that the element is to process a simple payload; if that's not possible, we should at least log a more meaningful error, or log a warning to tell the user to use Collection<Object> instead of Collection<?>.