Can't use payload-expression that's a Map when the Map's key is not a String.

args[0]: Map<Enum,Object> (actually the enum key doesn't matter, the bug exists as long as the Map key isn't a String.
args[1]: Some POJO

toList: returns List<Object>

<int:gateway service-interface="com.http.test.TestService" default-request-channel="requestChannel">
<int:method name="send" payload-expression="@payloadUnifier.toList(#args[0], #args[1])" />
</int:gateway>

The issue:

GatewayMethodInboundMessageMapper:

line: else if (Map.class.isAssignableFrom(methodParameter.getParameterType()))

evaluates the Map as a header ignoring the payload-expression.