Declare a Map<String,Object>. When trying to put a map literal into that map, assignments in property style (map.key = ...) and subscript style (map[key] = ...) fail with
[Static type checking] - No such property: ... for class: java.lang.Object
First assigning to a local variable gives no such problems. If the Map were to contain objects of a specific type, coercion from map to that particular type would make sense. This coercion from Map to Object makes to sense. A Map is an Object and should simply become the new value for that particular key, which is just what is happening with regular compilation.
Same for Map<String,?> and Map<String,? extends Object