In this case you are declaring a new K with your method declaration. Basically the code is equivalent to this code
Now here M might be resolved to something not compatible with Number, like lets say String. But Test class can only have a type which is a Number. So you'll have to bound M to a number.
Here first of all the type declaration in the method is useless. So we'll strip it out first
Now the return type would clearly work. As for the parameter, since you are using a wildcard syntax, so compile would be able to maintain type safety here. Lets actually put some code into the class to see how
Basically you can't make changes to s variable based on its type as the type is not known. So even though ? super K seems to be breaking the bounds of K, it will still result in type safe code which is the goal of generics...