score:1

As said in comments, this solution doesn't work

If I understood your problem well, you can also assign default values, as explained in details in what does it mean assign "_" to a field in scala?. You can find more infos in 4.2 Variable Declarations and Definitions of the The Scala Language Specification

So you can simply do:

def unwrap[T](iface: Class[T]): T = _

which will set unwrap with null without the type mismatch.

score:6

The "correct" solution is to do something which will immediately fail. Like so:

def unwrap[T](iface: Class[T]): T = sys.error("unimplemented")

In scala 2.10, this would have been implemented as:

def unwrap[T](iface: Class[T]): T = ???

Because there is a new method in Predef called ???. This works because an expression of the form throw new Exception has the type Nothing, which is a subtype of any type (it is called bottom in type-theoretical circles).

The reason that this is correct is that it is much better to fail instantly with an error, rather than use a null which may fail later and obfuscate the cause.

score:26

This is because T could be a non-nullable type. It works when you enforce T to be a nullable type:

def unwrap[T >: Null](iface: Class[T]): T = null

unwrap(classOf[String]) // compiles

unwrap(classOf[Int]) // does not compile, because Int is not nullable

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