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
null without the type mismatch.
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
???. 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.
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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