How to prevent compiler from choosing the least generic type?

问题

I have a method that looks up some storage for an instance of a particular class:

def lookup[T >: Null: ClassTag]: T = {
  // Print what class tag we got:
  System.out.println(implicitly[ClassTag[T]].toString);
  null; // details omitted, just return null
}

It works well, but the problem is that if I don't provide an explicit type, the compiler chooses Null for T, and of course it doesn't work then:

def print(msg: String) = { /* ... */ }

print(lookup);

prints Null and of course nothing is found. Clearly the compiler infers the least generic type possible.

If I add an explicit type like

print(lookup[String]);

it works fine. But this is very error-prone. I'd like to either:

1. Let the compiler to always choose the most generic possible type, instead of the least generic possible one. So in print(lookup) the most generic possible type is String, so I'd like the compiler to infer String for T. Or
2. Force somehow that an explicit type is always present and issue a compile-time error when I write just something like print(lookup).

Is any of this possible?

回答1:

1) Most generic type is AnyRef.

2) You could use =!= from this answer:

scala> def lookup[T >: Null: ClassTag](implicit guard: T =!= Null ): T = {
     |   null; // details omitted, just return null
     | }
lookup: [T >: Null](implicit evidence$1: scala.reflect.ClassTag[T], implicit guard: =!=[T,Null])T

scala> lookup
<console>:11: error: ambiguous implicit values:
 both method equal in trait LowerPriorityImplicits of type [A]=> =!=[A,A]
 and method nequal in object =!= of type [A, B](implicit same: =:=[A,B])=!=[A,B]
 match expected type =!=[Null,Null]
              lookup
              ^

scala> lookup[String]
res3: String = null

来源：https://stackoverflow.com/questions/16425502/how-to-prevent-compiler-from-choosing-the-least-generic-type