Add nullable type params test

Author: noti0na1

tests/explicit-nulls/neg/type-param.scala

@@ -0,0 +1,22 @@
+/** We used to use `T >: Null <: AnyRef` to represent a reference type,
+ *  and `T` cannot be `Nothing`. However, with explicit nulls, this definition
+ *  is no longer valid, because `Null` is not a subtype of `AnyRef`.
+ *
+ *  For example:
+ *  ```scala
+ *  def nullOf[T >: Null <: AnyRef]: T = null
+ *  ```
+ *
+ * We can modify the definition as following to allow only nullable type paramters.
+ */
+
+ def nullOf[T >: Null <: AnyRef | Null]: T = null
+
+ def f = {
+   val s1 = nullOf[String] // error: Type argument String does not conform to lower bound Null
+   val s2 = nullOf[String | Null]  // ok
+
+   val n = nullOf[Null]
+   val i = nullOf[Int] // error: Type argument Int does not conform to upper bound AnyRef | Null
+   val a = nullOf[Any] // error: Type argument Any does not conform to upper bound AnyRef | Null
+ }