Fix problem with comparing Null and value classes

Consider:
```scala
val x: Int = 0
val y: Int | Null = x // during erasure, x is boxed here, and Int | Null becomes Object
val z: Int = y
```
The last line should be an error. But previously
```
C | Null <: C
```
succeeded for all classes `C` since it was deemed incorrectly that `Null` derivesFrom `C`.

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -264,9 +264,13 @@ object Types {
           // If the type is `T | Null` or `T | Nothing`, the class is != Nothing,
           // and `T` derivesFrom the class, then the OrType derivesFrom the class.
           // Otherwise, we need to check both sides derivesFrom the class.
-          if tp.tp1.isBottomType && cls != defn.NothingClass then
+          def isLowerBottomType(tp: Type) =
+            tp.isBottomType
+            && (tp.hasClassSymbol(defn.NothingClass)
+                || cls != defn.NothingClass && !cls.isValueClass)
+          if isLowerBottomType(tp.tp1) then
             loop(tp.tp2)
-          else if tp.tp2.isBottomType && cls != defn.NothingClass then
+          else if isLowerBottomType(tp.tp2) then
             loop(tp.tp1)
           else
             loop(tp.tp1) && loop(tp.tp2)

tests/neg/null-anyval.scala

@@ -0,0 +1,12 @@
+object Test:
+  val x: Int = 0
+  val y: Int | Null = x // during erasure, x is boxed here, and Int | Null becomes Object
+  val z0: Int = identity(y) // error
+  val z1: Int = identity[Int |  Null](y) // error
+  val z2: Int = y // error
+
+  class StrWrapper(x: String) extends AnyVal
+  val z3: StrWrapper = null  // error
+  val z4: O.T = null // error
+object O:
+  opaque type T = String