SI-5866 Support casting null to value classes

The fix now supports null.asInstanceOf[C] where C is a value class that wraps a primitive type.

src/compiler/scala/tools/nsc/transform/Erasure.scala

@@ -563,8 +563,16 @@ abstract class Erasure extends AddInterfaces
               case _ =>
                 val clazz = tref.sym
                 log("not boxed: "+tree)
-                val tree0 = adaptToType(tree, clazz.tpe)
-                cast(Apply(Select(tree0, clazz.derivedValueClassUnbox), List()), pt)
+                lazy val underlying = underlyingOfValueClass(clazz)
+                val tree0 =
+                  if (tree.tpe.typeSymbol == NullClass &&
+                      isPrimitiveValueClass(underlying.typeSymbol)) {
+                    // convert `null` directly to underlying type, as going
+                    // via the unboxed type would yield a NPE (see SI-5866)
+                    unbox1(tree, underlying)
+                  } else
+                    Apply(Select(adaptToType(tree, clazz.tpe), clazz.derivedValueClassUnbox), List())
+                cast(tree0, pt)
             }
           case _ =>
             pt.typeSymbol match {

test/files/run/t5866.check

@@ -0,0 +1,2 @@
+0.0
+Foo(0.0)

test/files/run/t5866.scala

@@ -0,0 +1,11 @@
+class Foo(val d: Double) extends AnyVal {
+  override def toString = s"Foo($d)"
+}
+object Test {
+  def main(args: Array[String]): Unit = {
+    val d: Double = null.asInstanceOf[Double]
+    println(d)
+    val f: Foo = null.asInstanceOf[Foo]
+    println(f)
+  }
+}