Fix #3340: Always throw when casting to scala.Nothing

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

@@ -570,6 +570,10 @@ class Definitions {
   lazy val ClassClass: ClassSymbol                = ctx.requiredClass("java.lang.Class")
   lazy val BoxedNumberClass: ClassSymbol          = ctx.requiredClass("java.lang.Number")
   lazy val ClassCastExceptionClass: ClassSymbol   = ctx.requiredClass("java.lang.ClassCastException")
+    lazy val ClassCastExceptionClass_stringConstructor: TermSymbol  = ClassCastExceptionClass.info.member(nme.CONSTRUCTOR).suchThat(_.info.firstParamTypes match {
+      case List(pt) => (pt isRef StringClass)
+      case _ => false
+    }).symbol.asTerm
   lazy val ArithmeticExceptionClass: ClassSymbol  = ctx.requiredClass("java.lang.ArithmeticException")
     lazy val ArithmeticExceptionClass_stringConstructor: TermSymbol  = ArithmeticExceptionClass.info.member(nme.CONSTRUCTOR).suchThat(_.info.firstParamTypes match {
       case List(pt) => (pt isRef StringClass)

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -244,6 +244,13 @@ object TypeTestsCasts {
           else if (isDerivedValueClass(testCls)) {
             expr // adaptToType in Erasure will do the necessary type adaptation
           }
+          else if (testCls eq defn.NothingClass) {
+            // In the JVM `x.asInstanceOf[Nothing]` would throw a class cast exception except when `x eq null`.
+            // To avoid this loophole we execute `x` and then regardless of the result throw a `ClassCastException`
+            val throwCCE = Throw(New(defn.ClassCastExceptionClass.typeRef, defn.ClassCastExceptionClass_stringConstructor,
+                Literal(Constant("Cannot cast to scala.Nothing")) :: Nil))
+            Block(expr :: Nil, throwCCE).withSpan(expr.span)
+          }
           else
             derivedTree(expr, defn.Any_asInstanceOf, testType)
         }

tests/pos/i828.scala

@@ -0,0 +1,3 @@
+object X {
+  val x: Int = null.asInstanceOf[Nothing]
+}

tests/run/i3340.check

@@ -0,0 +1,5 @@
+Test$.f1(i3340.scala:12)
+Test$.f2(i3340.scala:16)
+Test$.f3(i3340.scala:20)
+Test$.f4(i3340.scala:27)
+Test$.f5(i3340.scala:34)

tests/run/i3340.scala

@@ -0,0 +1,45 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    printlnStackLine(f1)
+    printlnStackLine(f2)
+    printlnStackLine(f3)
+    printlnStackLine(f4)
+    printlnStackLine(f5)
+  }
+
+  def f1: Unit = {
+    val a: Nothing =
+      null.asInstanceOf[Nothing] // throws here
+  }
+
+  def f2: Unit = {
+    null.asInstanceOf[Nothing] // throws here
+  }
+
+  def f3: Unit = {
+    null.asInstanceOf[Nothing] // throws here
+    ()
+  }
+
+
+  def f4: Unit = {
+    val n: Any = null
+    n.asInstanceOf[Nothing] // throws here
+    ()
+  }
+
+  def f5: Unit = {
+    val n: Any = null
+    val a: Nothing =
+      n.asInstanceOf[Nothing] // throws here
+    ()
+  }
+
+  def printlnStackLine(t: => Any): Unit = {
+    try t
+    catch {
+      case e: ClassCastException =>
+        println(e.getStackTrace.head)
+    }
+  }
+}

tests/run/i4410.scala

@@ -0,0 +1,7 @@
+object Test {
+  val a =
+    try null.asInstanceOf[Nothing]
+    catch { case e: ClassCastException if e.getMessage == "Cannot cast to scala.Nothing" => /* As expected */ }
+  def main(args: Array[String]): Unit = {
+  }
+}