Properly find constructors needed by the compiler

The compiler sometimes needs to invoke `ClassCastExceptionClass_stringConstructor`
in generated code. Handle this properly in the presence of explicit
nulls.

Author: abeln

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

@@ -660,12 +660,16 @@ class Definitions {
   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 List(pt) =>
+        val pt1 = if (ctx.settings.YexplicitNulls.value) pt.stripNull else pt
+        pt1 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)
+      case List(pt) =>
+        val pt1 = if (ctx.settings.YexplicitNulls.value) pt.stripNull else pt
+        pt1 isRef StringClass
       case _ => false
     }).symbol.asTerm
 

tests/explicit-nulls/run/instanceof-nothing.scala

@@ -0,0 +1,25 @@
+// Check that calling `asInstanceOf[Nothing]` throws a ClassCastException.
+// In particular, the compiler needs access to the right method to throw
+// the exception, and identifying the method uses some explicit nulls related
+// logic (see ClassCastExceptionClass in Definitions.scala).
+object Test {
+  def main(args: Array[String]): Unit = {
+    val x: String = "hello"
+    try {
+      val y: Nothing = x.asInstanceOf[Nothing]
+      assert(false)
+    } catch {
+      case e: ClassCastException =>
+        // ok
+    }
+
+    val n: Null = null
+    try {
+      val y: Nothing = n.asInstanceOf[Nothing]
+      assert(false)
+    } catch {
+      case e: ClassCastException =>
+        // ok
+    }
+  }
+}