Warn about nonsensical isInstanceOf tests with Nothing as the scrutinee type

Author: odersky

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

@@ -247,6 +247,8 @@ object TypeTestsCasts {
             if expr.tpe.isNotNull then constant(expr, Literal(Constant(true)))
             else expr.testNotNull
           else {
+            if expr.tpe.isBottomType then
+              report.warning(TypeTestAlwaysDiverges(expr.tpe, testType), tree.srcPos)
             val nestedCtx = ctx.fresh.setNewTyperState()
             val foundClsSyms = foundClasses(expr.tpe.widen, Nil)
             val sensical = checkSensical(foundClsSyms)(using nestedCtx)

tests/neg-custom-args/fatal-warnings/i14705.scala

@@ -0,0 +1,3 @@
+val n = Nil
+val b = n.head.isInstanceOf[String] // error
+