Try harder to avoid generating ClassTags for Nothing/Null subtypes

`typeSymbol` returns NoSymbol on an intersection type, so our previous check
wasn't good enough. See #1730 for background.

Author: smarter

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -34,7 +34,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
           case defn.ArrayOf(elemTp) =>
             val etag = typer.inferImplicitArg(defn.ClassTagClass.typeRef.appliedTo(elemTp), span)
             if etag.tpe.isError then EmptyTree else etag.select(nme.wrap)
-          case tp if hasStableErasure(tp) && !defn.isBottomClassAfterErasure(tp.typeSymbol) =>
+          case tp if hasStableErasure(tp) && !tp.isBottomTypeAfterErasure =>
             val sym = tp.typeSymbol
             val classTagModul = ref(defn.ClassTagModule)
             if defn.SpecialClassTagClasses.contains(sym) then

tests/neg/i1730.scala

@@ -0,0 +1,7 @@
+import scala.reflect.ClassTag
+
+@main def Test =
+  val x: Array[? <: String] = Array[Int & Nothing]() // error: No ClassTag available for Int & Nothing
+                                                     // (was: ClassCastException: [I cannot be cast to [Ljava.lang.String)
+  val y: Array[? <: Int] = Array[String & Nothing]() // error: No ClassTag available for String & Nothing
+                                                     // (was: ClassCastException: [Lscala.runtime.Nothing$; cannot be cast to [I)