Fix scala#4247: Do not crash if self is of Array type

Author: nicolasstucki

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

@@ -271,8 +271,12 @@ trait TypeAssigner {
   def assignType(tree: untpd.Select, qual: Tree)(implicit ctx: Context): Select = {
     def qualType = qual.tpe.widen
     def arrayElemType = {
-      val JavaArrayType(elemtp) = qualType
-      elemtp
+      qualType match {
+        case JavaArrayType(elemtp) => elemtp
+        case _ =>
+          ctx.error("Array type conflict with " + qual.symbol.name, tree.pos)
+          defn.NothingType
+      }
     }
     val p = nme.primitive
     val tp = tree.name match {

tests/neg/i4247.scala

@@ -0,0 +1,3 @@
+class Foo[U] { self : Array[U] & Nothing =>
+  val s = self(0) // error: Array type conflict with Foo
+}