diff --git a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
index 61502cddf482..62e8c14a01d8 100644
--- a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
+++ b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
@@ -271,14 +271,18 @@ 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 {
       case p.arrayApply => MethodType(defn.IntType :: Nil, arrayElemType)
       case p.arrayUpdate => MethodType(defn.IntType :: arrayElemType :: Nil, defn.UnitType)
-      case p.arrayLength => MethodType(Nil, defn.IntType)
+      case p.arrayLength => arrayElemType; MethodType(Nil, defn.IntType)
 
       // Note that we do not need to handle calls to Array[T]#clone() specially:
       // The JLS section 10.7 says "The return type of the clone method of an array type
diff --git a/tests/neg/i4247.scala b/tests/neg/i4247.scala
new file mode 100644
index 000000000000..12e6786ea0a5
--- /dev/null
+++ b/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
+}
diff --git a/tests/neg/i4247b.scala b/tests/neg/i4247b.scala
new file mode 100644
index 000000000000..e23a5ee410e7
--- /dev/null
+++ b/tests/neg/i4247b.scala
@@ -0,0 +1,3 @@
+class Foo[U] { self : Array[U] & Nothing =>
+  self(0) = ??? // error: Array type conflict with Foo
+}
diff --git a/tests/neg/i4247c.scala b/tests/neg/i4247c.scala
new file mode 100644
index 000000000000..4fe28f7d381c
--- /dev/null
+++ b/tests/neg/i4247c.scala
@@ -0,0 +1,3 @@
+class Foo[U] { self : Array[U] & Nothing =>
+  self.length // error: Array type conflict with Foo
+}