diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 0d3901e9c252..eb22e1bb682d 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -431,12 +431,8 @@ object Types {
         if (lsym isSubClass rsym) lsym
         else if (rsym isSubClass lsym) rsym
         else NoSymbol
-      case OrType(l, r) => // TODO does not conform to spec
-        val lsym = l.classSymbol
-        val rsym = r.classSymbol
-        if (lsym isSubClass rsym) rsym
-        else if (rsym isSubClass lsym) lsym
-        else NoSymbol
+      case tp: OrType =>
+        tp.join.classSymbol
       case _ =>
         NoSymbol
     }
diff --git a/tests/run/memoTest.scala b/tests/run/memoTest.scala
new file mode 100644
index 000000000000..460991eeaa59
--- /dev/null
+++ b/tests/run/memoTest.scala
@@ -0,0 +1,12 @@
+object Test extends App {
+
+  var opCache: Int | Null = null
+
+  def foo(x: Int) = {
+    if (opCache == null) opCache = x
+    opCache.asInstanceOf[Int] + 1
+  }
+
+  assert(foo(1) + foo(2) == 4)
+
+}
\ No newline at end of file