start on an upgrade test

compiler/src/test/scala/org/capnproto/EncodingTest.scala

@@ -68,6 +68,28 @@ class EncodingSuite extends FunSuite {
     }
   }
 
+  test("UpgradeStructInBuilder") {
+    val builder = new MessageBuilder();
+    val root = builder.initRoot(TestAnyPointer.factory);
+
+
+    val oldReader = {
+      val oldVersion = root.getAnyPointerField().initAsStruct(TestOldVersion.factory);
+      oldVersion.setOld1(123);
+      oldVersion.setOld2("foo");
+      val sub = oldVersion.initOld3();
+      sub.setOld1(456);
+      sub.setOld2("bar");
+      oldVersion
+    }
+
+    {
+      //val newVersion = root.getAnyPointerField().getAsStruct(TestNewVersion.factory);
+    }
+
+    //...
+  }
+
   test("Constants") {
     assert(Void.VOID == TestConstants.VOID_CONST);
     assert(true == TestConstants.BOOL_CONST);

compiler/src/test/schema/test.capnp

@@ -192,6 +192,25 @@ struct TestGroups {
   }
 }
 
+
+struct TestOldVersion {
+  # A subset of TestNewVersion.
+  old1 @0 :Int64;
+  old2 @1 :Text;
+  old3 @2 :TestOldVersion;
+}
+
+struct TestNewVersion {
+  # A superset of TestOldVersion.
+  old1 @0 :Int64;
+  old2 @1 :Text;
+  old3 @2 :TestNewVersion;
+  new1 @3 :Int64 = 987;
+  new2 @4 :Text = "baz";
+}
+
+struct TestEmptyStruct {}
+
 struct TestConstants {
   const voidConst      :Void    = void;
   const boolConst      :Bool    = true;