Add facility to check Java pickling in negative test

build.sbt

@@ -51,6 +51,7 @@ lazy val root: Project = (project in file(".")).
 
 /** Scala Pickling code */
 lazy val core: Project = (project in file("core")).
+  dependsOn(testUtil % "test->test").
   settings(commonSettings: _*).
   settings(
     name := "scala-pickling",
@@ -73,6 +74,9 @@ lazy val core: Project = (project in file("core")).
     }
   )
 
+lazy val testUtil: Project = (project in file("test-util")).
+  settings(commonSettings ++ noPublish: _*)
+
 lazy val sandbox: Project = (project in file("sandbox")).
   dependsOn(core).
   settings(commonSettings ++ noPublish: _*).

core/src/test/scala/pickling/NegativeCompilation.scala

@@ -61,9 +61,12 @@ object NegativeCompilation {
   }
 
   def toolboxClasspath = {
-    val f = new java.io.File(s"core/target/scala-${scalaBinaryVersion}/classes")
-    if (!f.exists) sys.error(s"output directory ${f.getAbsolutePath} does not exist.")
-    f.getAbsolutePath
+    val f0 = new java.io.File(s"core/target/scala-${scalaBinaryVersion}/classes")
+    val f1 = new java.io.File(s"test-util/target/scala-${scalaBinaryVersion}/test-classes")
+    val fs = Vector(f0, f1)
+    fs foreach { f => if (!f.exists) sys.error(s"output directory ${f.getAbsolutePath} does not exist.") }
+    val sep = sys.props("file.separator")
+    fs.map(_.getAbsolutePath).mkString(sep)
   }
 
   def quasiquotesJar: String = {

core/src/test/scala/pickling/neg/java-field-fail.scala

@@ -5,16 +5,16 @@ import NegativeCompilation._
 import org.scalatest.FunSuite
 
 class JavaFieldFailTest extends FunSuite {
-  test("main") {
+  test("x.pickle does not compile for FakeByte") {
     expectError("Cannot generate") {
       """import _root_.scala.pickling._
         |import _root_.scala.pickling.Defaults._
         |import _root_.scala.pickling.json._
         |import _root_.scala.pickling.static._
+        |import scala.pickling.javafieldfail.FakeByte
         |
-        |val x: java.lang.Byte = 10.toByte
-        |
-        |val p = x.pickle""".stripMargin
+        |val x: FakeByte = new FakeByte(10)
+        |val pkl = x.pickle""".stripMargin
     }
   }
 }

test-util/src/test/java/pickling/FakeByte.java

@@ -0,0 +1,14 @@
+package scala.pickling.javafieldfail;
+
+public final class FakeByte {
+    private static final long serialVersionUID = 1L;
+    private final byte value;
+
+    public FakeByte(byte value) {
+        this.value = value;
+    }
+
+    public byte byteValue() {
+        return value;
+    }
+}