.

upickle/core/src/upickle/core/Types.scala

@@ -32,6 +32,10 @@ trait Types{ types =>
       extends Visitor.Delegate[Any, T](other) with ReadWriter[T]
 
     def merge[T](tagKey: String, rws: ReadWriter[_ <: T]*): TaggedReadWriter[T] = {
+      assert(
+        rws.forall(_.isInstanceOf[TaggedReadWriter[_]]),
+        "Can only merge ReadWriters of case classes, not " + rws.filterNot(_.isInstanceOf[TaggedReadWriter[_]])
+      )
       new TaggedReadWriter.Node(tagKey, rws.asInstanceOf[Seq[TaggedReadWriter[T]]]:_*)
     }
 
@@ -108,6 +112,10 @@ trait Types{ types =>
       override def visitArray(length: Int, index: Int) = super.visitArray(length, index).asInstanceOf[ArrVisitor[Any, Z]]
     }
     def merge[T](tagKey: String, readers0: Reader[_ <: T]*): TaggedReader.Node[T] = {
+      assert(
+        readers0.forall(_.isInstanceOf[TaggedReader[_]]),
+        "Can only merge Readers of case classes, not " + readers0.filterNot(_.isInstanceOf[TaggedReader[_]])
+      )
       new TaggedReader.Node(tagKey, readers0.asInstanceOf[Seq[TaggedReader[T]]]:_*)
     }
 
@@ -147,6 +155,10 @@ trait Types{ types =>
       def write0[R](out: Visitor[_, R], v: U): R = src.write(out, f(v))
     }
     def merge[T](writers: Writer[_ <: T]*) = {
+      assert(
+        writers.forall(_.isInstanceOf[TaggedWriter[_]]),
+        "Can only merge Writers of case classes, not " + writers.filterNot(_.isInstanceOf[TaggedWriter[_]])
+      )
       new TaggedWriter.Node(writers.asInstanceOf[Seq[TaggedWriter[T]]]:_*)
     }
   }

upickle/test/src/upickle/FailureTests.scala

@@ -37,6 +37,26 @@ object WrongTag {
 
 }
 
+object TaggedCustomSerializer{
+
+  sealed trait BooleanOrInt
+
+  object BooleanOrInt {
+    implicit val rw: upickle.default.ReadWriter[BooleanOrInt] = upickle.default.macroRW
+  }
+
+  case class IsBoolean(val value: Boolean) extends BooleanOrInt
+
+  object IsBoolean {
+    implicit val rw: upickle.default.ReadWriter[IsBoolean] = upickle.default.readwriter[Boolean].bimap[IsBoolean](_.value, IsBoolean(_))
+  }
+
+  case class IsInt(val value: Int) extends BooleanOrInt
+
+  object IsInt {
+    implicit val rw: upickle.default.ReadWriter[IsInt] = upickle.default.readwriter[Int].bimap[IsInt](_.value, IsInt(_))
+  }
+}
 /**
 * Generally, every failure should be a Invalid.Json or a
 * InvalidData. If any assertion errors, match errors, number
@@ -273,5 +293,15 @@ object FailureTests extends TestSuite {
 //      upickle.default.read[Int]("10e-1") ==> 1
 //      upickle.default.read[Long]("10e-1") ==> 1
     }
+    test("taggedCustomSerializer"){
+      import upickle.default._
+
+      val error = intercept[java.lang.AssertionError] {
+        val x: TaggedCustomSerializer.BooleanOrInt = TaggedCustomSerializer.IsBoolean(false);
+        upickle.default.write(x)
+      }
+
+      assert(error.getMessage.startsWith("assertion failed: Can only merge Readers of case classes"))
+    }
   }
 }