shapeless - In scala 2.13, why sometimes TypeTags cannot be inferred? And how to construct one from a variable symbol?

Question

Here is a simple example in shapeless:

  it("from Witness") {

    val ttg = implicitly[TypeTag[Witness.Lt[String]]]
    val ctg = implicitly[ClassTag[Witness.Lt[String]]]
  }

  it("... from macro") {

    val ttg = implicitly[TypeTag[Witness.`1`.T]]
    val ctg = implicitly[ClassTag[Witness.`1`.T]]
  }

  it("... doesn't work") {

    val ttg = implicitly[TypeTag[w1.T]] // failed!
    val ctg = implicitly[ClassTag[w2.T]]
  }

The second and third it block have very similar bytecode (after the Witness.? macro has been invoked), yet one succeed and one failed:

[Error] /home/peng/git-spike/scalaspike/common/src/test/scala/com/tribbloids/spike/scala_spike/Reflection/InferTypeTag.scala:55: No TypeTag available for com.tribbloids.spike.scala_spike.Reflection.InferTypeTag.w1.T

What could have caused this? And how do I circumvent this problem?

Answer 1

If you switch on scalacOptions += "-Xlog-implicits" you'll see

val w1 = Witness(1)
val ttg = implicitly[TypeTag[w1.T]] // doesn't compile

//materializing requested reflect.runtime.universe.type.TypeTag[App.w1.T] using scala.reflect.api.`package`.materializeTypeTag[App.w1.T](scala.reflect.runtime.`package`.universe)

//scala.reflect.api.`package`.materializeTypeTag[App.w1.T](scala.reflect.runtime.`package`.universe) is not a valid implicit value for reflect.runtime.universe.TypeTag[App.w1.T] because:
//failed to typecheck the materialized tag: 
//cannot create a TypeTag referring to type shapeless.Witness.<refinement>.T local to the reifee: use WeakTypeTag instead

//No TypeTag available for App.w1.T

So try to use WeakTypeTag as recommended

val w1 = Witness(1)
val ttg2 = implicitly[WeakTypeTag[w1.T]] // compiles

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