Type tag passed to implicit macro of contravariant type is always Any

[scabug]

Make some traits as

trait InvariantTrait[T]
trait ContravariantTrait[-T]

Then make some implicit macros that nominally create instances of those traits:

  implicit def makeInvariantTrait[T]: InvariantTrait[T] = macro makeInvariantTraitImpl[T]
  def makeInvariantTraitImpl[T](context: whitebox.Context)(implicit T: context.WeakTypeTag[T]): context.Tree = {
    import context.universe._

    context.error(context.enclosingPosition, s"$T")

    EmptyTree
  }

  implicit def makeContravariantTrait[T]: ContravariantTrait[T] = macro makeContravariantTraitImpl[T]
  def makeContravariantTraitImpl[T](context: whitebox.Context)(implicit T: context.WeakTypeTag[T]): context.Tree = {
    import context.universe._

    context.error(context.enclosingPosition, s"$T")

    EmptyTree
  }

Then use the macros

implicitly[InvariantTrait[Int]]
implicitly[ContravariantTrait[Int]]

And compile with -Xlog-implicits to see the problems.

In the invariant case, the error is

Information:(24, 13) makeInvariantTrait is not a valid implicit value for InvariantTrait[Int] because:
WeakTypeTag[Int]
  implicitly[InvariantTrait[Int]]
            ^

In the contravariant case, the error is

Information:(24, 13) makeContravariantTrait is not a valid implicit value for ContravariantTrait[Int] because:
WeakTypeTag[Any]
  implicitly[ContravariantTrait[Int]]
            ^

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-8802?orig=1
Reporter: Owen Healy (ellbur)
Affected Versions: 2.11.2

[scabug]

Owen Healy (ellbur) said:
A workaround is to declare the implicit function as

implicit def makeContravariantTrait[T]: ContravariantTrait[T] = macro makeContravariantTraitImpl[ContravariantTrait[T]]

For some reason this passes the correct type parameter, not Any.