Detect explicitly spliced wildcard types

[nicolasstucki]

We are not allowed to splice wildcard types to avoid soundness issues. They are not checked when explicitly spliced:

import scala.quoted._
object Macro {
  def impl[A : Type](using qctx: QuoteContext): Unit = {
    import qctx.tasty._
    val tpe = typeOf[A].seal.asInstanceOf[quoted.Type[_ <: AnyRef]]
    '{ (a: ${tpe}) => ???} // should be an error
  }
}

import scala.quoted._
object Macro {
  def impl[A : Type](using qctx: QuoteContext): Unit = {
    import qctx.tasty._
    val tpe/*: quoted.Type[? <: AnyKind]*/ = typeOf[A].seal
    '{ f[$tpe] } // should be an error
  }
  def f[T <: AnyKind]: Unit = ()
}

[nicolasstucki]

See #8865 (comment)

The way to make this work is by creating an existential type X for the type such that tpe is typed as quoted.Type[X]

We have logic in the compiler to do this sort of things automatically for wildcard types it's called skolemization, maybe this could be extended to deal with quotes: https://github.com/lampepfl/dotty/blob/5d6d6aa3797d000800d68f027574aeaee47aa3c2/compiler/src/dotty/tools/dotc/typer/Typer.scala#L3211-L3216