codifies the state of the art wrt SI-8104

As it was discovered in SI-8104, whiteboxity doesn’t apply equally to
type parameters and type members of materialized type classes.

During implicit search and subsequent type inference, whitebox type parameters
are consistently erased to wildcards, whereas whitebox type members sometimes
remain as is and get in the way of signature conformance checks.

test/files/neg/t8104.check

@@ -0,0 +1,4 @@
+Test_2.scala:20: error: could not find implicit value for parameter e: Generic.Aux[Test.C,(Int, Int)]
+  implicitly[Generic.Aux[C, (Int, Int)]]
+            ^
+one error found

test/files/neg/t8104/Macros_1.scala

@@ -0,0 +1,11 @@
+import scala.reflect.macros.WhiteboxContext
+
+object Macros {
+  def impl[T](c: WhiteboxContext)(implicit T: c.WeakTypeTag[T]) = {
+    import c.universe._
+    import definitions._
+    val fields = T.tpe.declarations.toList.collect{ case x: TermSymbol if x.isVal && x.isCaseAccessor => x }
+    val Repr = appliedType(TupleClass(fields.length).asType.toType, fields.map(_.typeSignature))
+    q"new Generic[$T]{ type Repr = $Repr }"
+  }
+}

test/files/neg/t8104/Test_2.scala

@@ -0,0 +1,21 @@
+trait Generic[T] { type Repr }
+object Generic {
+  type Aux[T, Repr0] = Generic[T] { type Repr = Repr0 }
+  import scala.language.experimental.macros
+  implicit def materializeGeneric[T]: Generic[T] = macro Macros.impl[T]
+}
+
+object Test extends App {
+  case class C(x: Int, y: Int)
+
+  import scala.reflect.runtime.universe._
+  def reprify[T, Repr](x: T)(implicit generic: Generic.Aux[T, Repr], tag: TypeTag[Repr]) = println(tag)
+  reprify(C(40, 2))
+
+  // this is a compilation error at the moment as explained in SI-8104
+  // because matchesPt in implicit search says that depoly(<type of materializeGeneric>) isn't a subtype of Generic.Aux[C, (Int, Int)]
+  // which is rightfully so, because depoly only replaces type parameters, not type members with wildcard types
+  // however in the future we might want to relax the matchesPt check, so this might start compiling
+  // therefore, if you've broken this test, then you should be happy, because most likely you've just enabled an interesting use case!
+  implicitly[Generic.Aux[C, (Int, Int)]]
+}

test/files/run/t8104.check

@@ -0,0 +1 @@
+TypeTag[(Int, Int)]

test/files/run/t8104/Macros_1.scala

@@ -0,0 +1,11 @@
+import scala.reflect.macros.WhiteboxContext
+
+object Macros {
+  def impl[T](c: WhiteboxContext)(implicit T: c.WeakTypeTag[T]) = {
+    import c.universe._
+    import definitions._
+    val fields = T.tpe.declarations.toList.collect{ case x: TermSymbol if x.isVal && x.isCaseAccessor => x }
+    val Repr = appliedType(TupleClass(fields.length).asType.toType, fields.map(_.typeSignature))
+    q"new Generic[$T]{ type Repr = $Repr }"
+  }
+}

test/files/run/t8104/Test_2.scala

@@ -0,0 +1,16 @@
+trait Generic[T] { type Repr }
+object Generic {
+  type Aux[T, Repr0] = Generic[T] { type Repr = Repr0 }
+  import scala.language.experimental.macros
+  implicit def materializeGeneric[T, Repr]: Generic.Aux[T, Repr] = macro Macros.impl[T]
+}
+
+object Test extends App {
+  case class C(x: Int, y: Int)
+
+  import scala.reflect.runtime.universe._
+  def reprify[T, Repr](x: T)(implicit generic: Generic.Aux[T, Repr], tag: TypeTag[Repr]) = println(tag)
+  reprify(C(40, 2))
+
+  implicitly[Generic.Aux[C, (Int, Int)]]
+}