SI-9442 Fix the uncurry-erasure types

Using the "uncurry-erased" type (the one after the uncurry phase) can
lead to incorrect tree transformations. For example, compiling:
```
  def foo(c: Ctx)(l: c.Tree): Unit = {
    val l2: c.Tree = l
  }
```
Results in the following AST:
```
  def foo(c: Ctx, l: Ctx#Tree): Unit = {
    val l$1: Ctx#Tree = l.asInstanceOf[Ctx#Tree]
    val l2: c.Tree = l$1 // no, not really, it's not.
  }
```
Of course, this is incorrect, since `l$1` has type `Ctx#Tree`, which is
not a subtype of `c.Tree`.

So what we need to do is to use the pre-uncurry type when creating
`l$1`, which is `c.Tree` and is correct. Now, there are two
additional problems:

1. when varargs and byname params are involved, the uncurry
   transformation desugares these special cases to actual
   typerefs, eg:
   ```
          T*  ~> Seq[T] (Scala-defined varargs)
          T*  ~> Array[T] (Java-defined varargs)
          =>T ~> Function0[T] (by name params)
   ```
   we use the DesugaredParameterType object (defined in
   scala.reflect.internal.transform.UnCurry) to redo this desugaring
   manually here
2. the type needs to be normalized, since `gen.mkCast` checks this
   (no HK here, just aliases have to be expanded before handing the
   type to `gen.mkAttributedCast`, which calls `gen.mkCast`)

2 files changed, 16 insertions, 8 deletions

diff --git a/src/reflect/scala/reflect/internal/transform/UnCurry.scala b/src/reflect/scala/reflect/internal/transform/UnCurry.scala
index abea8bed9f..85e3ac60e8 100644
--- a/src/reflect/scala/reflect/internal/transform/UnCurry.scala
+++ b/src/reflect/scala/reflect/internal/transform/UnCurry.scala
@@ -40,19 +40,27 @@ trait UnCurry {
           apply(MethodType(h.cloneSymbol.resetFlag(IMPLICIT) :: t, restpe))
         case NullaryMethodType(restpe) =>
           apply(MethodType(List(), restpe))
-        case TypeRef(pre, ByNameParamClass, arg :: Nil) =>
-          apply(functionType(List(), arg))
-        case TypeRef(pre, RepeatedParamClass, arg :: Nil) =>
-          apply(seqType(arg))
-        case TypeRef(pre, JavaRepeatedParamClass, arg :: Nil) =>
-          apply(arrayType(
-            if (isUnboundedGeneric(arg)) ObjectTpe else arg))
+        case DesugaredParameterType(desugaredTpe) =>
+          apply(desugaredTpe)
         case _ =>
           expandAlias(mapOver(tp))
       }
     }
   }
 
+  object DesugaredParameterType {
+    def unapply(tpe: Type): Option[Type] = tpe match {
+      case TypeRef(pre, ByNameParamClass, arg :: Nil) =>
+        Some(functionType(List(), arg))
+      case TypeRef(pre, RepeatedParamClass, arg :: Nil) =>
+        Some(seqType(arg))
+      case TypeRef(pre, JavaRepeatedParamClass, arg :: Nil) =>
+        Some(arrayType(if (isUnboundedGeneric(arg)) ObjectTpe else arg))
+      case _ =>
+        None
+      }
+  }
+
   private val uncurryType = new TypeMap {
     def apply(tp0: Type): Type = {
       val tp = expandAlias(tp0)
diff --git a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
index ea213cadd9..5477bdd6d4 100644
--- a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
+++ b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
@@ -444,7 +444,7 @@ trait JavaUniverseForce { self: runtime.JavaUniverse  =>
     definitions.ScalaValueClassesNoUnit
     definitions.ScalaValueClasses
 
-
+    uncurry.DesugaredParameterType
     erasure.GenericArray
     erasure.scalaErasure
     erasure.specialScalaErasure