Fix regression around type skolems and if exprs.

If we start with:

    async({
      val res = await[S[_$1 with String]](s);
      if (true)
        await[Int](0)
      res
    })

Typechecking the application (before macro expansion) yields
(where the trees are printed in the form `expr{tpe}`):

    async[S[_$1#5738 with String#137]]({
      val res: S[_$1#5490 with String] forSome { type _$1#5490 } =
        await[S[_$1#5487 with String]](
          s{S[_$1#5487 with String]}
        ){S[_$1#5487 with String]};
      if (true)
        await(0)
      else
        ()
      res{S[_$1#5738 with String]}
    }{S[_$1#5738 with String]}){S[_$1#5738 with String]}

Note that the type of the second last line contains a skolemized
symbol `_$1#5738` of the existential `_$1#5490`. This is created
by this case in `Typer#adapt`:

    case et @ ExistentialType(_, _) if ((mode & (EXPRmode | LHSmode)) == EXPRmode) =>
      adapt(tree setType et.skolemizeExistential(context.owner, tree), mode, pt, original)

Our ANF rewrites part of this code to:

  <synthetic> val await$1: S[_$1#5487 with String] = await[S[_$1#5487 with String]](awaitable$1);
  val res: S[_$1#5490 with String] forSome { type _$1 } = await$1;

And later, the state machine transformation splits the last line into
a blank field and an assignment. Typechecking the `Assign` node
led to the an type error.

This commit manually attributes the types to the `Assign` node so
as to avoid these problem.

It also reigns in an overeager rewriting of `If` nodes in the
ANF transform, which was due to a bug in the label detection
logic introduced in 4fc5463538.

Thanks to @gnovark for yet another devilish test case and
analysis of the problem with label detection.

I worked on a more principled fix on:

  https://github.com/retronym/async/compare/ticket/79-2?expand=1

in which I try to use `repackExistential` to convert skolemized
types to existentials for use as the types of synthetic vals
introduced by the ANF transform. This ran into a deeper problem
with existential subtyping in the compiler itself though.

2 files changed, 7 insertions, 4 deletions

diff --git a/src/main/scala/scala/async/internal/AsyncTransform.scala b/src/main/scala/scala/async/internal/AsyncTransform.scala
index c8d2234..a24a823 100644
--- a/src/main/scala/scala/async/internal/AsyncTransform.scala
+++ b/src/main/scala/scala/async/internal/AsyncTransform.scala
@@ -156,9 +156,12 @@ trait AsyncTransform {
         case ValDef(_, _, _, rhs) if liftedSyms(tree.symbol) =>
           atOwner(currentOwner) {
             val fieldSym = tree.symbol
-            val set = Assign(gen.mkAttributedStableRef(fieldSym.owner.thisType, fieldSym), transform(rhs))
-            changeOwner(set, tree.symbol, currentOwner)
-            localTyper.typedPos(tree.pos)(set)
+            val lhs = atPos(tree.pos) {
+              gen.mkAttributedStableRef(fieldSym.owner.thisType, fieldSym)
+            }
+            val assign = treeCopy.Assign(tree, lhs, transform(rhs)).setType(definitions.UnitTpe)
+            changeOwner(assign, tree.symbol, currentOwner)
+            assign
           }
         case _: DefTree if liftedSyms(tree.symbol)           =>
           EmptyTree
diff --git a/src/main/scala/scala/async/internal/TransformUtils.scala b/src/main/scala/scala/async/internal/TransformUtils.scala
index f228e1d..bef52f1 100644
--- a/src/main/scala/scala/async/internal/TransformUtils.scala
+++ b/src/main/scala/scala/async/internal/TransformUtils.scala
@@ -100,7 +100,7 @@ private[async] trait TransformUtils {
       case ld: LabelDef => ld.symbol
     }.toSet
     t.exists {
-      case rt: RefTree => !(labelDefs contains rt.symbol)
+      case rt: RefTree => rt.symbol != null && rt.symbol.isLabel && !(labelDefs contains rt.symbol)
       case _ => false
     }
   }