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 4fc5463.

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.

Author: retronym

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

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
     }
   }

src/test/scala/scala/async/run/ifelse4/IfElse4.scala

@@ -0,0 +1,63 @@
+/*
+ * Copyright (C) 2012-2014 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package scala.async
+package run
+package ifelse4
+
+import language.{reflectiveCalls, postfixOps}
+import scala.concurrent.{Future, ExecutionContext, future, Await}
+import scala.concurrent.duration._
+import scala.async.Async.{async, await}
+import org.junit.Test
+
+
+class TestIfElse4Class {
+
+  import ExecutionContext.Implicits.global
+  
+  class F[A]
+  class S[A](val id: String)
+  trait P
+ 
+  case class K(f: F[_])
+
+  def result[A](f: F[A]) = async {
+    new S[A with P]("foo")
+  }
+    
+  def run(k: K) = async {
+    val res = await(result(k.f))
+    // these triggered a crash with mismatched existential skolems
+    //  found   : S#10272[_$1#10308 with String#137] where type _$1#10308
+    //  required: S#10272[_$1#10311 with String#137] forSome { type _$1#10311 }
+
+    // This variation of the crash could be avoided by fixing the over-eager
+    // generation of states in `If` nodes, which was caused by a bug in label
+    // detection code.
+    if(true) {
+      identity(res)
+    }
+
+    // This variation remained after the aforementioned fix, however.
+    // It was fixed by manually typing the `Assign(liftedField, rhs)` AST,
+    // which is how we avoid these problems through the rest of the ANF transform.
+    if(true) {
+      identity(res)
+      await(result(k.f))
+    }
+    res
+  }
+}
+
+class IfElse4Spec {
+
+  @Test
+  def `await result with complex type containing skolem`() {
+    val o = new TestIfElse4Class
+    val fut = o.run(new o.K(null))
+    val res = Await.result(fut, 2 seconds)
+    res.id mustBe ("foo")
+  }
+}