Allow lazy vals without await in the initializer

We were incorrectly typechecking the `ClassDef` of the state machine
in the macro in a way that discarded the resulting trees, and only
kept around the symbol.

The led to the the macro engine retypechecking
that node, which somehow led to duplicated lazy val initiaializer
`DefDef`-s in the template, which manifest as a `VerifyError`.

This commit:

  - rescues the typechecked `ClassDef` node from the eager
    typechecking by the macro
  - loosens the restriction on lazy vals in async blocks. They are
    still prohibited if they contain an await on the RHS
  - Adds a test that shows evalution is indeed lazy.

Fixes #52

2 files changed, 12 insertions, 6 deletions

diff --git a/src/main/scala/scala/async/internal/AsyncAnalysis.scala b/src/main/scala/scala/async/internal/AsyncAnalysis.scala
index ffbc04d..7be6b43 100644
--- a/src/main/scala/scala/async/internal/AsyncAnalysis.scala
+++ b/src/main/scala/scala/async/internal/AsyncAnalysis.scala
@@ -60,9 +60,8 @@ trait AsyncAnalysis {
           super.traverse(tree)
         case Return(_)                                        =>
           c.abort(tree.pos, "return is illegal within a async block")
-        case ValDef(mods, _, _, _) if mods.hasFlag(Flag.LAZY) =>
-          // TODO lift this restriction
-          c.abort(tree.pos, "lazy vals are illegal within an async block")
+        case DefDef(mods, _, _, _, _, _) if mods.hasFlag(Flag.LAZY) && containsAwait =>
+          reportUnsupportedAwait(tree, "lazy val initalizer")
         case CaseDef(_, guard, _) if guard exists isAwait     =>
           // TODO lift this restriction
           reportUnsupportedAwait(tree, "pattern guard")
diff --git a/src/main/scala/scala/async/internal/AsyncTransform.scala b/src/main/scala/scala/async/internal/AsyncTransform.scala
index cf9dd1c..aae9dea 100644
--- a/src/main/scala/scala/async/internal/AsyncTransform.scala
+++ b/src/main/scala/scala/async/internal/AsyncTransform.scala
@@ -54,11 +54,10 @@ trait AsyncTransform {
       }
 
       val tryToUnit = appliedType(definitions.FunctionClass(1), futureSystemOps.tryType[Any], typeOf[Unit])
-      val template = Template(List(tryToUnit, typeOf[() => Unit]).map(TypeTree(_)), emptyValDef, body)
+      val template = Template(List(tryToUnit, typeOf[() => Unit]).map(TypeTree(_)), emptyValDef, body).setType(NoType)
 
       val t = ClassDef(NoMods, name.stateMachineT, Nil, template)
-      typingTransform(atPos(macroPos)(Block(t :: Nil, Literal(Constant(())))))((tree, api) => api.typecheck(tree))
-      t
+      typecheckClassDef(t)
     }
 
     val stateMachineClass = stateMachine.symbol
@@ -211,4 +210,12 @@ trait AsyncTransform {
     }
     result
   }
+
+  def typecheckClassDef(cd: ClassDef): ClassDef = {
+    val Block(cd1 :: Nil, _) = typingTransform(atPos(macroPos)(Block(cd :: Nil, Literal(Constant(())))))(
+      (tree, api) =>
+        api.typecheck(tree)
+    )
+    cd1.asInstanceOf[ClassDef]
+  }
 }