[backport] 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.

(cherry picked from commit cc4587b1985519f7049d0feb0783d8e22c10f792)

Conflicts:
	src/main/scala/scala/async/internal/AsyncAnalysis.scala
	src/main/scala/scala/async/internal/AsyncTransform.scala

1 files changed, 2 insertions, 3 deletions

diff --git a/src/main/scala/scala/async/internal/AsyncAnalysis.scala b/src/main/scala/scala/async/internal/AsyncAnalysis.scala
index 76c2dba..274603f 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(_)                                        =>
           abort(tree.pos, "return is illegal within a async block")
-        case ValDef(mods, _, _, _) if mods.hasFlag(Flag.LAZY) =>
-          // TODO lift this restriction
-          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")