Make async blackbox / throw a meaningful exception from await

src/main/scala/scala/async/Async.scala

@@ -15,7 +15,7 @@ package scala.async
 import scala.language.experimental.macros
 import scala.concurrent.{ExecutionContext, Future}
 import scala.annotation.compileTimeOnly
-import scala.reflect.macros.whitebox
+import scala.reflect.macros.blackbox
 
 /**
  * Async blocks provide a direct means to work with [[scala.concurrent.Future]].
@@ -60,11 +60,9 @@ object Async {
    * in the `onComplete` handler of `awaitable`, and will *not* block a thread.
    */
   @compileTimeOnly("[async] `await` must be enclosed in an `async` block")
-  def await[T](awaitable: Future[T]): T = ??? // No implementation here, as calls to this are translated to `onComplete` by the macro.
+  def await[T](awaitable: Future[T]): T = throw new UnsupportedOperationException("Calls to Async.await should have been treated as compiler intrinsics and rewritten!")
 
-  def asyncImpl[T: c.WeakTypeTag](c: whitebox.Context)
-                                          (body: c.Tree)
-                                          (execContext: c.Tree): c.Tree = {
+  def asyncImpl[T: c.WeakTypeTag](c: blackbox.Context)(body: c.Tree)(execContext: c.Tree): c.Tree = {
     import c.universe._
     if (!c.compilerSettings.contains("-Xasync")) {
       c.abort(c.macroApplication.pos, "The async requires the compiler option -Xasync (supported only by Scala 2.12.12+ / 2.13.3+)")

src/test/scala/scala/async/FutureSpec.scala

@@ -102,7 +102,7 @@ class FutureSpec {
       val a = await(future0.mapTo[Int])
       val b = await((Future { (a * 2).toString }).mapTo[Int])
       val c = await(Future { (7 * 2).toString })
-      b + "-" + c
+      b.toString + "-" + c
     }
 
     Await.result(future1, defaultTimeout) mustBe ("10-14")
@@ -129,7 +129,7 @@ class FutureSpec {
       Res(a: Int) <- asyncReq(Req("Hello"))
       Res(b: Int) <- asyncReq(Req(a))
       Res(c: Int) <- asyncReq(Req(7))
-    } yield b + "-" + c
+    } yield b.toString + "-" + c
 
     Await.result(future1, defaultTimeout) mustBe ("10-14")
     intercept[NoSuchElementException] { Await.result(future2, defaultTimeout) }

src/test/scala/scala/async/TestUtil.scala

@@ -59,7 +59,7 @@ object TestUtil {
   }
 
   implicit class objectops(obj: Any) {
-    def mustBe(other: Any): Unit = assert(obj == other, obj + " is not " + other)
+    def mustBe(other: Any): Unit = assert(obj == other, obj.toString + " is not " + other)
 
     def mustEqual(other: Any): Unit = mustBe(other)
   }