[backport] Avoid dead code warning with async(throw T)

By declararing the parameter of `async` as by-name.

Fixes #150 (the bug in the original ticket.)

(cherry picked from commit 4b1dbee)

Author: retronym

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

@@ -42,7 +42,7 @@ object Async {
    * Run the block of code `body` asynchronously. `body` may contain calls to `await` when the results of
    * a `Future` are needed; this is translated into non-blocking code.
    */
-  def async[T](body: T)(implicit execContext: ExecutionContext): Future[T] = macro internal.ScalaConcurrentAsync.asyncImpl[T]
+  def async[T](body: => T)(implicit execContext: ExecutionContext): Future[T] = macro internal.ScalaConcurrentAsync.asyncImpl[T]
 
   /**
    * Non-blocking await the on result of `awaitable`. This may only be used directly within an enclosing `async` block.

src/main/scala/scala/async/internal/AsyncId.scala

@@ -13,7 +13,7 @@ object AsyncId extends AsyncBase {
   lazy val futureSystem = IdentityFutureSystem
   type FS = IdentityFutureSystem.type
 
-  def async[T](body: T) = macro asyncIdImpl[T]
+  def async[T](body: => T) = macro asyncIdImpl[T]
 
   def asyncIdImpl[T: c.WeakTypeTag](c: Context)(body: c.Expr[T]): c.Expr[T] = asyncImpl[T](c)(body)(c.literalUnit)
 }

src/test/scala/scala/async/run/WarningsSpec.scala

@@ -31,6 +31,25 @@ class WarningsSpec {
     })
   }
 
+  @Test
+  // https://github.com/scala/async/issues/74
+  def noDeadCodeWarningForAsyncThrow() {
+    val global = mkGlobal("-cp ${toolboxClasspath} -Yrangepos -Ywarn-dead-code -Xfatal-warnings")
+    // was: "a pure expression does nothing in statement position; you may be omitting necessary parentheses"
+    val source =
+      """
+        | class Test {
+        |   import scala.async.Async._
+        |   import scala.concurrent.ExecutionContext.Implicits.global
+        |   async { throw new Error() }
+        | }
+      """.stripMargin
+    val run = new global.Run
+    val sourceFile = global.newSourceFile(source)
+    run.compileSources(sourceFile :: Nil)
+    assert(!global.reporter.hasErrors, global.reporter.asInstanceOf[StoreReporter].infos)
+  }
+
   @Test
   def noDeadCodeWarning() {
     val global = mkGlobal("-cp ${toolboxClasspath} -Yrangepos -Ywarn-dead-code -Xfatal-warnings")