SI-8001 spurious "pure expression does nothing" warning

`isPureExprForWarningPurposes` doesn’t warn on `()`, but `(): Unit`
leaves it confused. This patch fixes the problem.

The fix is important in the context of the recent split between blackbox
and whitebox macros. Macro engines wrap expansions of blackbox macros
in type ascriptions, so a macro that expands into `()` would actually
produce `(): Unit`, which would trigger a spurious warning. Thanks
@milessabin for spotting this problem!

2 files changed, 14 insertions, 0 deletions

diff --git a/test/files/pos/t8001/Macros_1.scala b/test/files/pos/t8001/Macros_1.scala
new file mode 100644
index 0000000000..1f8dab51c1
--- /dev/null
+++ b/test/files/pos/t8001/Macros_1.scala
@@ -0,0 +1,10 @@
+import scala.language.experimental.macros
+import scala.reflect.macros.BlackboxContext
+
+object Macros {
+  def foo = macro impl
+  def impl(c: BlackboxContext) = {
+    import c.universe._
+    q"()"
+  }
+}
\ No newline at end of file
diff --git a/test/files/pos/t8001/Test_2.scala b/test/files/pos/t8001/Test_2.scala
new file mode 100644
index 0000000000..6d72d96070
--- /dev/null
+++ b/test/files/pos/t8001/Test_2.scala
@@ -0,0 +1,4 @@
+object Test extends App {
+  Macros.foo
+  (): Unit
+}
\ No newline at end of file