Fix block construction/deconstruction asymmetry

Deconstruction of blocks in case clauses uncovered asymmetry between
construction and deconstruction of blocks:

   tree match { case cq"$pat => ..$cases" => cq"$pat => ..$cases" }

Such an identity-like transformation used to produce an incorrect tree due
to the fact that zero-element block was mistakingly associated with
empty tree. Such association was used as a solution to block flatenning:

   val ab = q"a; b"
   q"..$ab; c"       // ==> q"a; b; c"

   val a = q"a"
   q"..$a; c"        // ==> q"a; c"

   val empty = q""
   q"..$empty; c"    // ==> q"c"

This commit changes meaning of zero-element block to a be a synthetic unit
instead. This is consistent with parsing of `{}`, cases, ifs and
non-abstract empty-bodied methods. A local tweak to block flattenning is
used to flatten empty tree as empty list instead.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/TreeGen.scala b/src/reflect/scala/reflect/internal/TreeGen.scala
index 6011289baf..9066c73393 100644
--- a/src/reflect/scala/reflect/internal/TreeGen.scala
+++ b/src/reflect/scala/reflect/internal/TreeGen.scala
@@ -452,7 +452,7 @@ abstract class TreeGen {
 
   /** Create block of statements `stats`  */
   def mkBlock(stats: List[Tree]): Tree =
-    if (stats.isEmpty) Literal(Constant(()))
+    if (stats.isEmpty) mkSyntheticUnit()
     else if (!stats.last.isTerm) Block(stats, mkSyntheticUnit())
     else if (stats.length == 1) stats.head
     else Block(stats.init, stats.last)