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, 2 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/reflect/quasiquotes/Reifiers.scala b/src/compiler/scala/tools/reflect/quasiquotes/Reifiers.scala
index 4937bfc35d..481897d0ec 100644
--- a/src/compiler/scala/tools/reflect/quasiquotes/Reifiers.scala
+++ b/src/compiler/scala/tools/reflect/quasiquotes/Reifiers.scala
@@ -202,6 +202,8 @@ trait Reifiers { self: Quasiquotes =>
       // not to cause infinite recursion.
       case block @ SyntacticBlock(stats) if block.isInstanceOf[Block] =>
         reifyBuildCall(nme.SyntacticBlock, stats)
+      case SyntheticUnit() =>
+        reifyBuildCall(nme.SyntacticBlock, Nil)
       case Try(block, catches, finalizer) =>
         reifyBuildCall(nme.SyntacticTry, block, catches, finalizer)
       case Match(selector, cases) =>