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, 13 insertions, 6 deletions

diff --git a/test/files/scalacheck/quasiquotes/TermConstructionProps.scala b/test/files/scalacheck/quasiquotes/TermConstructionProps.scala
index 10ce1604b1..fd4d2e9c4b 100644
--- a/test/files/scalacheck/quasiquotes/TermConstructionProps.scala
+++ b/test/files/scalacheck/quasiquotes/TermConstructionProps.scala
@@ -103,7 +103,7 @@ object TermConstructionProps extends QuasiquoteProperties("term construction") {
 
   def blockInvariant(quote: Tree, trees: List[Tree]) =
     quote ≈ (trees match {
-      case Nil => q""
+      case Nil => q"{}"
       case _ :+ last if !last.isTerm => Block(trees, q"()")
       case head :: Nil => head
       case init :+ last => Block(init, last)
@@ -277,11 +277,18 @@ object TermConstructionProps extends QuasiquoteProperties("term construction") {
     assert(stats ≈ List(q"def x = 2", q"()"))
   }
 
-  property("empty-tree as block") = test {
-    val q"{ ..$stats1 }" = q" "
-    assert(stats1.isEmpty)
-    val stats2 = List.empty[Tree]
-    assert(q"{ ..$stats2 }" ≈ q"")
+  property("empty-tree is not a block") = test {
+    assertThrows[MatchError] {
+      val q"{ ..$stats1 }" = q" "
+    }
+  }
+
+  property("empty block is synthetic unit") = test {
+    val q"()" = q"{}"
+    val q"{..$stats}" = q"{}"
+    assert(stats.isEmpty)
+    assertEqAst(q"{..$stats}", "{}")
+    assertEqAst(q"{..$stats}", "()")
   }
 
   property("consistent variable order") = test {