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, 0 deletions

diff --git a/test/files/scalacheck/quasiquotes/TermDeconstructionProps.scala b/test/files/scalacheck/quasiquotes/TermDeconstructionProps.scala
index c77946c291..e96d1186f7 100644
--- a/test/files/scalacheck/quasiquotes/TermDeconstructionProps.scala
+++ b/test/files/scalacheck/quasiquotes/TermDeconstructionProps.scala
@@ -186,4 +186,17 @@ object TermDeconstructionProps extends QuasiquoteProperties("term deconstruction
     assert(init ≈ List(q"a", q"b"))
     assert(last ≈ q"c")
   }
+
+  property("si-8275 c") = test {
+    val cq"_ => ..$stats" = cq"_ =>"
+    assert(stats.isEmpty)
+    assertEqAst(q"{ case _ => ..$stats }", "{ case _ => }")
+  }
+
+  property("can't flatten type into block") = test {
+    assertThrows[IllegalArgumentException] {
+      val tpt = tq"List[Int]"
+      q"..$tpt; ()"
+    }
+  }
 }