SI-8306: handle SWITCH nodes with just default case

Handle properly SWITCH nodes that contain just a default case in optimizer
(ConstantOptimization). SWITCH with just default case is expressed as a
node with empty tags and exactly one label. We can handle such nodes
easily by introducing a shortcut in logic that computes reachableLabels.

Add a test case which triggers patmat to generate SWITCH node with just
a default case.

Fixes SI-8306.

2 files changed, 9 insertions, 0 deletions

diff --git a/test/files/pos/t8306.flags b/test/files/pos/t8306.flags
new file mode 100644
index 0000000000..49d036a887
--- /dev/null
+++ b/test/files/pos/t8306.flags
@@ -0,0 +1 @@
+-optimize
diff --git a/test/files/pos/t8306.scala b/test/files/pos/t8306.scala
new file mode 100644
index 0000000000..e04b054eb9
--- /dev/null
+++ b/test/files/pos/t8306.scala
@@ -0,0 +1,8 @@
+class Si8306 {
+  def foo: Int = 123
+  lazy val extension: Int =
+      foo match {
+          case idx if idx != -1 => 15
+          case _ => 17
+      }
+}