From 3fab4bc11156e11c5ef3b3a4f754276e003c7ea9 Mon Sep 17 00:00:00 2001
From: Marek Safar <marek.safar@gmail.com>
Date: Wed, 27 Jul 2016 11:40:49 +0200
Subject: [PATCH] [mcs] Detect constant switch statement fall-through. Fixes
 #42585

---
 mcs/errors/cs0163-2.cs | 13 +++++++++++++
 mcs/mcs/statement.cs   | 34 +++++++++++++++++++++++-----------
 2 files changed, 36 insertions(+), 11 deletions(-)
 create mode 100644 mcs/errors/cs0163-2.cs

diff --git a/mcs/errors/cs0163-2.cs b/mcs/errors/cs0163-2.cs
new file mode 100644
index 00000000000..945ae8ad094
--- /dev/null
+++ b/mcs/errors/cs0163-2.cs
@@ -0,0 +1,13 @@
+// CS0163: Control cannot fall through from one case label `case 1:' to another
+// Line: 9
+
+public class Program
+{
+	public static void Main ()
+	{
+		switch (1) {
+			case 1: {}
+			default: {}
+		}
+	}
+}
\ No newline at end of file
diff --git a/mcs/mcs/statement.cs b/mcs/mcs/statement.cs
index d0341cb3b63..703e7ba497f 100644
--- a/mcs/mcs/statement.cs
+++ b/mcs/mcs/statement.cs
@@ -5378,18 +5378,30 @@ namespace Mono.CSharp {
 						continue;
 					}
 
-					if (constant_label != null && constant_label != sl)
+					if (section_rc.IsUnreachable) {
+						//
+						// Common case. Previous label section end is unreachable as
+						// it ends with break, return, etc. For next section revert
+						// to reachable again unless we have constant switch block
+						//
+						section_rc = constant_label != null && constant_label != sl ?
+							Reachability.CreateUnreachable () :
+							new Reachability ();
+					} else if (prev_label != null) {
+						//
+						// Error case as control cannot fall through from one case label
+						//
+						sl.SectionStart = false;
+						s = new MissingBreak (prev_label);
+						s.MarkReachable (rc);
+						block.Statements.Insert (i - 1, s);
+						++i;
+					} else if (constant_label != null && constant_label != sl) {
+						//
+						// Special case for the first unreachable label in constant
+						// switch block
+						//
 						section_rc = Reachability.CreateUnreachable ();
-					else if (section_rc.IsUnreachable) {
-						section_rc = new Reachability ();
-					} else {
-						if (prev_label != null) {
-							sl.SectionStart = false;
-							s = new MissingBreak (prev_label);
-							s.MarkReachable (rc);
-							block.Statements.Insert (i - 1, s);
-							++i;
-						}
 					}
 
 					prev_label = sl;
-- 
2.25.1