SI-8731 warning if @switch is ignored

For matches with two or fewer cases, @switch is ignored. This should
not happen silently.

4 files changed, 26 insertions, 1 deletions

diff --git a/test/files/neg/t6902.scala b/test/files/neg/t6902.scala
index ce5ff8b6fb..627c324279 100644
--- a/test/files/neg/t6902.scala
+++ b/test/files/neg/t6902.scala
@@ -16,7 +16,7 @@ object Test {
   // at scala.reflect.internal.SymbolTable.abort(SymbolTable.scala:50)
   // at scala.tools.nsc.Global.abort(Global.scala:249)
   // at scala.tools.nsc.backend.jvm.GenASM$JPlainBuilder$jcode$.emitSWITCH(GenASM.scala:1850)
-  ((1: Byte): @unchecked @annotation.switch) match {
+  ((1: Byte): @unchecked) match {
     case 1 => 2
     case 1 => 3 // crash
   }
diff --git a/test/files/neg/t8731.check b/test/files/neg/t8731.check
new file mode 100644
index 0000000000..2a9af475fc
--- /dev/null
+++ b/test/files/neg/t8731.check
@@ -0,0 +1,9 @@
+t8731.scala:5: warning: matches with two cases or fewer are emitted using if-then-else instead of switch
+  def f(x: Int) = (x: @annotation.switch) match {
+                       ^
+t8731.scala:10: warning: could not emit switch for @switch annotated match
+  def g(x: Int) = (x: @annotation.switch) match {
+                       ^
+error: No warnings can be incurred under -Xfatal-warnings.
+two warnings found
+one error found
diff --git a/test/files/neg/t8731.flags b/test/files/neg/t8731.flags
new file mode 100644
index 0000000000..e8fb65d50c
--- /dev/null
+++ b/test/files/neg/t8731.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
\ No newline at end of file
diff --git a/test/files/neg/t8731.scala b/test/files/neg/t8731.scala
new file mode 100644
index 0000000000..d93fe706ad
--- /dev/null
+++ b/test/files/neg/t8731.scala
@@ -0,0 +1,15 @@
+class C {
+  // not a compile-time constant due to return type
+  final val K: Int = 20
+
+  def f(x: Int) = (x: @annotation.switch) match {
+    case K => 0
+    case 2 => 1
+  }
+
+  def g(x: Int) = (x: @annotation.switch) match {
+    case K => 0
+    case 2 => 1
+    case 3 => 2
+  }
+}