Refine switch warning

The test code counted the number of different types in all patterns,
but it needs to account for alternative patterns as well.

Author: odersky

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -967,11 +967,15 @@ object PatternMatcher {
           case Block(_, Match(_, cases)) => cases
           case _ => Nil
         }
-        def numConsts(cdefs: List[CaseDef]): Int = {
-          val tpes = cdefs.map(_.pat.tpe)
+        def numTypes(cdefs: List[CaseDef]): Int = {
+          def patTypes(pat: Tree): List[Type] = pat match {
+            case Alternative(pats) => pats.flatMap(patTypes)
+            case _ => pat.tpe :: Nil
+          }
+          val tpes = cdefs.flatMap(patTypes)
           tpes.toSet.size: Int // without the type ascription, testPickling fails because of #2840.
         }
-        if (numConsts(resultCases) < numConsts(original.cases))
+        if (numTypes(resultCases) < numTypes(original.cases))
           ctx.warning(UnableToEmitSwitch(), original.pos)
       case _ =>
     }

tests/neg-custom-args/i3561.scala

@@ -0,0 +1,16 @@
+class Test {
+  val Constant = 'Q' // OK if final
+  def tokenMe(ch: Char) = (ch: @annotation.switch) match { // error: could not emit switch
+    case ' ' => 1
+    case 'A' => 2
+    case '5' | Constant => 3
+    case '4' => 4
+  }
+
+  def test2(x: Any) = (x: @annotation.switch) match { // error: could not emit switch
+    case ' ' => 1
+    case 'A' => 2
+    case '5' | Constant => 3
+    case '4' => 4
+  }
+}