Clarify switch error message if there are too few cases

Author: odersky

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -2030,10 +2030,11 @@ object messages {
     }
   }
 
-  case class UnableToEmitSwitch()(implicit ctx: Context)
+  case class UnableToEmitSwitch(tooFewCases: Boolean)(implicit ctx: Context)
   extends Message(UnableToEmitSwitchID) {
     val kind = "Syntax"
-    val msg = hl"Could not emit switch for ${"@switch"} annotated match"
+    val tooFewStr = if (tooFewCases) " since there are not enough cases" else ""
+    val msg = hl"Could not emit switch for ${"@switch"} annotated match$tooFewStr"
     val explanation = {
       val codeExample =
         """val ConstantB = 'B'

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

@@ -48,6 +48,9 @@ object PatternMatcher {
 
   final val selfCheck = false // debug option, if on we check that no case gets generated twice
 
+  /** Minimal number of cases to emit a switch */
+  final val MinSwitchCases = 4
+
   /** Was symbol generated by pattern matcher? */
   def isPatmatGenerated(sym: Symbol)(implicit ctx: Context): Boolean =
     sym.is(Synthetic) &&
@@ -863,7 +866,7 @@ object PatternMatcher {
       plan match {
         case plan: TestPlan =>
           val switchCases = collectSwitchCases(plan)
-          if (switchCases.lengthCompare(4) >= 0) // at least 3 cases + default
+          if (switchCases.lengthCompare(MinSwitchCases) >= 0) // at least 3 cases + default
             Match(plan.scrutinee, emitSwitchCases(switchCases))
           else {
             /** Merge nested `if`s that have the same `else` branch into a single `if`.
@@ -982,7 +985,7 @@ object PatternMatcher {
           patmatch.println(i"original types: ${typesInCases(original.cases)}%, %")
           patmatch.println(i"switch types  : ${typesInCases(resultCases)}%, %")
           patmatch.println(i"tree = $result")
-          ctx.warning(UnableToEmitSwitch(), original.pos)
+          ctx.warning(UnableToEmitSwitch(numTypes(original.cases) < MinSwitchCases), original.pos)
         }
       case _ =>
     }

tests/neg-custom-args/i3561.scala

@@ -13,4 +13,10 @@ class Test {
     case '5' | Constant => 3
     case '4' => 4
   }
+
+  def test3(x: Any) = (x: @annotation.switch) match { // error: could not emit switch - too few cases
+    case 1 => 1
+    case 2 => 2
+    case x: String => 4
+  }
 }