Fix #9776: Don't issue a @switch warning when fewer than 4 cases

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

@@ -2029,10 +2029,9 @@ import transform.SymUtils._
           |whose behavior may have changed since version change."""
   }
 
-  class UnableToEmitSwitch(tooFewCases: Boolean)(using Context)
+  class UnableToEmitSwitch()(using Context)
   extends SyntaxMsg(UnableToEmitSwitchID) {
-    def tooFewStr: String = if (tooFewCases) " since there are not enough cases" else ""
-    def msg = em"Could not emit switch for ${hl("@switch")} annotated match$tooFewStr"
+    def msg = em"Could not emit switch for ${hl("@switch")} annotated match"
     def explain = {
       val codeExample =
         """val ConstantB = 'B'

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

@@ -976,30 +976,32 @@ object PatternMatcher {
     /** If match is switch annotated, check that it translates to a switch
      *  with at least as many cases as the original match.
      */
-    private def checkSwitch(original: Match, result: Tree) = original.selector match {
+    private def checkSwitch(original: Match, result: Tree) = original.selector match
       case Typed(_, tpt) if tpt.tpe.hasAnnotation(defn.SwitchAnnot) =>
-        val resultCases = result match {
+        val resultCases = result match
           case Match(_, cases) => cases
           case Block(_, Match(_, cases)) => cases
+          case Block((_: ValDef) :: Block(_, Match(_, cases)) :: Nil, _) => cases
           case _ => Nil
-        }
-        def typesInPattern(pat: Tree): List[Type] = pat match {
+        val caseThreshold =
+          if ValueClasses.isDerivedValueClass(tpt.tpe.typeSymbol) then 1
+          else MinSwitchCases
+        def typesInPattern(pat: Tree): List[Type] = pat match
           case Alternative(pats) => pats.flatMap(typesInPattern)
           case _ => pat.tpe :: Nil
-        }
         def typesInCases(cdefs: List[CaseDef]): List[Type] =
           cdefs.flatMap(cdef => typesInPattern(cdef.pat))
         def numTypes(cdefs: List[CaseDef]): Int =
           typesInCases(cdefs).toSet.size: Int // without the type ascription, testPickling fails because of #2840.
-        if (numTypes(resultCases) < numTypes(original.cases)) {
+        val numTypesInOriginal = numTypes(original.cases)
+        if numTypesInOriginal >= caseThreshold && numTypes(resultCases) < numTypesInOriginal then
           patmatch.println(i"switch warning for ${ctx.compilationUnit}")
           patmatch.println(i"original types: ${typesInCases(original.cases)}%, %")
           patmatch.println(i"switch types  : ${typesInCases(resultCases)}%, %")
           patmatch.println(i"tree = $result")
-          report.warning(UnableToEmitSwitch(numTypes(original.cases) < MinSwitchCases), original.srcPos)
-        }
+          report.warning(UnableToEmitSwitch(), original.srcPos)
       case _ =>
-    }
+    end checkSwitch
 
     val optimizations: List[(String, Plan => Plan)] = List(
       "mergeTests" -> mergeTests,

tests/neg-custom-args/fatal-warnings/i3561.scala

@@ -13,10 +13,4 @@ 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
-  }
 }

tests/neg-custom-args/fatal-warnings/i9776.scala

@@ -0,0 +1,59 @@
+import scala.annotation.switch
+
+sealed trait Fruit
+
+object Fruit {
+  case object Apple extends Fruit
+  case object Banana extends Fruit
+  case object Lemon extends Fruit
+  case object Lime extends Fruit
+  case object Orange extends Fruit
+
+  def isCitrus(fruit: Fruit): Boolean =
+    (fruit: @switch) match {  // error Could not emit switch for @switch annotated match
+      case Orange => true
+      case Lemon  => true
+      case Lime   => true
+      case _      => false
+    }
+}
+
+
+sealed trait TaggedFruit {
+  def tag: Int
+}
+
+object TaggedFruit {
+  case object Apple extends TaggedFruit {
+    val tag = 1
+  }
+  case object Banana extends TaggedFruit {
+    val tag = 2
+  }
+  case object Orange extends TaggedFruit {
+    val tag = 3
+  }
+
+  def isCitrus(fruit: TaggedFruit): Boolean =
+    (fruit.tag: @switch) match {  // error Could not emit switch for @switch annotated match
+      case Apple.tag => true
+      case 2         => true
+      case 3         => true
+      case _         => false
+  }
+
+  // fewer than four cases, so no warning
+  def succ1(fruit: TaggedFruit): Boolean =
+    (fruit.tag: @switch) match {
+      case 3             => false
+      case 2 | Apple.tag => true
+    }
+
+  // fewer than four cases, so no warning
+  def succ2(fruit: TaggedFruit): Boolean =
+    (fruit.tag: @switch) match {
+      case 3          => false
+      case 2          => true
+      case Apple.tag  => true
+  }
+}

tests/neg-custom-args/fatal-warnings/switches.scala

@@ -38,13 +38,15 @@ object Main {
   // thinks a val in an object is constant... so naive
   def fail1(c: Char) = (c: @switch @unchecked) match { // error: Could not emit switch for @switch annotated match
     case 'A'        => true
+    case 'B'        => true
     case Other.C1   => true
     case _          => false
   }
 
   // more naivete
   def fail2(c: Char) = (c: @unchecked @switch) match { // error: Could not emit switch for @switch annotated match
     case 'A'        => true
+    case 'B'        => true
     case Other.C3   => true
     case _          => false
   }
@@ -74,4 +76,25 @@ object Main {
     case 1 | 2 | 3 => true
     case _ => false
   }
+
+  case class IntAnyVal(x: Int) extends AnyVal
+
+  val Ten = IntAnyVal(10)
+  def fail5(x: IntAnyVal) = (x: @switch) match { // error: Could not emit switch for @switch annotated match
+    case IntAnyVal(1) => 0
+    case Ten           => 1
+    case IntAnyVal(100) => 2
+    case IntAnyVal(1000) => 3
+    case IntAnyVal(10000) => 4
+  }
+
+  // the generated lookupswitch covers only a subset of the cases
+  final val One = IntAnyVal(1)
+  def fail6(x: IntAnyVal) = (x: @switch) match { // error: Could not emit switch for @switch annotated match
+    case One          => 0
+    case IntAnyVal(10) => 1
+    case IntAnyVal(100) => 2
+    case IntAnyVal(1000) => 3
+    case IntAnyVal(10000) => 4
+  }
 }

tests/untried/neg/t5830.scala → tests/neg-custom-args/fatal-warnings/t5830.scala

@@ -3,7 +3,7 @@ import scala.annotation.switch
 class Test {
   def unreachable(ch: Char) = (ch: @switch) match {
     case 'a' => println("b") // ok
-    case 'a' => println("b") // unreachable
+    case 'a' => println("b") // error: unreachable case
     case 'c' =>
   }
 }

tests/pos-special/fatal-warnings/i9776.scala

@@ -0,0 +1,38 @@
+import scala.annotation.switch
+
+sealed trait Fruit
+
+object Fruit {
+  case object Apple extends Fruit
+  case object Banana extends Fruit
+  case object Orange extends Fruit
+
+  def isCitrus(fruit: Fruit): Boolean =
+    (fruit: @switch) match {
+      case Orange => true
+      case _      => false
+    }
+}
+
+
+sealed trait TaggedFruit {
+  def tag: Int
+}
+
+object TaggedFruit {
+  case object Apple extends TaggedFruit {
+    val tag = 1
+  }
+  case object Banana extends TaggedFruit {
+    val tag = 2
+  }
+  case object Orange extends TaggedFruit {
+    val tag = 3
+  }
+
+  def isCitrus(fruit: TaggedFruit): Boolean =
+    (fruit.tag: @switch) match {
+      case 3 => true
+      case _ => false
+  }
+}

tests/pos-special/fatal-warnings/switches.scala

@@ -33,8 +33,18 @@ class Test {
     case 1 | 2 | 3 => true
     case _ => false
   }
+
+  def test6(x: IntAnyVal) = (x: @switch) match {
+    case IntAnyVal(1) => 0
+    case IntAnyVal(10) => 1
+    case IntAnyVal(100) => 2
+    case IntAnyVal(1000) => 3
+    case IntAnyVal(10000) => 4
+  }
 }
 
+case class IntAnyVal(x: Int) extends AnyVal
+
 object Test {
   final val LF = '\u000A'
   final val CR = '\u000D'