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

Author: griggt

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

@@ -1969,10 +1969,9 @@ import ast.tpd
           |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

@@ -983,12 +983,13 @@ object PatternMatcher {
           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)) {
+        if (numTypes(resultCases) < numTypes(original.cases)
+            && numTypes(original.cases) >= MinSwitchCases) {
           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 _ =>
     }

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

@@ -14,7 +14,7 @@ class Test {
     case '4' => 4
   }
 
-  def test3(x: Any) = (x: @annotation.switch) match { // error: could not emit switch - too few cases
+  def test3(x: Any) = (x: @annotation.switch) match {
     case 1 => 1
     case 2 => 2
     case x: String => 4

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

@@ -36,14 +36,14 @@ object Main {
   // multiple annotations are processed correctly
 
   // 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
+  def fail1(c: Char) = (c: @switch @unchecked) match {
     case 'A'        => 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
+  def fail2(c: Char) = (c: @unchecked @switch) match {
     case 'A'        => true
     case Other.C3   => true
     case _          => false