Use new specced match types for scala.compiletime.int.S[n].

Author: sjrd

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -3340,6 +3340,13 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
 
               case _ =>
                 false
+
+          case MatchTypeCasePattern.CompileTimeS(argPattern) =>
+            natValue(scrut) match
+              case Some(scrutValue) if scrutValue != Int.MinValue =>
+                rec(argPattern, ConstantType(Constant(scrutValue - 1)), variance, scrutIsWidenedAbstract)
+              case _ =>
+                false
       end rec
 
       // This might not be needed

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -5062,6 +5062,7 @@ object Types {
     case Capture(num: Int, isWildcard: Boolean)
     case TypeTest(tpe: Type)
     case BaseTypeTest(classType: TypeRef, argPatterns: List[MatchTypeCasePattern], needsConcreteScrut: Boolean)
+    case CompileTimeS(argPattern: MatchTypeCasePattern)
 
     def isTypeTest: Boolean =
       this.isInstanceOf[TypeTest]
@@ -5126,6 +5127,14 @@ object Types {
                       (argPattern, tparam) => tparam.paramVarianceSign != 0 && argPattern.needsConcreteScrutInVariantPos
                     }
                     MatchTypeCasePattern.BaseTypeTest(tycon, argPatterns1, needsConcreteScrut)
+            else if defn.isCompiletime_S(tyconSym) && args.sizeIs == 1 then
+              val argPattern = rec(args.head, variance)
+              if argPattern == null then
+                null
+              else if argPattern.isTypeTest then
+                MatchTypeCasePattern.TypeTest(pat)
+              else
+                MatchTypeCasePattern.CompileTimeS(argPattern)
             else
               null