Merge pull request #7544 from dotty-staging/fix-#6606

Fix #6606: Do not compute S on negative Ints

Author: liufengyun

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

@@ -1022,7 +1022,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
     def compareS(tp: AppliedType, other: Type, fromBelow: Boolean): Boolean = tp.args match {
       case arg :: Nil =>
         natValue(arg) match {
-          case Some(n) =>
+          case Some(n) if n != Int.MaxValue =>
             val succ = ConstantType(Constant(n + 1))
             if (fromBelow) recur(other, succ) else recur(succ, other)
           case none =>

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

@@ -3598,7 +3598,8 @@ object Types {
         if (defn.isCompiletime_S(tycon.symbol) && args.length == 1)
           trace(i"normalize S $this", typr, show = true) {
             args.head.normalized match {
-              case ConstantType(Constant(n: Int)) => ConstantType(Constant(n + 1))
+              case ConstantType(Constant(n: Int)) if n >= 0 && n < Int.MaxValue =>
+                ConstantType(Constant(n + 1))
               case none => tryMatchAlias
             }
           }

library/src/scala/compiletime/package.scala

@@ -52,5 +52,15 @@ package object compiletime {
    */
   inline def summonFrom[T](f: Nothing => T) <: T = ???
 
-  type S[X <: Int] <: Int
+  /** Succesor of a natural number where zero is the type 0 and successors are reduced as if the definition was
+   *
+   *      type S[N <: Int] <: Int = N match {
+   *        case 0 => 1
+   *        case 1 => 2
+   *        case 2 => 3
+   *        ...
+   *        case 2147483646 => 2147483647
+   *      }
+   */
+  type S[N <: Int] <: Int
 }

tests/neg/i6606.scala

@@ -0,0 +1,8 @@
+val t0: Int = 0
+val t1: scala.compiletime.S[-1] = 0 // error
+val t2: scala.compiletime.S[Int] = 1
+val t3: scala.compiletime.S[Int] = 0 // error
+val t4: scala.compiletime.S[Int] = t0 // error
+
+val t5: scala.compiletime.S[2147483647] = -2147483648 // error
+val t6: scala.compiletime.S[-2147483648] = -2147483647 // error