Fix #6606: Do not compute S on negative Ints

Author: nicolasstucki

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

@@ -3552,7 +3552,7 @@ 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 => ConstantType(Constant(n + 1))
               case none => tryMatchAlias
             }
           }

tests/neg/i6606.scala

@@ -0,0 +1,5 @@
+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] = n // error