Fixes #8280

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

@@ -3780,22 +3780,22 @@ object Types {
 
     def tryCompiletimeConstantFold(implicit ctx: Context): Type = tycon match {
       case tycon: TypeRef if defn.isCompiletimeAppliedType(tycon.symbol) =>
-        def constValue(tp: Type): Option[Any] = tp match {
+        def constValue(tp: Type): Option[Any] = tp.dealias match {
           case ConstantType(Constant(n)) => Some(n)
           case _ => None
         }
 
-        def boolValue(tp: Type): Option[Boolean] = tp match {
+        def boolValue(tp: Type): Option[Boolean] = tp.dealias match {
           case ConstantType(Constant(n: Boolean)) => Some(n)
           case _ => None
         }
 
-        def intValue(tp: Type): Option[Int] = tp match {
+        def intValue(tp: Type): Option[Int] = tp.dealias match {
           case ConstantType(Constant(n: Int)) => Some(n)
           case _ => None
         }
 
-        def stringValue(tp: Type): Option[String] = tp match {
+        def stringValue(tp: Type): Option[String] = tp.dealias match {
           case ConstantType(Constant(n: String)) => Some(n)
           case _ => None
         }

tests/pos/singleton-ops-test-issue-8280.scala

@@ -0,0 +1,16 @@
+import scala.compiletime.ops.int._
+import scala.compiletime.S
+
+class Foo[T <: Int] {
+  def incP = new Foo[T + 1]
+  def incS = new Foo[S[T]]
+}
+object Foo {
+  def apply[T <: Int & Singleton](value : T) : Foo[T] = new Foo[T]
+}
+
+val fincS   : Foo[2] = Foo(1).incS
+val fincP1  : Foo[2] = Foo(1).incP
+val fincP2a = Foo(1).incP
+val fincP2b : Foo[2] = fincP2a
+val fincP3  : Foo[2] = (new Foo[1]).incP