Fix #7868: Normalize type parameter in ValueOf

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -861,7 +861,7 @@ trait Implicits { self: Typer =>
 
       formal.argTypes match {
         case arg :: Nil =>
-          fullyDefinedType(arg.dealias, "ValueOf argument", span) match {
+          fullyDefinedType(arg.dealias, "ValueOf argument", span).normalized match {
             case ConstantType(c: Constant) =>
               success(Literal(c))
             case TypeRef(_, sym) if sym == defn.UnitClass =>

compiler/test/dotc/pos-test-pickling.blacklist

@@ -22,6 +22,7 @@ typelevel0.scala
 matchtype.scala
 6322.scala
 i7087.scala
+i7868.scala
 
 # Opaque type
 i5720.scala

compiler/test/dotc/run-test-pickling.blacklist

@@ -22,4 +22,5 @@ t8905
 t10889
 macros-in-same-project1
 i5257.scala
+i7868.scala
 enum-java

tests/pos/i7868.scala

@@ -0,0 +1,38 @@
+import scala.compiletime._
+
+final case class Coproduct[+Set, +Value, Index <: Int](value: Value & Set, index: Index)
+
+object Coproduct {
+  opaque type +:[+A, +B] = A | B
+
+  trait At[+Set, -Value, Index <: Int] {
+    def cast: Value <:< Set
+  }
+
+  object At {
+
+    given atHead[Head, Tail]: At[Head +: Tail, Head, 0] {
+      def cast: Head <:< Head +: Tail = summon[Head <:< Head +: Tail]
+    }
+
+    given atTail[Head, Tail, Value, NextIndex <: Int](given atNext: At[Tail, Value, NextIndex]): At[Head +: Tail, Value, S[NextIndex]] {
+      val cast: Value <:< Head +: Tail = atNext.cast
+    }
+
+    given [A](given A): (() => A)= { () => summon[A]}
+  }
+
+  def upCast[A, B](a: A)(given erased evidence: (A <:< B)): B = a.asInstanceOf[B]
+
+  def from[Set, Value, Index <: Int](value: Value)(given erased at: At[Set, Value, Index]): (given ValueOf[Index]) => Coproduct[Set, Value, Index] = {
+    Coproduct[Set, Value, Index](upCast(value: Value)(given at.cast.liftCo[[+X] =>> Value & X]), valueOf[Index])
+  }
+
+}
+
+object Test extends App {
+  import Coproduct._
+
+  // Error: No singleton value available for scala.compiletime.S[scala.compiletime.S[(0 : Int)]].
+  val c = from[Set = Int +: String +: Seq[Double] +: Nothing](Nil)
+}

tests/run/i7868.scala

@@ -0,0 +1,8 @@
+import scala.compiletime.S
+
+object Test extends App {
+    def plusOne[I <: Int](given x: ValueOf[S[I]]): S[I] = x.value
+    def plusTwo[I <: Int](given x: ValueOf[S[S[I]]]): S[S[I]] = x.value
+    assert(plusOne[0] == 1)
+    assert(plusTwo[0] == 2)
+}