fix: use underlying of VCs to compute generic signature

Author: hamzaremmal

compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala

@@ -276,8 +276,9 @@ object GenericSignatures {
             else builder.append(defn.typeTag(sym.info))
           else if (sym.isDerivedValueClass) {
             if (unboxedVCs) {
-              val erasedUnderlying = fullErasure(tp)
-              jsig(erasedUnderlying, toplevel = toplevel, unboxedVCs = true)
+              val underlying = ValueClasses
+                .underlyingOfValueClass(sym.asClass, tp)
+              jsig(underlying, toplevel = toplevel, unboxedVCs = true)
             } else classSig(sym, pre, args)
           }
           else if (defn.isSyntheticFunctionClass(sym)) {

compiler/src/dotty/tools/dotc/transform/ValueClasses.scala

@@ -44,4 +44,9 @@ object ValueClasses {
   /** The unboxed type that underlies a derived value class */
   def underlyingOfValueClass(sym: ClassSymbol)(using Context): Type =
     valueClassUnbox(sym).info.resultType
+
+  /** The unboxed type that underlies a derived value class as seen from as seen from tpe*/
+  def underlyingOfValueClass(sym: ClassSymbol, tpe: Type)(using Context): Type =
+    valueClassUnbox(sym).info.resultType.asSeenFrom(tpe, sym)
+  
 }

tests/run/i24276.check

@@ -0,0 +1,4 @@
+public <A> scala.Option<Box<A>> Foo.bar(scala.Option<Box<A>>)
+public <A> A Foo.foo(A)
+public static <A> scala.Option<Box<A>> Foo.sbar(scala.Option<Box<A>>)
+public static <A> A Foo.sfoo(A)

tests/run/i24276.scala

@@ -0,0 +1,13 @@
+class Box[A](value: A) extends AnyVal
+
+class Foo:
+  def foo[A](a: A): Box[A] = Box(a)
+  def bar[A](opt: Option[Box[A]]): Option[Box[A]] = opt
+
+object Foo:
+  def sfoo[A](a: A): Box[A] = Box(a)
+  def sbar[A](opt: Option[Box[A]]): Option[Box[A]] = opt
+
+@main def Test =
+  for mtd <- classOf[Foo].getDeclaredMethods.sortBy(_.getName) do
+    println(mtd.toGenericString)