fix: use underlying of VCs to compute generic signature

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 +1,12 @@
+public <A> scala.Option<Box<A>> Foo.bar1(scala.Option<Box<A>>)
+public <A> scala.Option<IBox> Foo.bar2(scala.Option<IBox>)
+public <A> scala.Option<SBox> Foo.bar3(scala.Option<SBox>)
+public <A> A Foo.foo1(A)
+public int Foo.foo2(int)
+public java.lang.String Foo.foo3(java.lang.String)
+public static <A> scala.Option<Box<A>> Foo.sbar1(scala.Option<Box<A>>)
+public static <A> scala.Option<IBox> Foo.sbar2(scala.Option<IBox>)
+public static <A> scala.Option<SBox> Foo.sbar3(scala.Option<SBox>)
+public static <A> A Foo.sfoo1(A)
+public static int Foo.sfoo2(int)
+public static java.lang.String Foo.sfoo3(java.lang.String)

tests/run/i24276.scala

@@ -0,0 +1,25 @@
+// scalajs: --skip
+
+class Box[A](value: A) extends AnyVal
+class IBox(value: Int) extends AnyVal
+class SBox(value: String) extends AnyVal
+
+class Foo:
+  def foo1[A](a: A): Box[A] = Box(a)
+  def foo2(a: IBox): IBox = a
+  def foo3(a: SBox): SBox = a
+  def bar1[A](opt: Option[Box[A]]): Option[Box[A]] = opt
+  def bar2[A](opt: Option[IBox]): Option[IBox] = opt
+  def bar3[A](opt: Option[SBox]): Option[SBox] = opt
+
+object Foo:
+  def sfoo1[A](a: A): Box[A] = Box(a)
+  def sfoo2(a: IBox): IBox = a
+  def sfoo3(a: SBox): SBox = a
+  def sbar1[A](opt: Option[Box[A]]): Option[Box[A]] = opt
+  def sbar2[A](opt: Option[IBox]): Option[IBox] = opt
+  def sbar3[A](opt: Option[SBox]): Option[SBox] = opt
+
+@main def Test =
+  for mtd <- classOf[Foo].getDeclaredMethods.sortBy(_.getName) do
+    println(mtd.toGenericString)

tests/run/i8001/B_2.java

@@ -16,7 +16,7 @@ public static void test() {
 
     a.arr1(intArr);
     a.arr2(stringArr);
-    a.arr3(intArr);
+    a.arr3(intArr2);
     a.arr4(stringArr2);
 
     a.arrRef1(integerArr);