From 42edcf7bb5399bba68cdc75a96a293cfe3e59c86 Mon Sep 17 00:00:00 2001
From: Hamza Remmal <hamza@remmal.net>
Date: Tue, 28 Oct 2025 11:44:58 +0100
Subject: [PATCH] fix: use underlying of VCs to compute generic signature

---
 .../dotc/transform/GenericSignatures.scala    |  5 ++--
 .../tools/dotc/transform/ValueClasses.scala   |  5 ++++
 tests/run/i24276.check                        | 12 +++++++++
 tests/run/i24276.scala                        | 25 +++++++++++++++++++
 tests/run/i8001/B_2.java                      |  2 +-
 5 files changed, 46 insertions(+), 3 deletions(-)
 create mode 100644 tests/run/i24276.check
 create mode 100644 tests/run/i24276.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala b/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
index ed02587eb1f1..58d75e3c5524 100644
--- a/compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala
+++ b/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)) {
diff --git a/compiler/src/dotty/tools/dotc/transform/ValueClasses.scala b/compiler/src/dotty/tools/dotc/transform/ValueClasses.scala
index 5cdd5d8ded43..13a3ce7c08a3 100644
--- a/compiler/src/dotty/tools/dotc/transform/ValueClasses.scala
+++ b/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)
+  
 }
diff --git a/tests/run/i24276.check b/tests/run/i24276.check
new file mode 100644
index 000000000000..b1fd06bf794d
--- /dev/null
+++ b/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)
diff --git a/tests/run/i24276.scala b/tests/run/i24276.scala
new file mode 100644
index 000000000000..a03e06e8b8ff
--- /dev/null
+++ b/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)
diff --git a/tests/run/i8001/B_2.java b/tests/run/i8001/B_2.java
index bde79bae36fb..31346721ca36 100644
--- a/tests/run/i8001/B_2.java
+++ b/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);