Correct Java signature for value classes appearing in type arguments (#…

…20463)

As suggested in #10846 the fix to this issue should be to port
scala/scala#8127 to scala3

I started by adding the same tests as in the scala2 PR and then tried to
find the place where to do the fix by adding some log traces.
Unfortunately I am still pretty lost because this is my first time
looking at the compiler code.

Any tips where this needs to be fixed are very welcome. Meanwhile a few
questions:
* The scala2 fix was done in
`src/compiler/scala/tools/nsc/transform/Erasure.scala`, should I do the
fix for scala3 in
`compiler/src/dotty/tools/dotc/transform/Erasure.scala` as well?
* I think I need to do the fix around `ErasedValueType`, either when it
is created (in `TypeErasure#eraseDerivedValueClass`) or when it is used
(in `Erasure#unbox`).

Please also send me any pointers besides
https://dotty.epfl.ch/docs/contributing/index.html regarding compiler
contributions

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

@@ -64,7 +64,7 @@ object GenericSignatures {
       ps.foreach(boxedSig)
     }
 
-    def boxedSig(tp: Type): Unit = jsig(tp.widenDealias, primitiveOK = false)
+    def boxedSig(tp: Type): Unit = jsig(tp.widenDealias, unboxedVCs = false)
 
     /** The signature of the upper-bound of a type parameter.
      *
@@ -232,7 +232,7 @@ object GenericSignatures {
     }
 
     @noinline
-    def jsig(tp0: Type, toplevel: Boolean = false, primitiveOK: Boolean = true): Unit = {
+    def jsig(tp0: Type, toplevel: Boolean = false, unboxedVCs: Boolean = true): Unit = {
 
       val tp = tp0.dealias
       tp match {
@@ -241,7 +241,7 @@ object GenericSignatures {
           val erasedUnderlying = fullErasure(ref.underlying.bounds.hi)
           // don't emit type param name if the param is upper-bounded by a primitive type (including via a value class)
           if erasedUnderlying.isPrimitiveValueType then
-            jsig(erasedUnderlying, toplevel, primitiveOK)
+            jsig(erasedUnderlying, toplevel, unboxedVCs)
           else typeParamSig(ref.paramName.lastPart)
 
         case defn.ArrayOf(elemtp) =>
@@ -269,15 +269,14 @@ object GenericSignatures {
           else if (sym == defn.NullClass)
             builder.append("Lscala/runtime/Null$;")
           else if (sym.isPrimitiveValueClass)
-            if (!primitiveOK) jsig(defn.ObjectType)
+            if (!unboxedVCs) jsig(defn.ObjectType)
             else if (sym == defn.UnitClass) jsig(defn.BoxedUnitClass.typeRef)
             else builder.append(defn.typeTag(sym.info))
           else if (sym.isDerivedValueClass) {
-            val erasedUnderlying = fullErasure(tp)
-            if (erasedUnderlying.isPrimitiveValueType && !primitiveOK)
-              classSig(sym, pre, args)
-            else
-              jsig(erasedUnderlying, toplevel, primitiveOK)
+            if (unboxedVCs) {
+              val erasedUnderlying = fullErasure(tp)
+              jsig(erasedUnderlying, toplevel)
+            } else classSig(sym, pre, args)
           }
           else if (defn.isSyntheticFunctionClass(sym)) {
             val erasedSym = defn.functionTypeErasure(sym).typeSymbol
@@ -286,7 +285,7 @@ object GenericSignatures {
           else if sym.isClass then
             classSig(sym, pre, args)
           else
-            jsig(erasure(tp), toplevel, primitiveOK)
+            jsig(erasure(tp), toplevel, unboxedVCs)
 
         case ExprType(restpe) if toplevel =>
           builder.append("()")
@@ -339,23 +338,23 @@ object GenericSignatures {
           val (reprParents, _) = splitIntersection(parents)
           val repr =
             reprParents.find(_.typeSymbol.is(TypeParam)).getOrElse(reprParents.head)
-          jsig(repr, primitiveOK = primitiveOK)
+          jsig(repr, unboxedVCs = unboxedVCs)
 
         case ci: ClassInfo =>
           val tParams = tp.typeParams
           if (toplevel) polyParamSig(tParams)
           superSig(ci.typeSymbol, ci.parents)
 
         case AnnotatedType(atp, _) =>
-          jsig(atp, toplevel, primitiveOK)
+          jsig(atp, toplevel, unboxedVCs)
 
         case hktl: HKTypeLambda =>
-          jsig(hktl.finalResultType, toplevel, primitiveOK)
+          jsig(hktl.finalResultType, toplevel, unboxedVCs)
 
         case _ =>
           val etp = erasure(tp)
           if (etp eq tp) throw new UnknownSig
-          else jsig(etp, toplevel, primitiveOK)
+          else jsig(etp, toplevel, unboxedVCs)
       }
     }
     val throwsArgs = sym0.annotations flatMap ThrownException.unapply

tests/pos/i10347/C_2.java

@@ -1,5 +1,5 @@
 public class C_2 {
   String hi = A.foo().head();
-  String hy = A.bar().head();
+  String hy = A.bar().head().s();
   String hj = A.baz("").head();
 }

tests/run/i10846.check

@@ -0,0 +1,3 @@
+i10846.V: scala.Option<i10846.V>
+i10846.U: scala.Option<i10846.U>
+i10846.W: scala.Option<i10846.W<scala.Function1<java.lang.Object, java.lang.String>>>

tests/run/i10846/i10846.scala

@@ -0,0 +1,28 @@
+// scalajs: --skip
+
+package i10846 {
+  final class V(val x: Int) extends AnyVal
+  object V { def get: Option[V] = null }
+
+  final class U(val y: String) extends AnyVal
+  object U { def get: Option[U] = null }
+
+  final class W[T](val z: T) extends AnyVal
+  object W { def get: Option[W[Int => String]] = null }
+}
+
+
+object Test extends scala.App {
+  def check[T](implicit tt: reflect.ClassTag[T]): Unit = {
+    val companion = tt.runtimeClass.getClassLoader.loadClass(tt.runtimeClass.getName + '$')
+    val get = companion.getMethod("get")
+    assert(get.getReturnType == classOf[Option[_]])
+    println(s"${tt.runtimeClass.getName}: ${get.getGenericReturnType}")
+  }
+
+  import i10846._
+
+  check[V]
+  check[U]
+  check[W[_]]
+}