Using Scala 3 seriously breaks default value handing

Minimal example:

case class A(a: Int = 1)

import upickle.default._
implicit val rw: ReadWriter[A] = macroRW[A]

println(read[A]("{\"a\":2}"))
println(read[A]("{}"))

Expected (upickle 1.4.0, Scala 2.13.6):

A(2)
A(1)

Got (upickle 1.4.0, Scala 3.0.1):

A(2)
A(2)

God knows how many projects are silently corrupting their databases. Or leaking data from another user in their API calls. Since the parser is keeping and leaking state, everything is possible.

[jodersky]

Maven doesn’t work that way, you can’t unpublish something. Also, that would break even more people’s deployments. That being said, this is a very high prio issue of course.

[lihaoyi]

@jodersky if I’m not mistaken I think this is the fix

lihaoyi upickle$ git diff
diff --git a/build.sc b/build.sc
index 133b30f..8664795 100644
--- a/build.sc
+++ b/build.sc
@@ -14,7 +14,7 @@ import com.github.lolgab.mill.mima._
 val scala211  = "2.11.12"
 val scala212  = "2.12.13"
 val scala213  = "2.13.4"
-val scala3    = "3.0.0"
+val scala3    = "3.0.2"
 val scalaJS06 = "0.6.33"
 val scalaJS1  = "1.5.1"
 val scalaNative = "0.4.0"
diff --git a/implicits/src-3/upickle/implicits/CaseClassReader.scala b/implicits/src-3/upickle/implicits/CaseClassReader.scala
index de66b16..e21a6b1 100644
--- a/implicits/src-3/upickle/implicits/CaseClassReader.scala
+++ b/implicits/src-3/upickle/implicits/CaseClassReader.scala
@@ -11,9 +11,8 @@ trait CaseClassReaderPiece extends MacrosCommon:

     def visitorForKey(currentKey: String): Visitor[_, _]

-    private val builder = collection.mutable.Map.empty[String, Any]
-
     override def visitObject(length: Int, index: Int) = new ObjVisitor[Any, T] {
+      private val builder = collection.mutable.Map.empty[String, Any]
       var currentKey: String = null

       def subVisitor: Visitor[_, _] = visitorForKey(currentKey)
diff --git a/implicits/src-3/upickle/implicits/CaseClassWriter.scala b/implicits/src-3/upickle/implicits/CaseClassWriter.scala
index f2802c0..f49368c 100644
--- a/implicits/src-3/upickle/implicits/CaseClassWriter.scala
+++ b/implicits/src-3/upickle/implicits/CaseClassWriter.scala
@@ -14,8 +14,8 @@ trait CaseClassWriterPiece extends MacrosCommon:
       var n = 0
       for
         (name, _, value) <- elemsInfo(v)
-        defaultValue <- defaultParams.get(name)
-        if defaultValue != value || serializeDefaults
+        defaultValue = defaultParams.get(name)
+        if serializeDefaults || defaultValue.isEmpty || defaultValue.get != value
       do n += 1
       n
     end length

Looking at the Scala 3 macro code, it diverges from the Scala 2 implementation considerably; I haven’t benched it, but I bet the Scala 3 performance won’t be as good, as it seems to allocate a ton of stuff (it uses for-loops!? collections!?!? hashtables!?!?!?) that the Scala 2 version painstakingly avoids. We should definitely look into unifying them. They may do the same thing, but for a performance critical piece of code the Scala 2 version is what we want

Regardless, this small patch should fix this particular issue