Make annotation reading lazier in Scala2Unpickler

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -1011,9 +1011,9 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
    */
   protected def deferredAnnot(end: Int)(using Context): Annotation = {
     val start = readIndex
-    val atp = readTypeRef()
     val phase = ctx.phase
-    Annotation.deferred(atp.typeSymbol)(
+    Annotation.deferredSymAndTree(
+        atReadPos(start, () => atPhase(phase)(readTypeRef().typeSymbol)))(
         atReadPos(start, () => atPhase(phase)(readAnnotationContents(end))))
   }
 

sbt-dotty/sbt-test/scala2-compat/i12109/app/App.scala

@@ -0,0 +1,3 @@
+import i12109.*
+
+def test = JsonReaderDefaultValue

sbt-dotty/sbt-test/scala2-compat/i12109/build.sbt

@@ -0,0 +1,13 @@
+val scala3Version = sys.props("plugin.scalaVersion")
+val scala2Version = "2.13.5"
+
+lazy val lib = project.in(file("lib"))
+  .settings(
+    scalaVersion := scala2Version
+  )
+
+lazy val app = project.in(file("app"))
+  .dependsOn(lib)
+  .settings(
+    scalaVersion := scala3Version
+  )

sbt-dotty/sbt-test/scala2-compat/i12109/lib/Lib.scala

@@ -0,0 +1,10 @@
+package i12109
+
+object JsonReaderDefaultValue extends LowPriorityDefaultValue {
+  class ReaderDefaultValue extends scala.annotation.StaticAnnotation
+}
+
+trait LowPriorityDefaultValue {
+  @JsonReaderDefaultValue.ReaderDefaultValue
+  class NoDefaultValue
+}

sbt-dotty/sbt-test/scala2-compat/i12109/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/scala2-compat/i12109/test

@@ -0,0 +1 @@
+> app/compile