Patch to avoid crash in #16351 (#16354)

This needs follow up but I don't have the time.

 - figure out why we crash and what is the right fix
 - fix the test to compile lib with 2.13

Fixes #16351  (provisionally)

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

@@ -13,6 +13,7 @@ import collection.mutable
 import ast.Trees._
 import core.NameKinds.SuperArgName
 import SymUtils._
+import core.Decorators.*
 
 object HoistSuperArgs {
   val name: String = "hoistSuperArgs"
@@ -181,7 +182,9 @@ class HoistSuperArgs extends MiniPhase with IdentityDenotTransformer { thisPhase
 
     /** Hoist complex arguments in super call out of the class. */
     def hoistSuperArgsFromCall(superCall: Tree, cdef: DefDef, lifted: mutable.ListBuffer[Symbol]): Tree = superCall match
-      case Block(defs, expr) =>
+      case Block(defs, expr) if !expr.symbol.owner.is(Scala2x) =>
+        // MO: The guard avoids the crash for #16351.
+        // It would be good to dig deeper, but I won't have the time myself to do it.
         cpy.Block(superCall)(
           stats = defs.mapconserve {
             case vdef: ValDef =>

sbt-test/scala2-compat/i16351/app/App.scala

@@ -0,0 +1,8 @@
+package app
+
+import lib.*
+
+object App {
+  def main(args: Array[String]): Unit =
+    new Lib(Value("Foo"), b = 2) {}
+}

sbt-test/scala2-compat/i16351/build.sbt

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

sbt-test/scala2-compat/i16351/lib/lib.scala

@@ -0,0 +1,10 @@
+// Should be compiled with 2.13
+package lib
+
+class Value(val value: String)
+
+class Lib(
+  value: => Value,
+  a: Int = 0,
+  b: Int
+)

sbt-test/scala2-compat/i16351/test

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