Only nullfiy simple kinded types

compiler/src/dotty/tools/dotc/core/ImplicitNullInterop.scala

@@ -107,10 +107,13 @@ object ImplicitNullInterop {
         case tp: TypeRef if !tp.hasSimpleKind
           // We don't modify value types because they're non-nullable even in Java.
           || tp.symbol.isValueClass
+          || tp.isRef(defn.NullClass)
+          || tp.isRef(defn.NothingClass)
           // We don't modify unit types.
           || tp.isRef(defn.UnitClass)
           // We don't modify `Any` because it's already nullable.
           || tp.isRef(defn.AnyClass) => false
+        case tp: TypeParamRef if !tp.hasSimpleKind => false
         case _ => true
 
     // We don't nullify Java varargs at the top level.

tests/explicit-nulls/flexible-unpickle/Flexible_2.scala

@@ -1,6 +1,6 @@
 import unsafeNulls.Foo.*
 import unsafeNulls.Unsafe_1
-import unsafeNulls.{A, B, C, F, G, H, I, J, L, M, S, T, U, expects}
+import unsafeNulls.{A, B, C, F, G, H, I, J, L, M, N, S, T, U, expects}
 import scala.reflect.Selectable.reflectiveSelectable
 import scala.quoted.*
 
@@ -100,6 +100,11 @@ def Flexible_2() =
 
   val m: String = M.test(null)
 
+  // i23911
+  val n1: List[Map[String, Int]] = ???
+  val n2 = new N[List]()
+  val n3 = n2.accept[Any](n1)
+
   // i23845
   transparent inline def typeName[A]: String = ${typeNameMacro[A]}
 
@@ -109,3 +114,4 @@ def Flexible_2() =
     implicit val givenS: S[A] = ???
     expects(alphaTypeNameMacro[A])
   }
+

tests/explicit-nulls/flexible-unpickle/Unsafe_1.scala

@@ -73,6 +73,11 @@ object M {
   def test(input: => String): String = "foo " + input
 }
 
+
+class N[F[_]] {
+  def accept[A](arg: F[A]): Nothing = ???
+}
+
 class S[X]
 object S {  def show[X] = "dummyStr" }
 class T