Test stdlib-bootstrpped UnrolledBuffer null default getter (#18084)

Author: nicolasstucki

project/TastyMiMaFilters.scala

@@ -53,7 +53,9 @@ object TastyMiMaFilters {
     ProblemMatcher.make(ProblemKind.MissingParent, "scala.ScalaReflectionException$"),
     ProblemMatcher.make(ProblemKind.MissingParent, "scala.UninitializedFieldError$"),
 
-    // Problem: ConstantType for `null` versus `scala.Null`
+    // Probably OK: ConstantType for `null` versus `scala.Null`
+    // Calls to the default getter seem to link correctly.
+    // Tested in stdlib-bootstrapped/test/scala/collection/UnrolledBufferTest.scala
     ProblemMatcher.make(ProblemKind.IncompatibleTypeChange, "scala.collection.mutable.UnrolledBuffer.Unrolled.<init>$default$4"),
 
     // Problem: Missing type arguments with higher-kinded types

stdlib-bootstrapped-tasty-tests/src/Main.scala

@@ -17,6 +17,7 @@ object HelloWorld:
     testScala2ObjectParents()
     testScala2CaseClassUnderscoreMembers()
     testScalaNumberUnderlying()
+    scala.collection.mutable.UnrolledBufferTest.test()
   }
 
   def testScala2UnapplySignatures() = {

stdlib-bootstrapped-tasty-tests/src/UnrolledBufferTest.scala

@@ -0,0 +1,7 @@
+package scala.collection
+package mutable
+
+object UnrolledBufferTest:
+  def test(): Unit =
+    new UnrolledBuffer.Unrolled[Int](0, Array.empty[Int], null, null)
+    new UnrolledBuffer.Unrolled[Int](0, Array.empty[Int], null) // using default null argument