Encode parent trait private members in extraHash to propagate TraitPrivateMembersModified across external dependency

internal/zinc-core/src/main/scala/sbt/internal/inc/Incremental.scala

@@ -82,6 +82,8 @@ object Incremental {
 
     def previousAnalysisPruned: Analysis
 
+    def previousAnalysis: Analysis
+
     /**
      * @return true when the compilation cycle is compiling all the sources; false, otherwise.
      */
@@ -885,7 +887,18 @@ private final class AnalysisCallback(
       case d @ (DefinitionType.ClassDef | DefinitionType.Trait) =>
         val extraApiHash = {
           if (d != DefinitionType.Trait) apiHash
-          else HashAPI(_.hashAPI(classApi), includePrivateDefsInTrait = true)
+          else {
+            val currentExtraHash = HashAPI(_.hashAPI(classApi), includePrivateDefsInTrait = true)
+            incHandlerOpt match {
+              case Some(handler) =>
+                val analysis = handler.previousAnalysis
+                val parents = analysis.relations.inheritance.external.forward(className)
+                val parentsAPI = parents.map(analysis.apis.externalAPI)
+                val parentsHashes = parentsAPI.map(_.extraHash())
+                parentsHashes.fold(currentExtraHash)(_ ^ _)
+              case None => currentExtraHash
+            }
+          }
         }
 
         classApis(className) = ApiInfo(apiHash, extraApiHash, savedClassApi)

internal/zinc-core/src/main/scala/sbt/internal/inc/IncrementalCommon.scala

@@ -98,6 +98,7 @@ private[inc] abstract class IncrementalCommon(
         invalidatedSources,
         classfileManager,
         pruned,
+        previous,
         classesToRecompile,
         profiler.registerCycle(
           invalidatedClasses,
@@ -148,6 +149,7 @@ private[inc] abstract class IncrementalCommon(
         invalidatedSources: Set[VirtualFile],
         classFileManager: XClassFileManager,
         pruned: Analysis,
+        override val previousAnalysis: Analysis,
         classesToRecompile: Set[String],
         registerCycle: (Set[String], APIChanges, Set[String], Boolean) => Unit
     ) extends IncrementalCallback(classFileManager) {

internal/zinc-scripted/src/test/scala/sbt/internal/inc/IncHandler.scala

@@ -508,7 +508,8 @@ case class ProjectStructure(
     compile(i).map { analysis =>
       discoverMainClasses(Some(analysis.apis)) match {
         case Seq(mainClassName) =>
-          val cp = ((i.si.allJars.map(_.toPath) :+ classesDir) ++ outputJar).map(_.toAbsolutePath)
+          val jars = i.si.allJars.map(_.toPath)
+          val cp = (jars ++ (unmanagedJars :+ output) ++ internalClasspath).map(_.toAbsolutePath)
           val loader = ClasspathUtil.makeLoader(cp, i.si, baseDirectory)
           val buffer = new ByteArrayOutputStream(8192)
           val oldOut = System.out

zinc/src/sbt-test/source-dependencies/trait-private-val-transitive-inheritance/a/A.scala

@@ -0,0 +1,5 @@
+package a
+
+trait A {
+  private val a1: String = ""
+}

zinc/src/sbt-test/source-dependencies/trait-private-val-transitive-inheritance/a/changes/A.scala

@@ -0,0 +1,5 @@
+package a
+
+trait A {
+  private val a2: String = ""
+}

zinc/src/sbt-test/source-dependencies/trait-private-val-transitive-inheritance/b/B.scala

@@ -0,0 +1,3 @@
+package b
+
+trait B extends a.A

zinc/src/sbt-test/source-dependencies/trait-private-val-transitive-inheritance/build.json

@@ -0,0 +1,8 @@
+{
+  "projects": [
+    {"name": "main", "dependsOn": ["a", "b","c"], "scalaVersion": "2.13.12"},
+    {"name": "c", "dependsOn": ["a","b"], "scalaVersion": "2.13.12"},
+    {"name": "b", "dependsOn": ["a"], "scalaVersion": "2.13.12"},
+    {"name": "a", "scalaVersion": "2.13.12"}
+  ]
+}

zinc/src/sbt-test/source-dependencies/trait-private-val-transitive-inheritance/c/C.scala

@@ -0,0 +1,3 @@
+package c
+
+class C extends b.B

zinc/src/sbt-test/source-dependencies/trait-private-val-transitive-inheritance/main/Main.scala

@@ -0,0 +1,4 @@
+object Main extends App {
+  val impl = new c.C()
+  println("OK")
+}

zinc/src/sbt-test/source-dependencies/trait-private-val-transitive-inheritance/test

@@ -0,0 +1,3 @@
+> main/run
+$ copy-file a/changes/A.scala a/A.scala
+> main/run