fix scala#12634 - port sbt/zinc#979

Add sealedDescendants method to
SymDenotation - recursive children
of a symbol

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

@@ -1614,6 +1614,69 @@ object SymDenotations {
 
       annotations.collect { case Annotation.Child(child) => child }.reverse
     end children
+
+    /** Recursively assemble all children of this symbol, Preserves order of insertion.
+     */
+    final def sealedStrictDescendants(using Context): List[Symbol] =
+
+      @tailrec
+      def findLvlN(
+        explore: mutable.ArrayDeque[Symbol],
+        seen: util.HashSet[Symbol],
+        acc: mutable.ListBuffer[Symbol]
+      ): List[Symbol] =
+        if explore.isEmpty then
+          acc.toList
+        else
+          val sym      = explore.head
+          val explore1 = explore.dropInPlace(1)
+          val lvlN     = sym.children
+          val notSeen  = lvlN.filterConserve(!seen.contains(_))
+          if notSeen.isEmpty then
+            findLvlN(explore1, seen, acc)
+          else
+            findLvlN(explore1 ++= notSeen, {seen ++= notSeen; seen}, acc ++= notSeen)
+      end findLvlN
+
+      /** Scans through `explore` to see if there are recursive children.
+       *  If a symbol in `explore` has children that are not contained in
+       *  `lvl1`, fallback to `findLvlN`, or else return `lvl1`.
+       *
+       *  Avoids allocating if there are no recursive children in `lvl1`.
+       *  Allocates 1 HashSet if there are recursive children in `lvl1`, but we have seen them all.
+       *  Unbounded if `lvl1` has recursive children that are unseen.
+       */
+      def findLvl2(
+        lvl1: List[Symbol], explore: List[Symbol], seenOrNull: util.HashSet[Symbol] | Null
+      ): List[Symbol] = explore match
+        case sym :: explore1 =>
+          val lvl2 = sym.children
+          if lvl2.isEmpty then // no children, scan rest of explore1
+            findLvl2(lvl1, explore1, seenOrNull)
+          else // check if we have seen the children before
+            val seen = // initialise the seen set if not already
+              if seenOrNull != null then seenOrNull
+              else util.HashSet.from(lvl1)
+            val notSeen = lvl2.filterConserve(!seen.contains(_))
+            if notSeen.isEmpty then // we found children, but we had already seen them, scan the rest of explore1
+              findLvl2(lvl1, explore1, seen)
+            else // found unseen recursive children, we should fallback to the loop
+              findLvlN(
+                explore = mutable.ArrayDeque.from(explore1).appendAll(notSeen),
+                seen = {seen ++= notSeen; seen},
+                acc = mutable.ListBuffer.from(lvl1).appendAll(notSeen)
+              )
+        case nil =>
+          lvl1
+      end findLvl2
+
+      val lvl1 = children
+      findLvl2(lvl1, lvl1, seenOrNull = null)
+    end sealedStrictDescendants
+
+    /** Same as `sealedStrictDescendants` but prepends this symbol as well.
+     */
+    final def sealedDescendants(using Context): List[Symbol] = this.symbol :: sealedStrictDescendants
   }
 
   /** The contents of a class definition during a period

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -221,7 +221,7 @@ private class ExtractAPICollector(using Context) extends ThunkHolder {
     val modifiers = apiModifiers(sym)
     val anns = apiAnnotations(sym).toArray
     val topLevel = sym.isTopLevelClass
-    val childrenOfSealedClass = sym.children.sorted(classFirstSort).map(c =>
+    val childrenOfSealedClass = sym.sealedDescendants.sorted(classFirstSort).map(c =>
       if (c.isClass)
         apiType(c.typeRef)
       else

compiler/src/dotty/tools/dotc/util/HashSet.scala

@@ -7,6 +7,11 @@ object HashSet:
    */
   inline val DenseLimit = 8
 
+  def from[T](xs: IterableOnce[T]): HashSet[T] =
+    val set = new HashSet[T]()
+    set ++= xs
+    set
+
 /** A hash set that allows some privileged protected access to its internals
  *  @param  initialCapacity  Indicates the initial number of slots in the hash table.
  *                           The actual number of slots is always a power of 2, so the

sbt-test/source-dependencies/sealed-extends-sealed/A.scala

@@ -0,0 +1,4 @@
+sealed trait Z
+sealed trait A extends Z
+class B extends A
+class C extends A

sbt-test/source-dependencies/sealed-extends-sealed/App.scala

@@ -0,0 +1,6 @@
+object App {
+	def foo(z: Z) = z match {
+		case _: B =>
+		case _: C =>
+	}
+}

sbt-test/source-dependencies/sealed-extends-sealed/changes/A.scala

@@ -0,0 +1,5 @@
+sealed trait Z
+sealed trait A extends Z
+class B extends A
+class C extends A
+class D extends A

sbt-test/source-dependencies/sealed-extends-sealed/project/DottyInjectedPlugin.scala

@@ -0,0 +1,12 @@
+import sbt._
+import Keys._
+
+object DottyInjectedPlugin extends AutoPlugin {
+  override def requires = plugins.JvmPlugin
+  override def trigger = allRequirements
+
+  override val projectSettings = Seq(
+    scalaVersion := sys.props("plugin.scalaVersion"),
+    scalacOptions ++= Seq("-source:3.0-migration", "-Xfatal-warnings")
+  )
+}

sbt-test/source-dependencies/sealed-extends-sealed/test

@@ -0,0 +1,8 @@
+> compile
+
+# Introduce a new class C that also extends A
+$ copy-file changes/A.scala A.scala
+
+# App.scala needs recompiling because the pattern match in it
+# is no longer exhaustive, which emits a warning
+-> compile