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 +1614,26 @@ object SymDenotations {
 
       annotations.collect { case Annotation.Child(child) => child }.reverse
     end children
+
+    /** Recursively assemble all children of this symbol, including this symbol.
+     *  Preserves order of insertion.
+     */
+    final def sealedDescendants(using Context): List[Symbol] =
+      @tailrec
+      def loop(syms: List[Symbol], acc: List[Symbol]): List[Symbol] = syms match
+        case sym :: syms1 =>
+          val notSeen = sym.children.filterConserve(!acc.contains(_))
+          if notSeen.isEmpty then
+            loop(syms1, acc)
+          else
+            loop(syms1 ::: notSeen, acc ::: notSeen)
+        case nil =>
+          acc
+      end loop
+
+      val lvl1 = children
+      loop(lvl1, this.symbol::lvl1)
+    end sealedDescendants
   }
 
   /** 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

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