Record anonymous child classes for exhaustivity check

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -842,10 +842,7 @@ class TreeUnpickler(reader: TastyReader,
       if (!sym.isType) { // Only terms might have leaky aliases, see the documentation of `checkNoPrivateLeaks`
         sym.info = ta.avoidPrivateLeaks(sym, tree.pos)
       }
-      if ((sym.isClass || sym.is(CaseVal)) && sym.isLocal)
-        // Child annotations for local classes and enum values are not pickled, so
-        // need to be re-established here.
-        sym.registerIfChild(late = true)
+
       sym.defTree = tree
 
       if (ctx.mode.is(Mode.ReadComments)) {

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -8,6 +8,7 @@ import Symbols._
 import SymDenotations._
 import Names._
 import NameOps._
+import StdNames._
 import NameKinds._
 import Flags._
 import Annotations._
@@ -132,16 +133,17 @@ class SymUtils(val self: Symbol) extends AnyVal {
 
   /** If this symbol is an enum value or a named class, register it as a child
    *  in all direct parent classes which are sealed.
-   *   @param  @late  If true, register only inaccessible children (all others are already
-   *                  entered at this point).
    */
-  def registerIfChild(late: Boolean = false)(implicit ctx: Context): Unit = {
+  def registerIfChild()(implicit ctx: Context): Unit = {
     def register(child: Symbol, parent: Type) = {
       val cls = parent.classSymbol
-      if (cls.is(Sealed) && (!late || child.isInaccessibleChildOf(cls)))
-        cls.addAnnotation(Annotation.Child(child))
+      if (cls.is(Sealed)) {
+        if ((child.isInaccessibleChildOf(cls) || child.isAnonymousClass) && !self.hasAnonymousChild)
+          cls.addAnnotation(Annotation.Child(cls))
+        else cls.addAnnotation(Annotation.Child(child))
+      }
     }
-    if (self.isClass && !self.isAnonymousClass)
+    if (self.isClass && !self.isEnumAnonymClass)
       self.asClass.classParents.foreach { parent =>
         val child = if (self.is(Module)) self.sourceModule else self
         register(child, parent)
@@ -150,6 +152,10 @@ class SymUtils(val self: Symbol) extends AnyVal {
       register(self, self.info)
   }
 
+  /** Does this symbol refer to anonymous classes synthesized by enum desugaring? */
+  def isEnumAnonymClass(implicit ctx: Context): Boolean =
+    self.isAnonymousClass && (self.owner.name.eq(nme.DOLLAR_NEW) || self.owner.is(CaseVal))
+
   /** Is this symbol defined locally (i.e. at some level owned by a term) and
    *  defined in a different toplevel class than its supposed parent class `cls`?
    *  Such children are not pickled, and have to be reconstituted manually.
@@ -163,6 +169,9 @@ class SymUtils(val self: Symbol) extends AnyVal {
       case Annotation.Child(child) => child
     }
 
+  def hasAnonymousChild(implicit ctx: Context): Boolean =
+    children.exists(_ `eq` self)
+
   /** Is symbol directly or indirectly owned by a term symbol? */
   @tailrec final def isLocal(implicit ctx: Context): Boolean = {
     val owner = self.owner

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -730,6 +730,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
     val res =
       (tp.classSymbol.is(Sealed) &&
         tp.classSymbol.is(AbstractOrTrait) &&
+        !tp.classSymbol.hasAnonymousChild &&
         tp.classSymbol.children.nonEmpty ) ||
       dealiasedTp.isInstanceOf[OrType] ||
       (dealiasedTp.isInstanceOf[AndType] && {
@@ -849,6 +850,8 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
           if (mergeList) "_: _*" else "_: List"
         else if (scalaConsType.isRef(sym))
           if (mergeList) "_, _: _*"  else "List(_, _: _*)"
+        else if (tp.classSymbol.is(Sealed) && tp.classSymbol.hasAnonymousChild)
+          "_: " + showType(tp) + " (anonymous)"
         else if (tp.classSymbol.is(CaseClass) && !hasCustomUnapply(tp.classSymbol))
         // use constructor syntax for case class
           showType(tp) + params(tp).map(_ => "_").mkString("(", ", ", ")")

tests/patmat/anonym.check

@@ -0,0 +1 @@
+10: Pattern Match Exhaustivity: _: Base (anonymous)

tests/patmat/anonym.scala

@@ -0,0 +1,21 @@
+// spurious dead-code
+class Foo {
+  def bar(x: Either[String, Base]): Unit = {
+    x match {
+      case Left(s) => println("l")
+      case Right(Base(s)) => println("r")
+    }
+  }
+
+  def bar(x: Base): Unit = x match {
+    case Zero =>
+  }
+
+  def anonymous: Base = new Base("bla") {}
+}
+
+sealed abstract case class Base(value: String) {
+  override def toString = value // ok?
+}
+
+object Zero extends Base("zero")

tests/patmat/t9677.check

@@ -0,0 +1 @@
+18: Pattern Match Exhaustivity: _: Base (anonymous)

tests/patmat/t9677.scala

@@ -15,7 +15,7 @@ object ExhaustiveMatchWarning {
 
   def test: Unit = {
     val b: Base = A("blabla")
-    b match {
+    b match {    // A.apply creates anonymous class <: Base
       case A.Root => println("Root")
       case path: A => println("Not root")
     }