SI-9408 Record known subclasses of local classes

Using the same facility that we use to record subclasses of
sealed classes, record the subclasses of term-owned ("local")
classes.

I have changed existing callers of `children` to use `sealedChildren`
so we don't start using this new information in pattern matching
and type pattern checkability analysis.

The following commit will build on this to infer finality of local
classes in the context of outer pointer elision in the constructors
phase.

Author: retronym

src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala

@@ -138,7 +138,7 @@ trait TreeAndTypeAnalysis extends Debugging {
 
           if(grouped) {
             def enumerateChildren(sym: Symbol) = {
-              sym.children.toList
+              sym.sealedChildren.toList
                 .sortBy(_.sealedSortName)
                 .filterNot(x => x.isSealed && x.isAbstractClass && !isPrimitiveValueClass(x))
             }

src/compiler/scala/tools/nsc/typechecker/Checkable.scala

@@ -212,8 +212,8 @@ trait Checkable {
     )
     /** Are all children of these symbols pairwise irreconcilable? */
     def allChildrenAreIrreconcilable(sym1: Symbol, sym2: Symbol) = (
-      sym1.children.toList forall (c1 =>
-        sym2.children.toList forall (c2 =>
+      sym1.sealedChildren.toList forall (c1 =>
+        sym2.sealedChildren.toList forall (c2 =>
           areIrreconcilableAsParents(c1, c2)
         )
       )

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -1694,6 +1694,8 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
               psym addChild context.owner
             else
               pending += ParentSealedInheritanceError(parent, psym)
+          if (psym.isLocalToBlock && !phase.erasedTypes)
+            psym addChild context.owner
           val parentTypeOfThis = parent.tpe.dealias.typeOfThis
 
           if (!(selfType <:< parentTypeOfThis) &&

src/reflect/scala/reflect/internal/Symbols.scala

@@ -980,7 +980,7 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     private def isNotOverridden = (
       owner.isClass && (
            owner.isEffectivelyFinal
-        || owner.isSealed && owner.children.forall(c => c.isEffectivelyFinal && (overridingSymbol(c) == NoSymbol))
+        || (owner.isSealed && owner.sealedChildren.forall(c => c.isEffectivelyFinal && (overridingSymbol(c) == NoSymbol)))
       )
     )
 
@@ -2495,14 +2495,15 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     def associatedFile: AbstractFile = enclosingTopLevelClass.associatedFile
     def associatedFile_=(f: AbstractFile) { abort("associatedFile_= inapplicable for " + this) }
 
-    /** If this is a sealed class, its known direct subclasses.
+    /** If this is a sealed or local class, its known direct subclasses.
      *  Otherwise, the empty set.
      */
     def children: Set[Symbol] = Set()
+    final def sealedChildren: Set[Symbol] = if (!isSealed) Set.empty else children
 
     /** Recursively assemble all children of this symbol.
      */
-    def sealedDescendants: Set[Symbol] = children.flatMap(_.sealedDescendants) + this
+    final def sealedDescendants: Set[Symbol] = if (!isSealed) Set(this) else children.flatMap(_.sealedDescendants) + this
 
     @inline final def orElse(alt: => Symbol): Symbol = if (this ne NoSymbol) this else alt
     @inline final def andAlso(f: Symbol => Unit): Symbol = { if (this ne NoSymbol) f(this) ; this }