Fix #18484: Query every legal child of sealed class on children call

Sometimes, a macro or Mirror can be called/generated for a type that
may have not had its children registered by natural typechecking
procedures of the compiler. This meant that, in those cases, the Mirror
could not have been generated correctly, and macro could not obtain the
children of the class.

This was partially fixed in a previous PR by introducing a separate
procedure for querying children, outside of the regular typechecking
procedure. However, they was only queried in the owner symbol or in the
companion class, when non-enum sealed classes specifically, can
appear anywhere in the file. This commit aims to rectify this by
searching for any class that appears in the same file as the sealed
type.

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

@@ -1688,7 +1688,7 @@ object SymDenotations {
      */
     def children(using Context): List[Symbol] =
 
-      def completeChildrenIn(owner: Symbol)(using Context) =
+      def completeChildrenIn(owner: Symbol, recursively: Boolean)(using Context): Unit =
         // Possible children are: classes extending Scala classes and
         // Scala or Java enum values that are defined in owner.
         // If owner is a package, we complete only
@@ -1698,20 +1698,28 @@ object SymDenotations {
           && (!owner.is(Package)
              || sym.originDenotation.infoOrCompleter.match
                   case _: SymbolLoaders.SecondCompleter => sym.associatedFile == this.symbol.associatedFile
-                  case _ => false)
+                  case _: ClassInfo if recursively && sym.isValidInCurrentRun =>
+                    sym.is(Package) || sym.associatedFile == this.symbol.associatedFile
+                  case other => false)
 
         if owner.isClass then
           for c <- owner.info.decls.toList if maybeChild(c) do
             c.ensureCompleted()
+            if recursively then completeChildrenIn(c, recursively)
       end completeChildrenIn
 
       if is(Sealed) || isAllOf(JavaEnumTrait) then
         if !is(ChildrenQueried) then
-          // Make sure all visible children are completed, so that
-          // they show up in Child annotations. A possible child is visible if it
-          // is defined in the same scope as `cls` or in the companion object of `cls`.
-          completeChildrenIn(owner)
-          completeChildrenIn(companionClass)
+          if !is(Enum) then // non-enum sealed class
+            // Make sure all children are completed, so that
+            // they show up in Child annotations.
+            completeChildrenIn(defn.RootClass, recursively = true)
+          else
+            // Make sure all visible children are completed, so that
+            // they show up in Child annotations. A possible child is visible if it
+            // is defined in the same scope as `cls` or in the companion object of `cls`.
+            completeChildrenIn(owner, recursively = false)
+            completeChildrenIn(companionClass, recursively = false)
           setFlag(ChildrenQueried)
 
       annotations.collect { case Annotation.Child(child) => child }.reverse

tests/pos/i18484/Macro_1.scala

@@ -0,0 +1,11 @@
+object Macro {
+  import scala.quoted.*
+
+  def subtypesImpl[A: Type](using quotes: Quotes): Expr[String] = {
+    import quotes.reflect.*
+    val a = TypeRepr.of[A].typeSymbol.children
+    '{""}
+  }
+
+  inline def subtypes[A]: String = ${ subtypesImpl[A] }
+}

tests/pos/i18484/Test_2.scala

@@ -0,0 +1,21 @@
+class Test {
+  def test(): Unit = {
+    scala.compiletime.testing.typeCheckErrors("Macro.subtypes[top.inner.Inner.Shape]")
+  }
+}
+
+package top {
+  package inner {
+    object Inner:
+      sealed trait Shape
+      case class Circle(center: Int, rad: Double) extends Shape
+      object Inner2:
+        case class Circle2(center: Int, rad: Double) extends Shape
+  }
+  case class Circle3(center: Int, rad: Double) extends inner.Inner.Shape
+}
+case class Circle4(center: Int, rad: Double) extends top.inner.Inner.Shape
+
+package top2 {
+  case class Circle5(center: Int, rad: Double) extends top.inner.Inner.Shape
+}