Support Mirrors for local and inner classes.

- update child accessibility check for anonymous
  mirrors in whyNotGenericSum. Now check the
  prefix at the callsite can access the child.
- for sum mirrors, compute a new prefix for each
  child from the callsite prefix of the parent,
  see TypeOps.childPrefix. For each child,
  subsititute its prefix at definition with the
  childPrefix using asSeenFrom. For polymorphic
  classes, perform the subsitution on the
  constructor before inferring constraints.
- add tests for issues 13332, 13935, 11174, 12328
- add tests for local/inner classes taken from
  Shapeless for its Generic type, backed
  by mirrors

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

@@ -868,4 +868,73 @@ object TypeOps:
   def stripTypeVars(tp: Type)(using Context): Type =
     new StripTypeVarsMap().apply(tp)
 
+  /** computes a prefix for `child`, derived from its common prefix with `pre`
+   *  - `pre` is assumed to be the prefix of `parent` at a given callsite.
+   *  - `child` is assumed to be the sealed child of `parent`, and reachable according to `whyNotGenericSum`.
+   */
+  def childPrefix(pre: Type, parent: Symbol, child: Symbol)(using Context): Type =
+    // Example, given this class hierarchy, we can see how this should work
+    // when summoning a mirror for `wrapper.Color`:
+    //
+    // package example
+    // object Outer3:
+    //   class Wrapper:
+    //     sealed trait Color
+    //   val wrapper = new Wrapper
+    //   object Inner:
+    //     case object Red extends wrapper.Color
+    //     case object Green extends wrapper.Color
+    //     case object Blue extends wrapper.Color
+    //
+    //   summon[Mirror.SumOf[wrapper.Color]]
+    //                       ^^^^^^^^^^^^^
+    //       > pre = example.Outer3.wrapper.type
+    //       > parent = sealed trait example.Outer3.Wrapper.Color
+    //       > child = module val example.Outer3.Innner.Red
+    //       > parentOwners = [example, Outer3, Wrapper] // computed from definition
+    //       > childOwners = [example, Outer3, Inner] // computed from definition
+    //       > parentRest = [Wrapper] // strip common owners from `childOwners`
+    //       > childRest = [Inner] // strip common owners from `parentOwners`
+    //       > commonPrefix = example.Outer3.type // i.e. parentRest has only 1 element, use 1st subprefix of `pre`.
+    //       > childPrefix = example.Outer3.Inner.type // select all symbols in `childRest` from `commonPrefix`
+
+    /** unwind the prefix into a sequence of sub-prefixes, selecting the one at `limit`
+     *  @return `NoType` if there is an unrecognised prefix type.
+     */
+    def subPrefixAt(pre: Type, limit: Int): Type =
+      def go(pre: Type, limit: Int): Type =
+        if limit == 0 then pre // EXIT: No More prefix
+        else pre match
+          case pre: ThisType          => go(pre.tref.prefix, limit - 1)
+          case pre: TermRef           => go(pre.prefix, limit - 1)
+          case _:SuperType | NoPrefix => pre.ensuring(limit == 1) // EXIT: can't rewind further than this
+          case _                      => NoType // EXIT: unrecognized prefix
+      go(pre, limit)
+    end subPrefixAt
+
+    /** Successively select each symbol in the `suffix` from `pre`, such that they are reachable. */
+    def selectAll(pre: Type, suffix: Seq[Symbol]): Type =
+      suffix.foldLeft(pre)((pre, sym) =>
+        pre.select(
+          if sym.isType && sym.is(Module) then sym.sourceModule
+          else sym
+        )
+      )
+
+    def stripCommonPrefix(xs: List[Symbol], ys: List[Symbol]): (List[Symbol], List[Symbol]) = (xs, ys) match
+      case (x :: xs1, y :: ys1) if x eq y => stripCommonPrefix(xs1, ys1)
+      case _ => (xs, ys)
+
+    val (parentRest, childRest) = stripCommonPrefix(
+      parent.owner.ownersIterator.toList.reverse,
+      child.owner.ownersIterator.toList.reverse
+    )
+
+    val commonPrefix = subPrefixAt(pre, parentRest.size) // unwind parent owners up to common prefix
+
+    if commonPrefix.exists then selectAll(commonPrefix, childRest)
+    else NoType
+
+  end childPrefix
+
 end TypeOps

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

@@ -26,9 +26,7 @@ class PostInlining extends MacroTransform, IdentityDenotTransformer:
     override def transform(tree: Tree)(using Context): Tree =
       super.transform(tree) match
         case tree1: Template
-        if tree1.hasAttachment(ExtendsSingletonMirror)
-          || tree1.hasAttachment(ExtendsProductMirror)
-          || tree1.hasAttachment(ExtendsSumMirror) =>
+        if tree1.hasAttachment(ExtendsSingletonMirror) || tree1.hasAttachment(ExtendsSumOrProductMirror) =>
           synthMbr.addMirrorSupport(tree1)
         case tree1 => tree1
 

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

@@ -163,28 +163,42 @@ object SymUtils:
     *     and also the location of the generated mirror.
     *   - all of its children are generic products, singletons, or generic sums themselves.
     */
-    def whyNotGenericSum(using Context): String =
+    def whyNotGenericSum(pre: Type)(using Context): String =
       if (!self.is(Sealed))
         s"it is not a sealed ${self.kindString}"
       else if (!self.isOneOf(AbstractOrTrait))
         "it is not an abstract class"
       else {
         val children = self.children
         val companionMirror = self.useCompanionAsSumMirror
+        val ownerScope = if pre.isInstanceOf[SingletonType] then pre.classSymbol else NoSymbol
         def problem(child: Symbol) = {
 
-          def isAccessible(sym: Symbol): Boolean =
-            (self.isContainedIn(sym) && (companionMirror || ctx.owner.isContainedIn(sym)))
-            || sym.is(Module) && isAccessible(sym.owner)
+          def accessibleMessage(sym: Symbol): String =
+            def inherits(sym: Symbol, scope: Symbol): Boolean =
+              !scope.is(Package) && (scope.derivesFrom(sym) || inherits(sym, scope.owner))
+            def isVisibleToParent(sym: Symbol): Boolean =
+              self.isContainedIn(sym) || sym.is(Module) && isVisibleToParent(sym.owner)
+            def isVisibleToScope(sym: Symbol): Boolean =
+              def isReachable: Boolean = ctx.owner.isContainedIn(sym)
+              def isMemberOfPrefix: Boolean =
+                ownerScope.exists && inherits(sym, ownerScope)
+              isReachable || isMemberOfPrefix || sym.is(Module) && isVisibleToScope(sym.owner)
+            if !isVisibleToParent(sym) then i"to its parent $self"
+            else if !companionMirror && !isVisibleToScope(sym) then i"to call site ${ctx.owner}"
+            else ""
+          end accessibleMessage
+
+          val childAccessible = accessibleMessage(child.owner)
 
           if (child == self) "it has anonymous or inaccessible subclasses"
-          else if (!isAccessible(child.owner)) i"its child $child is not accessible"
+          else if (!childAccessible.isEmpty) i"its child $child is not accessible $childAccessible"
           else if (!child.isClass) "" // its a singleton enum value
           else {
             val s = child.whyNotGenericProduct
             if s.isEmpty then s
             else if child.is(Sealed) then
-              val s = child.whyNotGenericSum
+              val s = child.whyNotGenericSum(pre)
               if s.isEmpty then s
               else i"its child $child is not a generic sum because $s"
             else
@@ -195,7 +209,7 @@ object SymUtils:
         else children.map(problem).find(!_.isEmpty).getOrElse("")
       }
 
-    def isGenericSum(using Context): Boolean = whyNotGenericSum.isEmpty
+    def isGenericSum(pre: Type)(using Context): Boolean = whyNotGenericSum(pre).isEmpty
 
     /** If this is a constructor, its owner: otherwise this. */
     final def skipConstructor(using Context): Symbol =

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

@@ -18,14 +18,15 @@ import NullOpsDecorator._
 
 object SyntheticMembers {
 
+  enum MirrorImpl:
+    case OfProduct(pre: Type)
+    case OfSum(childPres: List[Type])
+
   /** Attachment marking an anonymous class as a singleton case that will extend from Mirror.Singleton */
   val ExtendsSingletonMirror: Property.StickyKey[Unit] = new Property.StickyKey
 
   /** Attachment recording that an anonymous class should extend Mirror.Product */
-  val ExtendsProductMirror: Property.StickyKey[Unit] = new Property.StickyKey
-
-  /** Attachment recording that an anonymous class should extend Mirror.Sum */
-  val ExtendsSumMirror: Property.StickyKey[Unit] = new Property.StickyKey
+  val ExtendsSumOrProductMirror: Property.StickyKey[MirrorImpl] = new Property.StickyKey
 }
 
 /** Synthetic method implementations for case classes, case objects,
@@ -484,32 +485,41 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
    *  type MirroredMonoType = C[?]
    *  ```
    */
-  def fromProductBody(caseClass: Symbol, param: Tree)(using Context): Tree = {
-    val (classRef, methTpe) =
-      caseClass.primaryConstructor.info match {
+  def fromProductBody(caseClass: Symbol, param: Tree, optInfo: Option[MirrorImpl.OfProduct])(using Context): Tree =
+    def extractParams(tpe: Type): List[Type] =
+      tpe.asInstanceOf[MethodType].paramInfos
+
+    def computeFromCaseClass: (Type, List[Type]) =
+      val (baseRef, baseInfo) =
+        val rawRef = caseClass.typeRef
+        val rawInfo = caseClass.primaryConstructor.info
+        optInfo match
+          case Some(info) =>
+            (rawRef.asSeenFrom(info.pre, caseClass.owner), rawInfo.asSeenFrom(info.pre, caseClass.owner))
+          case _ =>
+            (rawRef, rawInfo)
+      baseInfo match
         case tl: PolyType =>
           val (tl1, tpts) = constrained(tl, untpd.EmptyTree, alwaysAddTypeVars = true)
           val targs =
             for (tpt <- tpts) yield
               tpt.tpe match {
                 case tvar: TypeVar => tvar.instantiate(fromBelow = false)
               }
-          (caseClass.typeRef.appliedTo(targs), tl.instantiate(targs))
+          (baseRef.appliedTo(targs), extractParams(tl.instantiate(targs)))
         case methTpe =>
-          (caseClass.typeRef, methTpe)
-      }
-    methTpe match {
-      case methTpe: MethodType =>
-        val elems =
-          for ((formal, idx) <- methTpe.paramInfos.zipWithIndex) yield {
-            val elem =
-              param.select(defn.Product_productElement).appliedTo(Literal(Constant(idx)))
-                .ensureConforms(formal.translateFromRepeated(toArray = false))
-            if (formal.isRepeatedParam) ctx.typer.seqToRepeated(elem) else elem
-          }
-        New(classRef, elems)
-    }
-  }
+          (baseRef, extractParams(methTpe))
+    end computeFromCaseClass
+
+    val (classRefApplied, paramInfos) = computeFromCaseClass
+    val elems =
+      for ((formal, idx) <- paramInfos.zipWithIndex) yield
+        val elem =
+          param.select(defn.Product_productElement).appliedTo(Literal(Constant(idx)))
+            .ensureConforms(formal.translateFromRepeated(toArray = false))
+        if (formal.isRepeatedParam) ctx.typer.seqToRepeated(elem) else elem
+    New(classRefApplied, elems)
+  end fromProductBody
 
   /** For an enum T:
    *
@@ -527,24 +537,36 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
    *  a wildcard for each type parameter. The normalized type of an object
    *  O is O.type.
    */
-  def ordinalBody(cls: Symbol, param: Tree)(using Context): Tree =
-    if (cls.is(Enum)) param.select(nme.ordinal).ensureApplied
-    else {
+  def ordinalBody(cls: Symbol, param: Tree, optInfo: Option[MirrorImpl.OfSum])(using Context): Tree =
+    if cls.is(Enum) then
+      param.select(nme.ordinal).ensureApplied
+    else
+      def computeChildTypes: List[Type] =
+        def rawRef(child: Symbol): Type =
+          if (child.isTerm) child.reachableTermRef else child.reachableRawTypeRef
+        optInfo match
+          case Some(info) => info
+            .childPres
+            .lazyZip(cls.children)
+            .map((pre, child) => rawRef(child).asSeenFrom(pre, child.owner))
+          case _ =>
+            cls.children.map(rawRef)
+      end computeChildTypes
+      val childTypes = computeChildTypes
       val cases =
-        for ((child, idx) <- cls.children.zipWithIndex) yield {
-          val patType = if (child.isTerm) child.reachableTermRef else child.reachableRawTypeRef
+        for (patType, idx) <- childTypes.zipWithIndex yield
           val pat = Typed(untpd.Ident(nme.WILDCARD).withType(patType), TypeTree(patType))
           CaseDef(pat, EmptyTree, Literal(Constant(idx)))
-        }
+
       Match(param.annotated(New(defn.UncheckedAnnot.typeRef, Nil)), cases)
-    }
+  end ordinalBody
 
   /** - If `impl` is the companion of a generic sum, add `deriving.Mirror.Sum` parent
    *    and `MirroredMonoType` and `ordinal` members.
    *  - If `impl` is the companion of a generic product, add `deriving.Mirror.Product` parent
    *    and `MirroredMonoType` and `fromProduct` members.
-   *  - If `impl` is marked with one of the attachments ExtendsSingletonMirror, ExtendsProductMirror,
-   *    or ExtendsSumMirror, remove the attachment and generate the corresponding mirror support,
+   *  - If `impl` is marked with one of the attachments ExtendsSingletonMirror or ExtendsSumOfProductMirror,
+   *    remove the attachment and generate the corresponding mirror support,
    *    On this case the represented class or object is referred to in a pre-existing `MirroredMonoType`
    *    member of the template.
    */
@@ -581,30 +603,33 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
     }
     def makeSingletonMirror() =
       addParent(defn.Mirror_SingletonClass.typeRef)
-    def makeProductMirror(cls: Symbol) = {
+    def makeProductMirror(cls: Symbol, optInfo: Option[MirrorImpl.OfProduct]) = {
       addParent(defn.Mirror_ProductClass.typeRef)
       addMethod(nme.fromProduct, MethodType(defn.ProductClass.typeRef :: Nil, monoType.typeRef), cls,
-        fromProductBody(_, _).ensureConforms(monoType.typeRef))  // t4758.scala or i3381.scala are examples where a cast is needed
+        fromProductBody(_, _, optInfo).ensureConforms(monoType.typeRef))  // t4758.scala or i3381.scala are examples where a cast is needed
     }
-    def makeSumMirror(cls: Symbol) = {
+    def makeSumMirror(cls: Symbol, optInfo: Option[MirrorImpl.OfSum]) = {
       addParent(defn.Mirror_SumClass.typeRef)
       addMethod(nme.ordinal, MethodType(monoType.typeRef :: Nil, defn.IntType), cls,
-        ordinalBody(_, _))
+        ordinalBody(_, _, optInfo))
     }
 
     if (clazz.is(Module)) {
       if (clazz.is(Case)) makeSingletonMirror()
-      else if (linked.isGenericProduct) makeProductMirror(linked)
-      else if (linked.isGenericSum) makeSumMirror(linked)
+      else if (linked.isGenericProduct) makeProductMirror(linked, None)
+      else if (linked.isGenericSum(NoType)) makeSumMirror(linked, None)
       else if (linked.is(Sealed))
-        derive.println(i"$linked is not a sum because ${linked.whyNotGenericSum}")
+        derive.println(i"$linked is not a sum because ${linked.whyNotGenericSum(NoType)}")
     }
     else if (impl.removeAttachment(ExtendsSingletonMirror).isDefined)
       makeSingletonMirror()
-    else if (impl.removeAttachment(ExtendsProductMirror).isDefined)
-      makeProductMirror(monoType.typeRef.dealias.classSymbol)
-    else if (impl.removeAttachment(ExtendsSumMirror).isDefined)
-      makeSumMirror(monoType.typeRef.dealias.classSymbol)
+    else
+      impl.removeAttachment(ExtendsSumOrProductMirror).match
+        case Some(prodImpl: MirrorImpl.OfProduct) =>
+          makeProductMirror(monoType.typeRef.dealias.classSymbol, Some(prodImpl))
+        case Some(sumImpl: MirrorImpl.OfSum) =>
+          makeSumMirror(monoType.typeRef.dealias.classSymbol, Some(sumImpl))
+        case _ =>
 
     cpy.Template(impl)(parents = newParents, body = newBody)
   }