Fix 13493: compute union of child types for mirror

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -17,6 +17,8 @@ import transform.SyntheticMembers._
 import util.Property
 import annotation.{tailrec, constructorOnly}
 
+import scala.collection.mutable
+
 /** Synthesize terms for special classes */
 class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
   import ast.tpd._
@@ -339,7 +341,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
     if acceptable(mirroredType) && cls.isGenericSum(if useCompanion then cls.linkedClass else ctx.owner) then
       val elemLabels = cls.children.map(c => ConstantType(Constant(c.name.toString)))
 
-      def solve(sym: Symbol): Type = sym match
+      def solve(target: Type)(sym: Symbol): Type = sym match
         case childClass: ClassSymbol =>
           assert(childClass.isOneOf(Case | Sealed))
           if childClass.is(Module) then
@@ -350,36 +352,50 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
                 // Compute the the full child type by solving the subtype constraint
                 // `C[X1, ..., Xn] <: P`, where
                 //
-                //   - P is the current `mirroredType`
+                //   - P is the current `targetPart`
                 //   - C is the child class, with type parameters X1, ..., Xn
                 //
                 // Contravariant type parameters are minimized, all other type parameters are maximized.
-                def instantiate(using Context) =
-                  val poly = constrained(info, untpd.EmptyTree)._1
+                def instantiate(targetPart: Type)(using Context) =
+                  val poly = constrained(info)
                   val resType = poly.finalResultType
-                  val target = mirroredType match
-                    case tp: HKTypeLambda => tp.resultType
-                    case tp => tp
-                  resType <:< target
+                  resType <:< targetPart // record constraints
                   val tparams = poly.paramRefs
                   val variances = childClass.typeParams.map(_.paramVarianceSign)
                   val instanceTypes = tparams.lazyZip(variances).map((tparam, variance) =>
                     TypeComparer.instanceType(tparam, fromBelow = variance < 0))
                   resType.substParams(poly, instanceTypes)
-                instantiate(using ctx.fresh.setExploreTyperState().setOwner(childClass))
+
+                def instantiateAll(using Context): Type =
+
+                  // instantiate for each part of a union type, compute lub of the results
+                  def loop(explore: List[Type], acc: mutable.ListBuffer[Type]): Type = explore match
+                    case OrType(tp1, tp2) :: rest => loop(tp1 :: tp2 :: rest, acc                   )
+                    case tp               :: rest => loop(rest              , acc += instantiate(tp))
+                    case _                        => TypeComparer.lub(acc.toList)
+
+                  def instantiateLub(tp1: Type, tp2: Type): Type =
+                    loop(tp1 :: tp2 :: Nil, new mutable.ListBuffer[Type])
+
+                  target match
+                    case OrType(tp1, tp2) => instantiateLub(tp1, tp2)
+                    case _ => instantiate(target)
+
+                instantiateAll(using ctx.fresh.setExploreTyperState().setOwner(childClass))
               case _ =>
                 childClass.typeRef
         case child => child.termRef
       end solve
 
       val (monoType, elemsType) = mirroredType match
         case mirroredType: HKTypeLambda =>
+          val target = mirroredType.resultType
           val elems = mirroredType.derivedLambdaType(
-            resType = TypeOps.nestedPairs(cls.children.map(solve))
+            resType = TypeOps.nestedPairs(cls.children.map(solve(target)))
           )
           (mkMirroredMonoType(mirroredType), elems)
-        case _ =>
-          val elems = TypeOps.nestedPairs(cls.children.map(solve))
+        case target =>
+          val elems = TypeOps.nestedPairs(cls.children.map(solve(target)))
           (mirroredType, elems)
 
       val mirrorType =

tests/pos/i13493.scala

@@ -0,0 +1,43 @@
+import deriving.Mirror
+
+sealed trait Box[T]
+object Box
+
+case class Child[T](t: T) extends Box[T]
+
+object MirrorK1:
+  type Of[F[_]] = Mirror { type MirroredType[A] = F[A] }
+
+def testSums =
+
+  val foo = summon[Mirror.Of[Option[Int] | Option[String]]]
+  summon[foo.MirroredElemTypes =:= (None.type, Some[Int] | Some[String])]
+
+  val bar = summon[Mirror.Of[Box[Int] | Box[String]]]
+  summon[bar.MirroredElemTypes =:= ((Child[Int] | Child[String]) *: EmptyTuple)]
+
+  val qux = summon[Mirror.Of[Option[Int | String]]]
+  summon[qux.MirroredElemTypes =:= (None.type, Some[Int | String])]
+
+  val bip = summon[Mirror.Of[Box[Int | String]]]
+  summon[bip.MirroredElemTypes =:= (Child[Int | String] *: EmptyTuple)]
+
+  val bap = summon[MirrorK1.Of[[X] =>> Box[X] | Box[Int] | Box[String]]]
+  summon[bap.MirroredElemTypes[Boolean] =:= ((Child[Boolean] | Child[Int] | Child[String]) *: EmptyTuple)]
+
+
+def testProducts =
+  val foo = summon[Mirror.Of[Some[Int] | Some[String]]]
+  summon[foo.MirroredElemTypes =:= ((Int | String) *: EmptyTuple)]
+
+  val bar = summon[Mirror.Of[Child[Int] | Child[String]]]
+  summon[bar.MirroredElemTypes =:= ((Int | String) *: EmptyTuple)]
+
+  val qux = summon[Mirror.Of[Some[Int | String]]]
+  summon[foo.MirroredElemTypes =:= ((Int | String) *: EmptyTuple)]
+
+  val bip = summon[Mirror.Of[Child[Int | String]]]
+  summon[bip.MirroredElemTypes =:= ((Int | String) *: EmptyTuple)]
+
+  val bap = summon[MirrorK1.Of[[X] =>> Child[X] | Child[Int] | Child[String]]]
+  summon[bap.MirroredElemTypes[Boolean] =:= ((Boolean | Int | String) *: EmptyTuple)]