Refine constraining scheme for result types

 - Take MatchTypes into account
 - Add test cases for tests that failed originally and would otherwise fail again
   after this PR. i19415.scala is fixed by the MatchType extension. i19749.scala was
   fixed by adding a `transparent`.

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

@@ -131,13 +131,21 @@ object ProtoTypes {
 
       constFoldException(pt) || {
         if Inlines.isInlineable(meth) then
-          // Stricter behaviour in 3.4+: do not apply `wildApprox` to non-transparent inlines
+          // Stricter behavisour in 3.4+: do not apply `wildApprox` to non-transparent inlines
+          // unless their return type is a MatchType. In this case there's no reason
+          // not to constrain type variables in the expected type. For transparent inlines
+          // we do not want to constrain type variables in the expected type since the
+          // actual return type might be smaller after instantiation. For inlines returning
+          // MatchTypes we do not want to constrain because the MatchType might be more
+          // specific after instantiation. TODO: Should we also use Wildcards for non-inline
+          // methods returning MatchTypes?
           if Feature.sourceVersion.isAtLeast(SourceVersion.`3.4`) then
-            if meth.is(Transparent) then
+            if meth.is(Transparent) || mt.resultType.isMatchAlias then
               constrainResult(mt, wildApprox(pt))
               // do not constrain the result type of transparent inline methods
               true
             else
+              println(i"OLD for $meth: $mt vs $pt")
               constrainResult(mt, pt)
           else
             // Best-effort to fix https://github.com/scala/scala3/issues/9685 in the 3.3.x series

tests/pos/i19415.scala

@@ -0,0 +1,24 @@
+def Test = {
+  val left: Parser[String] = ???
+  val right: Parser[Int] = ???
+  val both = left && right
+
+  val works = both.map(Ior.Both.apply)
+  val fails = (left && right).map(Ior.Both.apply)
+}
+
+trait Parser[T]:
+  final def &&[T2](other: Parser[T2])(implicit zip: Zip[T, T2]): Parser[zip.Out] = ???
+  final def map[T2](f: T => T2): Parser[T2] = ???
+
+infix trait Ior[+A, +B]
+object Ior:
+  final case class Both[+A, +B](a: A, b: B) extends (A Ior B)
+
+trait Zip[In1, In2]:
+  type Out
+
+object Zip {
+  type Out[In1, In2, O] = Zip[In1, In2] { type Out = O }
+  implicit def zip2[_1, _2]: Zip.Out[_1, _2, (_1, _2)] = ???
+}

tests/pos/i19479.scala

@@ -0,0 +1,54 @@
+case class Person(id: Int)
+
+class GeodeContinuousSourceSpec {
+  summon[PdxEncoder[Person]]
+}
+
+trait PdxEncoder[A] {
+  def encode(a: A): Boolean
+}
+
+object PdxEncoder extends ObjectEncoder {
+  implicit def intEncoder: PdxEncoder[Int] = ???
+}
+
+trait ObjectEncoder {
+  given emptyTupleEncoder: PdxEncoder[EmptyTuple] = ???
+
+  given tupleEncoder[K <: String, H, T <: Tuple](using
+      m: ValueOf[K],
+      hEncoder: PdxEncoder[H],
+      tEncoder: PdxEncoder[T]
+  ): PdxEncoder[FieldType[K, H] *: T] = ???
+
+  given objectEncoder[A, Repr <: Tuple](using
+      gen: LabelledGeneric.Aux[A, Repr],
+      tupleEncoder: PdxEncoder[Repr]
+  ): PdxEncoder[A] = ???
+}
+
+import scala.deriving.Mirror
+
+private type FieldType[K, +V] = V & KeyTag[K, V]
+private type KeyTag[K, +V]
+private type ZipWith[T1 <: Tuple, T2 <: Tuple, F[_, _]] <: Tuple = (T1, T2) match {
+  case (h1 *: t1, h2 *: t2) => F[h1, h2] *: ZipWith[t1, t2, F]
+  case (EmptyTuple, ?)      => EmptyTuple
+  case (?, EmptyTuple)      => EmptyTuple
+  case _                    => Tuple
+}
+
+private trait LabelledGeneric[A] {
+  type Repr
+}
+
+private object LabelledGeneric {
+  type Aux[A, R] = LabelledGeneric[A] { type Repr = R }
+
+  transparent inline given productInst[A <: Product](using
+      m: Mirror.ProductOf[A]
+  ): LabelledGeneric.Aux[A, ZipWith[m.MirroredElemLabels, m.MirroredElemTypes, FieldType]] =
+    new LabelledGeneric[A] {
+      type Repr = Tuple & ZipWith[m.MirroredElemLabels, m.MirroredElemTypes, FieldType]
+    }
+}