Fix Mirror.Product for type lambdas

We don't need to guess how the type parameters should be substituted.
Instead we can use the result type directly to obtain the member type.
This exposed another issue - the mirrored type contained type variables.
To get rid of them we apply `Type.stripTypeVar` recursively.

Author: joroKr21

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

@@ -829,4 +829,11 @@ object TypeOps:
   def nestedPairs(ts: List[Type])(using Context): Type =
     ts.foldRight(defn.EmptyTupleModule.termRef: Type)(defn.PairClass.typeRef.appliedTo(_, _))
 
+  class StripTypeVarsMap(using Context) extends TypeMap:
+    def apply(tp: Type) = mapOver(tp).stripTypeVar
+
+  /** Apply [[Type.stripTypeVar]] recursively. */
+  def stripTypeVars(tp: Type)(using Context): Type =
+    new StripTypeVarsMap().apply(tp)
+
 end TypeOps

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

@@ -249,21 +249,12 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
           val cls = mirroredType.classSymbol
           val accessors = cls.caseAccessors.filterNot(_.isAllOf(PrivateLocal))
           val elemLabels = accessors.map(acc => ConstantType(Constant(acc.name.toString)))
+          val nestedPairs = TypeOps.nestedPairs(accessors.map(mirroredType.resultType.memberInfo(_).widenExpr))
           val (monoType, elemsType) = mirroredType match
             case mirroredType: HKTypeLambda =>
-              def accessorType(acc: Symbol) =
-                if cls.typeParams.hasSameLengthAs(mirroredType.paramRefs) then
-                  acc.info.subst(cls.typeParams, mirroredType.paramRefs)
-                else
-                  acc.info
-              val elems =
-                mirroredType.derivedLambdaType(
-                  resType = TypeOps.nestedPairs(accessors.map(accessorType))
-                )
-              (mkMirroredMonoType(mirroredType), elems)
+              (mkMirroredMonoType(mirroredType), mirroredType.derivedLambdaType(resType = nestedPairs))
             case _ =>
-              val elems = TypeOps.nestedPairs(accessors.map(mirroredType.memberInfo(_).widenExpr))
-              (mirroredType, elems)
+              (mirroredType, nestedPairs)
           val elemsLabels = TypeOps.nestedPairs(elemLabels)
           checkRefinement(formal, tpnme.MirroredElemTypes, elemsType, span)
           checkRefinement(formal, tpnme.MirroredElemLabels, elemsLabels, span)
@@ -344,7 +335,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       (using Context): Tree =
     if checkFormal(formal) then
       formal.member(tpnme.MirroredType).info match
-        case TypeBounds(mirroredType, _) => synth(mirroredType.stripTypeVar, formal, span)
+        case TypeBounds(mirroredType, _) => synth(TypeOps.stripTypeVars(mirroredType), formal, span)
         case other => EmptyTree
     else EmptyTree
 

tests/pos/i13859.scala

@@ -0,0 +1,31 @@
+import scala.deriving.*
+
+object Test:
+  type Kind1[C, O[_]] = C {
+    type MirroredType[X] = O[X]
+    type MirroredMonoType = O[Any]
+    type MirroredElemTypes[_] <: Tuple
+  }
+
+  type Kind2[C, O[_, _]] = C {
+    type MirroredType[X, Y] = O[X, Y]
+    type MirroredMonoType = O[Any, Any]
+    type MirroredElemTypes[_, _] <: Tuple
+  }
+
+  type Test[X] = (X, Boolean)
+  type Swap[X, Y] = (Y, X)
+
+  locally {
+    val x = summon[Kind1[Mirror.Product, Test]]
+    x: Mirror.Product {
+      type MirroredElemTypes[X] = (X, Boolean)
+    }
+  }
+
+  locally {
+    val x = summon[Kind2[Mirror.Product, Swap]]
+    x: Mirror.Product {
+      type MirroredElemTypes[X, Y] = (Y, X)
+    }
+  }