Fix Mirror.Product for type lambdas

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)
+    }
+  }