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.

Author: joroKr21

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)

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)
+    }
+  }