Fix Tuple.toList return type

This method failed for any subtype of `Tuple`.
It only worked for `Tuple` and `TupleN` as these define their own `toList`.

Fixes scala#12721

Author: nicolasstucki

library/src/scala/Tuple.scala

@@ -12,9 +12,8 @@ sealed trait Tuple extends Product {
     runtime.Tuples.toArray(this)
 
   /** Create a copy this tuple as a List */
-  inline def toList: List[Union[this.type]] =
-    this.productIterator.toList
-      .asInstanceOf[List[Union[this.type]]]
+  inline def toList[This >: this.type <: Tuple]: List[Union[This]] =
+    this.productIterator.toList.asInstanceOf[List[Union[This]]]
 
   /** Create a copy this tuple as an IArray */
   inline def toIArray: IArray[Object] =