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

compiler/test/dotc/pos-test-pickling.blacklist

@@ -41,6 +41,7 @@ i5720.scala
 # Tuples
 toexproftuple.scala
 i7580.scala
+i12721.scala
 
 # Nullability
 nullable.scala

compiler/test/dotc/run-test-pickling.blacklist

@@ -22,6 +22,7 @@ tuple-zip.scala
 tuples1.scala
 tuples1a.scala
 tuples1b.scala
+toList.scala
 typeCheckErrors.scala
 typeclass-derivation-doc-example.scala
 typeclass-derivation1.scala

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] =

tests/pos/i12721.scala

@@ -0,0 +1,7 @@
+def bar(t: Any): Int = 1
+def foo(t: AnyRef): Unit =
+  (??? : Tuple).toList.map(bar)
+  (??? : EmptyTuple).toList.map(bar)
+  (??? : NonEmptyTuple).toList.map(bar)
+  (??? : *:[?, ?]).toList.map(bar)
+  (??? : (Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int)).toList.map(bar)