Fix scala#15992: Map on Tuples must widen its keys.

Author: anatoliykmetyuk

library/src/scala/Tuple.scala

@@ -56,8 +56,8 @@ sealed trait Tuple extends Product {
    *  If the tuple is of the form `a1 *: ... *: Tuple` (that is, the tail is not known
    *  to be the cons type.
    */
-  inline def map[F[_]](f: [t] => t => F[t]): Map[this.type, F] =
-    runtime.Tuples.map(this, f).asInstanceOf[Map[this.type, F]]
+  inline def map[This >: this.type <: Tuple, F[_]](f: [t] => t => F[t]): Map[This, F] =
+    runtime.Tuples.map(this, f).asInstanceOf[Map[This, F]]
 
   /** Given a tuple `(a1, ..., am)`, returns the tuple `(a1, ..., an)` consisting
    *  of its first n elements.

tests/pos/i15992.scala

@@ -0,0 +1,4 @@
+def widen[T](x: T): T = x
+
+def f[T <: Tuple](t: T): Unit =
+  val tl: Tuple.Map[T, List] = widen(t).map([X] => (x: X) => List(x))