Refine the bounds of the Tuple.Filter type lambda predicate ..

to only require it be defined on the elements of the tuple.

This is one of the ongoing proposed tuple improvements, addressing #19175.

As carefully pointed out by @sjrd, this _is_ a potential breaking change.
See tests/neg/tuple-filter-compat.scala for an example.

This is not an unprecedented change however,
the analogous improvements were made to `Tuple.{Map, FlatMap}` in 28a695e.

Author: EugeneFlesselle

library/src/scala/Tuple.scala

@@ -166,7 +166,7 @@ object Tuple {
    *  ```
    *  @syntax markdown
    */
-  type Filter[Tup <: Tuple, P[_] <: Boolean] <: Tuple = Tup match {
+  type Filter[Tup <: Tuple, P[_ <: Union[Tup]] <: Boolean] <: Tuple = Tup match {
     case EmptyTuple => EmptyTuple
     case h *: t => P[h] match {
       case true => h *: Filter[t, P]

tests/neg/tuple-filter-compat.scala

@@ -0,0 +1,12 @@
+
+type OldFilter[Tup <: Tuple, P[_] <: Boolean] = Nothing
+type NewFilter[Tup <: Tuple, P[_ <: Tuple.Union[Tup]] <: Boolean] = Nothing
+
+trait A:
+  type X >: OldFilter <: OldFilter
+
+trait B1 extends A:
+  type X = OldFilter // ok
+
+trait B2 extends A:
+  type X = NewFilter // error: breaking change

tests/pos/tuple-filter.scala

@@ -1,10 +1,16 @@
+import scala.compiletime.ops.int.<
+
 type P[x] <: Boolean = x match {
   case 3 => false
   case _ => true
 }
+
 type RejectAll[x] = false
 
+type Pos[X <: Int] = 0 < X
+
 def Test =
   summon[Tuple.Filter[(1, 2, 3, 4), P] =:= (1, 2, 4)]
   summon[Tuple.Filter[(1, 2, 3, 4), RejectAll] =:= EmptyTuple]
   summon[Tuple.Filter[EmptyTuple, P] =:= EmptyTuple]
+  summon[Tuple.Filter[(1, -2, 3, -4), Pos] =:= (1, 3)]