Add Filter type to Tuple

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

@@ -12,6 +12,7 @@ reference
 scala-days-2019-slides
 i7048e.scala
 i8052.scala
+tuple-filter.scala
 
 # Stale symbol: package object scala
 seqtype-cycle

library/src/scala/Tuple.scala

@@ -125,6 +125,24 @@ object Tuple {
     case h *: t => Concat[F[h], FlatMap[t, F]]
   }
 
+  /** Filters out those members of the tuple for which the predicate `P` returns `false`.
+   *  A predicate `P[X]` is a type that can be either `true` or `false`. For example:
+   *  ```
+   *  type IsString[x] = x match {
+   *    case String => true
+   *    case _ => false
+   *  }
+   *  Filter[(1, "foo", 2, "bar"), IsString] =:= ("foo", "bar")
+   * ```
+   */
+  type Filter[Tup <: Tuple, P[_] <: Boolean] <: Tuple = Tup match {
+    case EmptyTuple => EmptyTuple
+    case h *: t => P[h] match {
+      case true => h *: Filter[t, P]
+      case false => Filter[t, P]
+    }
+  }
+
   /** Given two tuples, `A1 *: ... *: An * At` and `B1 *: ... *: Bn *: Bt`
    *  where at least one of `At` or `Bt` is `EmptyTuple` or `Tuple`,
    *  returns the tuple type `(A1, B1) *: ... *: (An, Bn) *: Ct`

tests/neg/tuple-filter.scala

@@ -0,0 +1,10 @@
+type P[x] <: Boolean = x match {
+  case 3 => false
+  case _ => true
+}
+type RejectAll[x] = false
+
+def Test =
+  summon[Tuple.Filter[(1, 2, 3, 4), P] =:= EmptyTuple] // error
+  summon[Tuple.Filter[(1, 2, 3, 4), RejectAll] =:= (1, 2, 3)] // error
+  summon[Tuple.Filter[EmptyTuple, P] =:= (1, 2, 3)] // error

tests/pos/tuple-filter.scala

@@ -0,0 +1,10 @@
+type P[x] <: Boolean = x match {
+  case 3 => false
+  case _ => true
+}
+type RejectAll[x] = false
+
+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]