Add Filter type to the tuple

Author: anatoliykmetyuk

library/src/scala/Tuple.scala

@@ -125,6 +125,15 @@ 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`. */
+  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 +1,6 @@
+type P[x] <: Boolean = x match {
+  case 3 => false
+  case _ => true
+}
+val tup: (1, 2, 3) = (1, 2, 3)
+val tup2: Tuple.Filter[(1, 2, 3, 4), P] = tup  // error

tests/pos/tuple-filter.scala

@@ -0,0 +1,6 @@
+type P[x] <: Boolean = x match {
+  case 3 => false
+  case _ => true
+}
+val tup: (1, 2, 4) = (1, 2, 4)
+val tup2: Tuple.Filter[(1, 2, 3, 4), P] = tup