Combine cases of Tuple.Zip disjoint from (h1 *: t1, h2 *: t2) (#21287)

If we reach the second case of `Zip[T1 <: Tuple, T2 <: Tuple]`, then we
know `(T1, T2)` is disjoint from `(NonEmptyTuple, NonEmptyTuple)`, from
which we can conclude at least one of the two is an `EmptyTuple`.

Addressing #19175

Author: EugeneFlesselle

library/src/scala/Tuple.scala

@@ -205,15 +205,12 @@ object Tuple {
   }
 
   /** 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`
-   *  where `Ct` is `EmptyTuple` if `At` or `Bt` is `EmptyTuple`, otherwise `Ct` is `Tuple`.
+   *  where at least one of `At` or `Bt` is `EmptyTuple`,
+   *  returns the tuple type `(A1, B1) *: ... *: (An, Bn) *: EmptyTuple`.
    */
   type Zip[T1 <: Tuple, T2 <: Tuple] <: Tuple = (T1, T2) match {
     case (h1 *: t1, h2 *: t2) => (h1, h2) *: Zip[t1, t2]
-    case (EmptyTuple, _) => EmptyTuple
-    case (_, EmptyTuple) => EmptyTuple
-    case _ => Tuple
+    case _ => EmptyTuple
   }
 
   /** Converts a tuple `(F[T1], ..., F[Tn])` to `(T1,  ... Tn)` */

tests/pos/tuple-zip.scala

@@ -0,0 +1,20 @@
+
+import scala.Tuple.Zip
+
+type A
+type B
+type C
+
+def Test =
+
+  summon[Zip[A *: B *: C *: EmptyTuple, C *: B *: A *: EmptyTuple] =:= (A, C) *: (B, B) *: (C, A) *: EmptyTuple]
+
+  summon[Zip[A *: B *: EmptyTuple, C *: B *: A *: EmptyTuple] =:= (A, C) *: (B, B) *: EmptyTuple]
+  summon[Zip[A *: B *: C *: EmptyTuple, C *: B *: EmptyTuple] =:= (A, C) *: (B, B) *: EmptyTuple]
+
+  summon[Zip[A *: B *: C *: Tuple, C *: B *: A *: Tuple] =:= (A, C) *: (B, B) *: (C, A) *: Zip[Tuple, Tuple]]
+  summon[Zip[A *: B *: C *: Tuple, C *: B *: A *: Tuple] <:< (A, C) *: (B, B) *: (C, A) *: Tuple]
+
+  summon[Zip[A *: B *: Tuple, C *: B *: A *: Tuple] =:= (A, C) *: (B, B) *: Zip[Tuple, A *: Tuple]]
+  summon[Zip[A *: B *: NonEmptyTuple, C *: B *: A *: Tuple] =:= (A, C) *: (B, B) *: Zip[NonEmptyTuple, A *: Tuple]]
+  summon[Zip[A *: B *: EmptyTuple, C *: B *: A *: Tuple] =:= (A, C) *: (B, B) *: EmptyTuple]