Make Tuple.Filter tests more elaborate

Author: anatoliykmetyuk

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

tests/neg/tuple-filter.scala

@@ -2,5 +2,9 @@ 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
+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

@@ -2,5 +2,9 @@ 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
+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]