Support UNKNOWN type in Map function (facebookincubator#9410)

Summary:
Pull Request resolved: facebookincubator#9410

Currently the key type of `MAP` is specified as `knownTypeVariable` which refuses to bind `UNKNOWN` type, while in Presto `MAP(UNKNOWN, UNKNOWN)` is valid.

Reviewed By: kagamiori

Differential Revision: D55902535

fbshipit-source-id: 8c57736859d9f726b1cb7aa43cec74d60990c630

velox/functions/prestosql/Map.cpp

@@ -278,7 +278,7 @@ class MapFunction : public exec::VectorFunction {
   static std::vector<std::shared_ptr<exec::FunctionSignature>> signatures() {
     // array(K), array(V) -> map(K,V)
     return {exec::FunctionSignatureBuilder()
-                .knownTypeVariable("K")
+                .typeVariable("K")
                 .typeVariable("V")
                 .returnType("map(K,V)")
                 .argumentType("array(K)")

velox/functions/prestosql/tests/MapTest.cpp

@@ -420,4 +420,21 @@ TEST_F(MapTest, nestedNullInKeys) {
       "map key cannot be indeterminate");
 }
 
+TEST_F(MapTest, unknownType) {
+  // MAP(ARRAY[], ARRAY[])
+  auto emptyArrayVector = makeArrayVector<UnknownValue>({{}});
+  auto expectedMap = makeMapVector<UnknownValue, UnknownValue>({{}});
+  auto result = evaluate(
+      "map(c0, c1)", makeRowVector({emptyArrayVector, emptyArrayVector}));
+  assertEqualVectors(expectedMap, result);
+
+  // MAP(ARRAY[null], ARRAY[null])
+  auto elementVector = makeNullableFlatVector<UnknownValue>({std::nullopt});
+  auto nullArrayVector = makeArrayVector({0}, elementVector);
+  VELOX_ASSERT_THROW(
+      evaluate(
+          "map(c0, c1)", makeRowVector({nullArrayVector, nullArrayVector})),
+      "map key cannot be null");
+}
+
 } // namespace