Add folly::hasher for empty tuples

Summary:
This ensures that we can use any tuple type in maps/other places where
hashes are useful.  Since there's only one value for an empty tuple, just
return 0.

Reviewed By: ot

Differential Revision: D14898947

fbshipit-source-id: 8fefab46df32a6cc56e43b02346bbefe4d4ec567

folly/hash/Hash.h

@@ -367,6 +367,10 @@ struct Hash {
   size_t operator()(const T& t, const Ts&... ts) const {
     return hash::hash_128_to_64((*this)(t), (*this)(ts...));
   }
+
+  size_t operator()() const noexcept {
+    return 0;
+  }
 };
 
 // IsAvalanchingHasher<H, K> extends std::integral_constant<bool, V>.

folly/hash/test/HashTest.cpp

@@ -495,6 +495,15 @@ TEST(Hash, std_tuple) {
   EXPECT_EQ("bar", m[t]);
 }
 
+TEST(Hash, std_empty_tuple) {
+  std::unordered_map<std::tuple<>, std::string, folly::Hash> m;
+  m[{}] = "foo";
+  EXPECT_EQ(m[{}], "foo");
+
+  folly::hasher<std::tuple<>> h;
+  EXPECT_EQ(h({}), 0);
+}
+
 TEST(Hash, enum_type) {
   const auto hash = folly::Hash();