Removing broken specialization of std::hash<std::tuple>

Summary: ^

Test Plan: Unit tests

Reviewed By: delong.j@fb.com

FB internal diff: D543586

folly/Hash.h

@@ -338,15 +338,6 @@ namespace std {
       return folly::hash::hash_combine(x.first, x.second);
     }
   };
-
-  // Same as above, but for arbitrary tuples.
-  template <typename... Ts>
-  class hash<std::tuple<Ts...> > {
-  public:
-    size_t operator()(const Ts&... ts) const {
-      return folly::hash::hash_combine(ts...);
-    }
-  };
 } // namespace std
 
 #endif

folly/test/HashTest.cpp

@@ -19,6 +19,7 @@
 #include <gtest/gtest.h>
 #include <stdint.h>
 #include <unordered_map>
+#include <utility>
 
 using namespace folly::hash;
 
@@ -172,6 +173,16 @@ TEST(Hash, hasher) {
   EXPECT_EQ(get_default(m, 4), 5);
 }
 
+TEST(Hash, pair) {
+  auto a = std::make_pair(1, 2);
+  auto b = std::make_pair(3, 4);
+  auto c = std::make_pair(1, 2);
+  EXPECT_EQ(hash_combine(a),
+            hash_combine(c));
+  EXPECT_NE(hash_combine(b),
+            hash_combine(c));
+}
+
 TEST(Hash, hash_combine) {
   EXPECT_NE(hash_combine(1, 2), hash_combine(2, 1));
 }