rlock() should work with const input

Summary: The instance method is permitted on `const` objects, so the ADL method should also accept a `const&` input.

Reviewed By: yfeldblum

Differential Revision: D8240295

fbshipit-source-id: 960aa5567afea3f52b91c1b83566820f5ecefc4d

folly/Synchronized.h

@@ -894,9 +894,9 @@ class SynchronizedWLocker : public SynchronizedLockerBase<Synchronized> {
   }
 };
 template <typename Synchronized>
-class SynchronizedRLocker : public SynchronizedLockerBase<Synchronized> {
+class SynchronizedRLocker : public SynchronizedLockerBase<Synchronized const> {
  public:
-  using SynchronizedLockerBase<Synchronized>::SynchronizedLockerBase;
+  using SynchronizedLockerBase<Synchronized const>::SynchronizedLockerBase;
   using LockedPtr = typename Synchronized::ConstLockedPtr;
 
   auto lock() {
@@ -1653,7 +1653,7 @@ auto wlock(Synchronized<Data, Mutex>& synchronized) {
   return detail::SynchronizedWLocker<Synchronized<Data, Mutex>>{synchronized};
 }
 template <typename Data, typename Mutex>
-auto rlock(Synchronized<Data, Mutex>& synchronized) {
+auto rlock(Synchronized<Data, Mutex> const& synchronized) {
   return detail::SynchronizedRLocker<Synchronized<Data, Mutex>>{synchronized};
 }
 template <typename Data, typename Mutex>

folly/test/SynchronizedTest.cpp

@@ -823,7 +823,7 @@ TEST(FollyLockTest, TestVariadicLockWithSynchronized) {
   {
     auto syncs = std::array<folly::Synchronized<int>, 3>{};
     auto& one = syncs[0];
-    auto& two = syncs[1];
+    auto const& two = syncs[1];
     auto& three = syncs[2];
     auto locks =
         lock(folly::wlock(one), folly::rlock(two), folly::wlock(three));