Fix DistributedMutex build failure under MSVC

Summary:
[Folly] Fix `DistributedMutex` build failure under MSVC, which fails to understand conversion from `std::integral_constant<T, ...>` to `T` in variable template parameter-list contexts.

Fixes #1164.

Reviewed By: aary

Differential Revision: D15819859

fbshipit-source-id: 1a2647e908fd9ad3362c4bea15d64db75a79f95d

folly/synchronization/DistributedMutex-inl.h

@@ -547,14 +547,14 @@ class TaskWithBigReturnValue {
   static_assert(sizeof(Waiter::storage_) < sizeof(ReturnType), "");
 };
 
+// we need to use std::integral_constant::value here as opposed to
+// std::integral_constant::operator T() because MSVC errors out with the
+// implicit conversion
 template <typename T, typename = std::enable_if_t<true>>
 constexpr const auto Sizeof = sizeof(T);
 template <typename T>
-constexpr const auto Sizeof<T, std::enable_if_t<std::is_void<T>{}>> = 0;
+constexpr const auto Sizeof<T, std::enable_if_t<std::is_void<T>::value>> = 0;
 
-// we need to use std::integral_constant::value here as opposed to
-// std::integral_constant::operator T() because MSVC errors out with the
-// implicit conversion
 template <typename Func, typename Waiter>
 using CoalescedTask = std::conditional_t<
     std::is_void<folly::invoke_result_t<const Func&>>::value,