fix CancellationState under MSVC

Summary: It uses `std::array<CancellationCallback, N>`. But `CancellationCallback` is not default-constructible, and `N` may be zero, and MSVC < v19.31 fails to support this combination.

Reviewed By: Gownta

Differential Revision: D64644003

fbshipit-source-id: 0012977e5ba90f826b5256d2ac3239483ce7cf75

CMakeLists.txt

@@ -1065,8 +1065,7 @@ if (BUILD_TESTS OR BUILD_BENCHMARKS)
       TEST atomic_unordered_map_test SOURCES AtomicUnorderedMapTest.cpp
       TEST base64_test SOURCES base64_test.cpp
       TEST buffered_atomic_test SOURCES BufferedAtomicTest.cpp
-      TEST cancellation_token_test WINDOWS_DISABLED
-        SOURCES CancellationTokenTest.cpp
+      TEST cancellation_token_test SOURCES CancellationTokenTest.cpp
       TEST chrono_test SOURCES ChronoTest.cpp
       TEST clock_gettime_wrappers_test SOURCES ClockGettimeWrappersTest.cpp
       TEST concurrent_bit_set_test SOURCES ConcurrentBitSetTest.cpp

folly/CancellationToken-inl.h

@@ -115,7 +115,16 @@ class FixedMergingCancellationState : public CancellationState {
   FOLLY_NODISCARD static CancellationStateTokenPtr create(Ts&&... tokens);
 
  private:
-  std::array<CancellationCallback, N> callbacks_;
+  // MSVC < v19.31 does not correctly support std::array<T, 0>; so we improvise
+  template <typename, size_t>
+  struct EmptyArray {
+    explicit EmptyArray(std::initializer_list<int>) noexcept {}
+  };
+  using CancellationCallbackArray = conditional_t<
+      !N,
+      EmptyArray<CancellationCallback, N>,
+      std::array<CancellationCallback, N>>;
+  CancellationCallbackArray callbacks_;
 };
 
 template <typename... Data>