add tests for sized deallocation of size zero

Summary:
Sized deallocation via ::operator delete with size == 0 is
broken in some allocators.  This diff adds a unit test that ensures we
are either disabling sized deallocator or running with a good allocator.

Reviewed By: interwq

Differential Revision: D10857654

fbshipit-source-id: 981c7bcc1b021d4cd1ea1725ccf510d1958842fd

folly/test/MemoryTest.cpp

@@ -41,6 +41,26 @@ TEST(allocateBytes, simple) {
   deallocateBytes(p, 10);
 }
 
+TEST(allocateBytes, zero) {
+  auto p = allocateBytes(0);
+  deallocateBytes(p, 0);
+}
+
+TEST(operatorNewDelete, zero) {
+  auto p = ::operator new(0);
+  EXPECT_TRUE(p != nullptr);
+  ::operator delete(p);
+}
+
+#if __cpp_sized_deallocation
+TEST(operatorNewDelete, sized_zero) {
+  std::size_t n = 0;
+  auto p = ::operator new(n);
+  EXPECT_TRUE(p != nullptr);
+  ::operator delete(p, n);
+}
+#endif
+
 TEST(aligned_malloc, examples) {
   auto trial = [](size_t align) {
     auto const ptr = aligned_malloc(1, align);