Fix CPP20 compile issues

Summary: Fix CPP20 compile issues

Reviewed By: ispeters

Differential Revision: D31900132

fbshipit-source-id: 3338b84cc6aff8f317aad8a93a3e1fba5fdc6fc0

folly/hash/test/HashTest.cpp

@@ -270,13 +270,14 @@ template <class T>
 class TestAlloc {
  public:
   using Alloc = std::allocator<T>;
-  using value_type = typename Alloc::value_type;
-
-  using pointer = typename Alloc::pointer;
-  using const_pointer = typename Alloc::const_pointer;
-  using reference = typename Alloc::reference;
-  using const_reference = typename Alloc::const_reference;
-  using size_type = typename Alloc::size_type;
+  using AllocTraits = std::allocator_traits<Alloc>;
+  using value_type = typename AllocTraits::value_type;
+
+  using pointer = typename AllocTraits::pointer;
+  using const_pointer = typename AllocTraits::const_pointer;
+  using reference = value_type&;
+  using const_reference = value_type const&;
+  using size_type = typename AllocTraits::size_type;
 
   using propagate_on_container_swap = std::true_type;
   using propagate_on_container_copy_assignment = std::true_type;