Revert D20443631: Fix compilation errors for C++20

Differential Revision:
D20443631

Original commit changeset: 03e8210a64a3

fbshipit-source-id: 42621768beab530619aba877015dd48d509bc62b

folly/FBString.h

@@ -1007,13 +1007,13 @@ class basic_fbstring {
   typedef T traits_type;
   typedef typename traits_type::char_type value_type;
   typedef A allocator_type;
-  typedef typename std::allocator_traits<A>::size_type size_type;
-  typedef typename std::allocator_traits<A>::difference_type difference_type;
+  typedef typename A::size_type size_type;
+  typedef typename A::difference_type difference_type;
 
-  typedef typename std::allocator_traits<A>::value_type& reference;
-  typedef typename std::allocator_traits<A>::value_type const& const_reference;
-  typedef typename std::allocator_traits<A>::pointer pointer;
-  typedef typename std::allocator_traits<A>::const_pointer const_pointer;
+  typedef typename A::reference reference;
+  typedef typename A::const_reference const_reference;
+  typedef typename A::pointer pointer;
+  typedef typename A::const_pointer const_pointer;
 
   typedef E* iterator;
   typedef const E* const_iterator;

folly/detail/Iterators.h

@@ -93,11 +93,9 @@ class IteratorFacade {
    * this and the `operator==(D const&) const` method above.
    */
 
-  template <
-      class D2,
-      std::enable_if_t<!std::is_same<D, D2>::value, int> = 0,
-      std::enable_if_t<std::is_convertible<D, D2>::value, int> = 0>
-  bool operator==(D2 const& rhs) const {
+  template <class D2>
+  typename std::enable_if<std::is_convertible<D, D2>::value, bool>::type
+  operator==(D2 const& rhs) const {
     return D2(asDerivedConst()) == rhs;
   }