Fix C++20 build issues

Summary: Fix C++20 build issues

Reviewed By: ispeters

Differential Revision: D31848258

fbshipit-source-id: 55c7029f371f5686e62817712f9900ba4a2b4d0d

folly/experimental/StringKeyedCommon.h

@@ -28,7 +28,8 @@ template <class Alloc>
 StringPiece stringPieceDup(StringPiece piece, const Alloc& alloc) {
   auto size = piece.size();
   auto keyDup =
-      typename Alloc::template rebind<char>::other(alloc).allocate(size);
+      typename std::allocator_traits<Alloc>::template rebind_alloc<char>(alloc)
+          .allocate(size);
   if (size) {
     memcpy(
         keyDup, piece.data(), size * sizeof(typename StringPiece::value_type));
@@ -38,8 +39,8 @@ StringPiece stringPieceDup(StringPiece piece, const Alloc& alloc) {
 
 template <class Alloc>
 void stringPieceDel(StringPiece piece, const Alloc& alloc) {
-  typename Alloc::template rebind<char>::other(alloc).deallocate(
-      const_cast<char*>(piece.data()), piece.size());
+  typename std::allocator_traits<Alloc>::template rebind_alloc<char>(alloc)
+      .deallocate(const_cast<char*>(piece.data()), piece.size());
 }
 
 } // namespace folly

folly/experimental/coro/test/GmockHelpersTest.cpp

@@ -135,6 +135,7 @@ TEST(CoroGTestHelpers, CoReturnByMoveWithImplicitConversionTest) {
   MockFoo mock;
 
   struct ImplicitToStringMoveOnly {
+    constexpr ImplicitToStringMoveOnly() noexcept = default;
     ImplicitToStringMoveOnly(const ImplicitToStringMoveOnly&) = delete;
     ImplicitToStringMoveOnly& operator=(const ImplicitToStringMoveOnly&) =
         delete;

folly/experimental/test/StringKeyedTest.cpp

@@ -59,7 +59,7 @@ struct MemoryLeakCheckerAllocator {
       : alloc_(other.allocator()) {}
 
   value_type* allocate(size_t n, const void* hint = nullptr) {
-    auto p = alloc_.allocate(n, hint);
+    auto p = std::allocator_traits<Alloc>::allocate(alloc_, n, hint);
     allocated += n * sizeof(value_type);
     return p;
   }
@@ -69,14 +69,19 @@ struct MemoryLeakCheckerAllocator {
     freed += n * sizeof(value_type);
   }
 
-  size_t max_size() const { return alloc_.max_size(); }
+  size_t max_size() const {
+    return std::allocator_traits<Alloc>::max_size(alloc_);
+  }
 
   template <class... Args>
   void construct(value_type* p, Args&&... args) {
-    alloc_.construct(p, std::forward<Args>(args)...);
+    std::allocator_traits<Alloc>::construct(
+        alloc_, p, std::forward<Args>(args)...);
   }
 
-  void destroy(value_type* p) { alloc_.destroy(p); }
+  void destroy(value_type* p) {
+    std::allocator_traits<Alloc>::destroy(alloc_, p);
+  }
 
   template <class U>
   struct rebind {