Export of internal Abseil changes

--
90ecacd2a3db96ee64ef23af37a80fad404e2b32 by Gennadiy Rozental <rogeeff@google.com>:

Fixes MSVC regression by making MSVC version of class Flag into an aggregate type.

PiperOrigin-RevId: 277767054

--
018f3b040df51d91a988fa146fee163721e605e9 by Abseil Team <absl-team@google.com>:

Change libstdc++ lacking std::unique_ptr check from a gcc version check to based on feature macros.

PiperOrigin-RevId: 277736042

--
475844775ae343e2414318f08549ee3fa6676a8d by CJ Johnson <johnsoncj@google.com>:

Pass allocator_type through allocator_traits before extracting the typedefs

PiperOrigin-RevId: 277730393

--
d843bc4bc30bf5b11af76db8beda8634b6111a62 by Abseil Team <absl-team@google.com>:

Convert the Waiter::Init() method to the default constructor and define
a destructor for the Waiter class.

Use placement new and delete with Waiter objects.

PiperOrigin-RevId: 277728823

--
1ba6edf421dd2dfe13c55970a03c99592cb6677d by Derek Mauro <dmauro@google.com>:

Use lowercase spelling for include of dbghelp.h
When cross-compiling under MinGW this is important

PiperOrigin-RevId: 277629783

--
cfc662a6fa357a84ddda8037156c7f26cee40c36 by Abseil Team <absl-team@google.com>:

Don't use atomic ops on waiter and wakeup counts in WIN32 waiter mode.

Port the new CONDVAR waiter mode code in CL 277366017 to the WIN32
waiter mode.

PiperOrigin-RevId: 277603611

--
833106542e61fa0832900adf3c1b2afc6890b94b by Abseil Team <absl-team@google.com>:

Add the PerThreadSem::Destroy() method.

For ABSL_WAITER_MODE_CONDVAR or ABSL_WAITER_MODE_SEM,
PerThreadSem::Destroy() is used to destroy the pthread mutex and
condition variable or the POSIX semaphore.
PiperOrigin-RevId: 277586675

--
7814da4a59106cf1e0e4db1a31b9592ebbd2094b by Samuel Benzaquen <sbenza@google.com>:

Enable the assertion in the iterator's operator* and operator->

PiperOrigin-RevId: 277563401
GitOrigin-RevId: 90ecacd2a3db96ee64ef23af37a80fad404e2b32
Change-Id: Ib19be3680da74f0b94055c9039115ec6bcaea7b0

13 files changed, 239 insertions, 144 deletions

diff --git a/absl/container/inlined_vector.h b/absl/container/inlined_vector.h
index 5138348a15..d5c67db598 100644
--- a/absl/container/inlined_vector.h
+++ b/absl/container/inlined_vector.h
@@ -69,9 +69,10 @@ class InlinedVector {
   static_assert(N > 0, "`absl::InlinedVector` requires an inlined capacity.");
 
   using Storage = inlined_vector_internal::Storage<T, N, A>;
-  using rvalue_reference = typename Storage::rvalue_reference;
-  using MoveIterator = typename Storage::MoveIterator;
+
   using AllocatorTraits = typename Storage::AllocatorTraits;
+  using RValueReference = typename Storage::RValueReference;
+  using MoveIterator = typename Storage::MoveIterator;
   using IsMemcpyOk = typename Storage::IsMemcpyOk;
 
   template <typename Iterator>
@@ -92,10 +93,10 @@ class InlinedVector {
   using value_type = typename Storage::value_type;
   using pointer = typename Storage::pointer;
   using const_pointer = typename Storage::const_pointer;
-  using reference = typename Storage::reference;
-  using const_reference = typename Storage::const_reference;
   using size_type = typename Storage::size_type;
   using difference_type = typename Storage::difference_type;
+  using reference = typename Storage::reference;
+  using const_reference = typename Storage::const_reference;
   using iterator = typename Storage::iterator;
   using const_iterator = typename Storage::const_iterator;
   using reverse_iterator = typename Storage::reverse_iterator;
@@ -563,7 +564,7 @@ class InlinedVector {
 
   // Overload of `InlinedVector::insert(...)` that inserts `v` at `pos` using
   // move semantics, returning an `iterator` to the newly inserted element.
-  iterator insert(const_iterator pos, rvalue_reference v) {
+  iterator insert(const_iterator pos, RValueReference v) {
     return emplace(pos, std::move(v));
   }
 
@@ -660,7 +661,7 @@ class InlinedVector {
 
   // Overload of `InlinedVector::push_back(...)` for inserting `v` at `end()`
   // using move semantics.
-  void push_back(rvalue_reference v) {
+  void push_back(RValueReference v) {
     static_cast<void>(emplace_back(std::move(v)));
   }
 
diff --git a/absl/container/inlined_vector_test.cc b/absl/container/inlined_vector_test.cc
index 080ea95665..2c9b0d0e03 100644
--- a/absl/container/inlined_vector_test.cc
+++ b/absl/container/inlined_vector_test.cc
@@ -1754,6 +1754,30 @@ TEST(AllocatorSupportTest, SizeAllocConstructor) {
   }
 }
 
+TEST(InlinedVectorTest, MinimumAllocatorCompilesUsingTraits) {
+  using T = int;
+  using A = std::allocator<T>;
+  using ATraits = absl::allocator_traits<A>;
+
+  struct MinimumAllocator {
+    using value_type = T;
+
+    value_type* allocate(size_t n) {
+      A a;
+      return ATraits::allocate(a, n);
+    }
+
+    void deallocate(value_type* p, size_t n) {
+      A a;
+      ATraits::deallocate(a, p, n);
+    }
+  };
+
+  absl::InlinedVector<T, 1, MinimumAllocator> vec;
+  vec.emplace_back();
+  vec.resize(0);
+}
+
 TEST(InlinedVectorTest, AbslHashValueWorks) {
   using V = absl::InlinedVector<int, 4>;
   std::vector<V> cases;
diff --git a/absl/container/internal/inlined_vector.h b/absl/container/internal/inlined_vector.h
index f678c88daf..84aa785ad6 100644
--- a/absl/container/internal/inlined_vector.h
+++ b/absl/container/internal/inlined_vector.h
@@ -37,16 +37,17 @@ using IsAtLeastForwardIterator = std::is_convertible<
     typename std::iterator_traits<Iterator>::iterator_category,
     std::forward_iterator_tag>;
 
-template <typename AllocatorType>
-using IsMemcpyOk = absl::conjunction<
-    std::is_same<std::allocator<typename AllocatorType::value_type>,
-                 AllocatorType>,
-    absl::is_trivially_copy_constructible<typename AllocatorType::value_type>,
-    absl::is_trivially_copy_assignable<typename AllocatorType::value_type>,
-    absl::is_trivially_destructible<typename AllocatorType::value_type>>;
-
-template <typename AllocatorType, typename ValueType, typename SizeType>
-void DestroyElements(AllocatorType* alloc_ptr, ValueType* destroy_first,
+template <typename AllocatorType,
+          typename ValueType =
+              typename absl::allocator_traits<AllocatorType>::value_type>
+using IsMemcpyOk =
+    absl::conjunction<std::is_same<AllocatorType, std::allocator<ValueType>>,
+                      absl::is_trivially_copy_constructible<ValueType>,
+                      absl::is_trivially_copy_assignable<ValueType>,
+                      absl::is_trivially_destructible<ValueType>>;
+
+template <typename AllocatorType, typename Pointer, typename SizeType>
+void DestroyElements(AllocatorType* alloc_ptr, Pointer destroy_first,
                      SizeType destroy_size) {
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
 
@@ -57,20 +58,25 @@ void DestroyElements(AllocatorType* alloc_ptr, ValueType* destroy_first,
     }
 
 #if !defined(NDEBUG)
-    // Overwrite unused memory with `0xab` so we can catch uninitialized usage.
-    //
-    // Cast to `void*` to tell the compiler that we don't care that we might be
-    // scribbling on a vtable pointer.
-    auto* memory_ptr = static_cast<void*>(destroy_first);
-    auto memory_size = sizeof(ValueType) * destroy_size;
-    std::memset(memory_ptr, 0xab, memory_size);
+    {
+      using ValueType = typename AllocatorTraits::value_type;
+
+      // Overwrite unused memory with `0xab` so we can catch uninitialized
+      // usage.
+      //
+      // Cast to `void*` to tell the compiler that we don't care that we might
+      // be scribbling on a vtable pointer.
+      void* memory_ptr = destroy_first;
+      auto memory_size = destroy_size * sizeof(ValueType);
+      std::memset(memory_ptr, 0xab, memory_size);
+    }
 #endif  // !defined(NDEBUG)
   }
 }
 
-template <typename AllocatorType, typename ValueType, typename ValueAdapter,
+template <typename AllocatorType, typename Pointer, typename ValueAdapter,
           typename SizeType>
-void ConstructElements(AllocatorType* alloc_ptr, ValueType* construct_first,
+void ConstructElements(AllocatorType* alloc_ptr, Pointer construct_first,
                        ValueAdapter* values_ptr, SizeType construct_size) {
   for (SizeType i = 0; i < construct_size; ++i) {
     ABSL_INTERNAL_TRY {
@@ -83,8 +89,8 @@ void ConstructElements(AllocatorType* alloc_ptr, ValueType* construct_first,
   }
 }
 
-template <typename ValueType, typename ValueAdapter, typename SizeType>
-void AssignElements(ValueType* assign_first, ValueAdapter* values_ptr,
+template <typename Pointer, typename ValueAdapter, typename SizeType>
+void AssignElements(Pointer assign_first, ValueAdapter* values_ptr,
                     SizeType assign_size) {
   for (SizeType i = 0; i < assign_size; ++i) {
     values_ptr->AssignNext(assign_first + i);
@@ -93,28 +99,29 @@ void AssignElements(ValueType* assign_first, ValueAdapter* values_ptr,
 
 template <typename AllocatorType>
 struct StorageView {
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
+  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using Pointer = typename AllocatorTraits::pointer;
+  using SizeType = typename AllocatorTraits::size_type;
 
-  pointer data;
-  size_type size;
-  size_type capacity;
+  Pointer data;
+  SizeType size;
+  SizeType capacity;
 };
 
 template <typename AllocatorType, typename Iterator>
 class IteratorValueAdapter {
-  using pointer = typename AllocatorType::pointer;
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using Pointer = typename AllocatorTraits::pointer;
 
  public:
   explicit IteratorValueAdapter(const Iterator& it) : it_(it) {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
+  void ConstructNext(AllocatorType* alloc_ptr, Pointer construct_at) {
     AllocatorTraits::construct(*alloc_ptr, construct_at, *it_);
     ++it_;
   }
 
-  void AssignNext(pointer assign_at) {
+  void AssignNext(Pointer assign_at) {
     *assign_at = *it_;
     ++it_;
   }
@@ -125,46 +132,45 @@ class IteratorValueAdapter {
 
 template <typename AllocatorType>
 class CopyValueAdapter {
-  using pointer = typename AllocatorType::pointer;
-  using const_pointer = typename AllocatorType::const_pointer;
-  using const_reference = typename AllocatorType::const_reference;
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using ValueType = typename AllocatorTraits::value_type;
+  using Pointer = typename AllocatorTraits::pointer;
+  using ConstPointer = typename AllocatorTraits::const_pointer;
 
  public:
-  explicit CopyValueAdapter(const_reference v) : ptr_(std::addressof(v)) {}
+  explicit CopyValueAdapter(const ValueType& v) : ptr_(std::addressof(v)) {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
+  void ConstructNext(AllocatorType* alloc_ptr, Pointer construct_at) {
     AllocatorTraits::construct(*alloc_ptr, construct_at, *ptr_);
   }
 
-  void AssignNext(pointer assign_at) { *assign_at = *ptr_; }
+  void AssignNext(Pointer assign_at) { *assign_at = *ptr_; }
 
  private:
-  const_pointer ptr_;
+  ConstPointer ptr_;
 };
 
 template <typename AllocatorType>
 class DefaultValueAdapter {
-  using pointer = typename AllocatorType::pointer;
-  using value_type = typename AllocatorType::value_type;
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using ValueType = typename AllocatorTraits::value_type;
+  using Pointer = typename AllocatorTraits::pointer;
 
  public:
   explicit DefaultValueAdapter() {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
+  void ConstructNext(AllocatorType* alloc_ptr, Pointer construct_at) {
     AllocatorTraits::construct(*alloc_ptr, construct_at);
   }
 
-  void AssignNext(pointer assign_at) { *assign_at = value_type(); }
+  void AssignNext(Pointer assign_at) { *assign_at = ValueType(); }
 };
 
 template <typename AllocatorType>
 class AllocationTransaction {
-  using value_type = typename AllocatorType::value_type;
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using Pointer = typename AllocatorTraits::pointer;
+  using SizeType = typename AllocatorTraits::size_type;
 
  public:
   explicit AllocationTransaction(AllocatorType* alloc_ptr)
@@ -180,11 +186,11 @@ class AllocationTransaction {
   void operator=(const AllocationTransaction&) = delete;
 
   AllocatorType& GetAllocator() { return alloc_data_.template get<0>(); }
-  pointer& GetData() { return alloc_data_.template get<1>(); }
-  size_type& GetCapacity() { return capacity_; }
+  Pointer& GetData() { return alloc_data_.template get<1>(); }
+  SizeType& GetCapacity() { return capacity_; }
 
   bool DidAllocate() { return GetData() != nullptr; }
-  pointer Allocate(size_type capacity) {
+  Pointer Allocate(SizeType capacity) {
     GetData() = AllocatorTraits::allocate(GetAllocator(), capacity);
     GetCapacity() = capacity;
     return GetData();
@@ -196,14 +202,15 @@ class AllocationTransaction {
   }
 
  private:
-  container_internal::CompressedTuple<AllocatorType, pointer> alloc_data_;
-  size_type capacity_ = 0;
+  container_internal::CompressedTuple<AllocatorType, Pointer> alloc_data_;
+  SizeType capacity_ = 0;
 };
 
 template <typename AllocatorType>
 class ConstructionTransaction {
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
+  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using Pointer = typename AllocatorTraits::pointer;
+  using SizeType = typename AllocatorTraits::size_type;
 
  public:
   explicit ConstructionTransaction(AllocatorType* alloc_ptr)
@@ -220,12 +227,12 @@ class ConstructionTransaction {
   void operator=(const ConstructionTransaction&) = delete;
 
   AllocatorType& GetAllocator() { return alloc_data_.template get<0>(); }
-  pointer& GetData() { return alloc_data_.template get<1>(); }
-  size_type& GetSize() { return size_; }
+  Pointer& GetData() { return alloc_data_.template get<1>(); }
+  SizeType& GetSize() { return size_; }
 
   bool DidConstruct() { return GetData() != nullptr; }
   template <typename ValueAdapter>
-  void Construct(pointer data, ValueAdapter* values_ptr, size_type size) {
+  void Construct(Pointer data, ValueAdapter* values_ptr, SizeType size) {
     inlined_vector_internal::ConstructElements(std::addressof(GetAllocator()),
                                                data, values_ptr, size);
     GetData() = data;
@@ -237,28 +244,29 @@ class ConstructionTransaction {
   }
 
  private:
-  container_internal::CompressedTuple<AllocatorType, pointer> alloc_data_;
-  size_type size_ = 0;
+  container_internal::CompressedTuple<AllocatorType, Pointer> alloc_data_;
+  SizeType size_ = 0;
 };
 
 template <typename T, size_t N, typename A>
 class Storage {
  public:
-  using allocator_type = A;
-  using value_type = typename allocator_type::value_type;
-  using pointer = typename allocator_type::pointer;
-  using const_pointer = typename allocator_type::const_pointer;
-  using reference = typename allocator_type::reference;
-  using const_reference = typename allocator_type::const_reference;
-  using rvalue_reference = typename allocator_type::value_type&&;
-  using size_type = typename allocator_type::size_type;
-  using difference_type = typename allocator_type::difference_type;
+  using AllocatorTraits = absl::allocator_traits<A>;
+  using allocator_type = typename AllocatorTraits::allocator_type;
+  using value_type = typename AllocatorTraits::value_type;
+  using pointer = typename AllocatorTraits::pointer;
+  using const_pointer = typename AllocatorTraits::const_pointer;
+  using size_type = typename AllocatorTraits::size_type;
+  using difference_type = typename AllocatorTraits::difference_type;
+
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using RValueReference = value_type&&;
   using iterator = pointer;
   using const_iterator = const_pointer;
   using reverse_iterator = std::reverse_iterator<iterator>;
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
   using MoveIterator = std::move_iterator<iterator>;
-  using AllocatorTraits = absl::allocator_traits<allocator_type>;
   using IsMemcpyOk = inlined_vector_internal::IsMemcpyOk<allocator_type>;
 
   using StorageView = inlined_vector_internal::StorageView<allocator_type>;
diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index 9079a73e52..bf0c03c44f 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -615,7 +615,7 @@ class raw_hash_set {
 
     // PRECONDITION: not an end() iterator.
     reference operator*() const {
-      /* To be enabled: assert_is_full(); */
+      assert_is_full();
       return PolicyTraits::element(slot_);
     }
 
diff --git a/absl/debugging/symbolize_win32.inc b/absl/debugging/symbolize_win32.inc
index 0fb4b80376..b0fb3b9a3e 100644
--- a/absl/debugging/symbolize_win32.inc
+++ b/absl/debugging/symbolize_win32.inc
@@ -17,10 +17,10 @@
 
 #include <windows.h>
 
-// MSVC header DbgHelp.h has a warning for an ignored typedef.
+// MSVC header dbghelp.h has a warning for an ignored typedef.
 #pragma warning(push)
 #pragma warning(disable:4091)
-#include <DbgHelp.h>
+#include <dbghelp.h>
 #pragma warning(pop)
 
 #pragma comment(lib, "dbghelp.lib")
diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index c0060b449a..356ddb6160 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h
@@ -67,17 +67,20 @@ namespace absl {
 template <typename T>
 using Flag = flags_internal::Flag<T>;
 #else
-// MSVC debug builds do not implement constexpr correctly for classes with
-// virtual methods. To work around this we adding level of indirection, so that
-// the class `absl::Flag` contains an `internal::Flag*` (instead of being an
-// alias to that class) and dynamically allocates an instance when necessary.
-// We also forward all calls to internal::Flag methods via trampoline methods.
-// In this setup the `absl::Flag` class does not have virtual methods and thus
-// MSVC is able to initialize it at link time. To deal with multiple threads
-// accessing the flag for the first time concurrently we use an atomic boolean
-// indicating if flag object is constructed. We also employ the double-checked
-// locking pattern where the second level of protection is a global Mutex, so
-// if two threads attempt to construct the flag concurrently only one wins.
+// MSVC debug builds do not implement initialization with constexpr constructors
+// correctly. To work around this we add a level of indirection, so that the
+// class `absl::Flag` contains an `internal::Flag*` (instead of being an alias
+// to that class) and dynamically allocates an instance when necessary. We also
+// forward all calls to internal::Flag methods via trampoline methods. In this
+// setup the `absl::Flag` class does not have constructor and virtual methods,
+// all the data members are public and thus MSVC is able to initialize it at
+// link time. To deal with multiple threads accessing the flag for the first
+// time concurrently we use an atomic boolean indicating if flag object is
+// initialized. We also employ the double-checked locking pattern where the
+// second level of protection is a global Mutex, so if two threads attempt to
+// construct the flag concurrently only one wins.
+// This solution is based on a recomendation here:
+// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html?childToView=648454#comment-648454
 
 namespace flags_internal {
 absl::Mutex* GetGlobalConstructionGuard();
@@ -86,16 +89,23 @@ absl::Mutex* GetGlobalConstructionGuard();
 template <typename T>
 class Flag {
  public:
+  // No constructor and destructor to ensure this is an aggregate type.
+  // Visual Studio 2015 still requires the constructor for class to be
+  // constexpr initializable.
+#if _MSC_VER <= 1900
   constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
                  const char* filename,
                  const flags_internal::FlagMarshallingOpFn marshalling_op,
-                 const flags_internal::InitialValGenFunc initial_value_gen)
+                 const flags_internal::InitialValGenFunc initial_value_gen,
+                 bool, void*)
       : name_(name),
         help_gen_(help_gen),
         filename_(filename),
         marshalling_op_(marshalling_op),
         initial_value_gen_(initial_value_gen),
-        inited_(false) {}
+        inited_(false),
+        impl_(nullptr) {}
+#endif
 
   flags_internal::Flag<T>* GetImpl() const {
     if (!inited_.load(std::memory_order_acquire)) {
@@ -113,7 +123,8 @@ class Flag {
     return impl_;
   }
 
-  // absl::Flag API
+  // Public methods of `absl::Flag<T>` are NOT part of the Abseil Flags API.
+  // See https://abseil.io/docs/cpp/guides/flags
   bool IsRetired() const { return GetImpl()->IsRetired(); }
   bool IsAbseilFlag() const { return GetImpl()->IsAbseilFlag(); }
   absl::string_view Name() const { return GetImpl()->Name(); }
@@ -126,9 +137,9 @@ class Flag {
   std::string Filename() const { return GetImpl()->Filename(); }
   std::string DefaultValue() const { return GetImpl()->DefaultValue(); }
   std::string CurrentValue() const { return GetImpl()->CurrentValue(); }
-  template <typename T1>
+  template <typename U>
   inline bool IsOfType() const {
-    return GetImpl()->template IsOfType<T1>();
+    return GetImpl()->template IsOfType<U>();
   }
   T Get() const { return GetImpl()->Get(); }
   bool AtomicGet(T* v) const { return GetImpl()->AtomicGet(v); }
@@ -138,7 +149,8 @@ class Flag {
   }
   void InvokeCallback() { GetImpl()->InvokeCallback(); }
 
- private:
+  // The data members are logically private, but they need to be public for
+  // this to be an aggregate type.
   const char* name_;
   const flags_internal::HelpGenFunc help_gen_;
   const char* filename_;
@@ -146,7 +158,7 @@ class Flag {
   const flags_internal::InitialValGenFunc initial_value_gen_;
 
   mutable std::atomic<bool> inited_;
-  mutable flags_internal::Flag<T>* impl_ = nullptr;
+  mutable flags_internal::Flag<T>* impl_;
 };
 #endif
 
@@ -310,17 +322,35 @@ void SetFlag(absl::Flag<T>* flag, const V& v) {
 // Note: Name of registrar object is not arbitrary. It is used to "grab"
 // global name for FLAGS_no<flag_name> symbol, thus preventing the possibility
 // of defining two flags with names foo and nofoo.
+#if !defined(_MSC_VER) || defined(__clang__)
 #define ABSL_FLAG_IMPL(Type, name, default_value, help)             \
   namespace absl /* block flags in namespaces */ {}                 \
   ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
   ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                   \
-  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name(                    \
+  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                    \
       ABSL_FLAG_IMPL_FLAGNAME(#name), &AbslFlagsWrapHelp##name,     \
       ABSL_FLAG_IMPL_FILENAME(),                                    \
       &absl::flags_internal::FlagMarshallingOps<Type>,              \
-      &AbslFlagsInitFlag##name);                                    \
+      &AbslFlagsInitFlag##name};                                    \
+  extern bool FLAGS_no##name;                                       \
+  bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
+#else
+// MSVC version uses aggregate initialization.
+#define ABSL_FLAG_IMPL(Type, name, default_value, help)             \
+  namespace absl /* block flags in namespaces */ {}                 \
+  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
+  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                   \
+  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                    \
+      ABSL_FLAG_IMPL_FLAGNAME(#name),                               \
+      &AbslFlagsWrapHelp##name,                                     \
+      ABSL_FLAG_IMPL_FILENAME(),                                    \
+      &absl::flags_internal::FlagMarshallingOps<Type>,              \
+      &AbslFlagsInitFlag##name,                                     \
+      false,                                                        \
+      nullptr};                                                     \
   extern bool FLAGS_no##name;                                       \
   bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
+#endif
 
 // ABSL_RETIRED_FLAG
 //
diff --git a/absl/memory/memory.h b/absl/memory/memory.h
index 5a4a1a1dfe..243a5dda6e 100644
--- a/absl/memory/memory.h
+++ b/absl/memory/memory.h
@@ -92,11 +92,12 @@ struct MakeUniqueResult<T[N]> {
 
 }  // namespace memory_internal
 
-// gcc 4.8 has __cplusplus at 201301 but doesn't define make_unique.  Other
-// supported compilers either just define __cplusplus as 201103 but have
-// make_unique (msvc), or have make_unique whenever __cplusplus > 201103 (clang)
+// gcc 4.8 has __cplusplus at 201301 but the libstdc++ shipped with it doesn't
+// define make_unique.  Other supported compilers either just define __cplusplus
+// as 201103 but have make_unique (msvc), or have make_unique whenever
+// __cplusplus > 201103 (clang).
 #if (__cplusplus > 201103L || defined(_MSC_VER)) && \
-    !(defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ == 8)
+    !(defined(__GLIBCXX__) && !defined(__cpp_lib_make_unique))
 using std::make_unique;
 #else
 // -----------------------------------------------------------------------------
diff --git a/absl/synchronization/internal/create_thread_identity.cc b/absl/synchronization/internal/create_thread_identity.cc
index ce3676a884..eef661f23d 100644
--- a/absl/synchronization/internal/create_thread_identity.cc
+++ b/absl/synchronization/internal/create_thread_identity.cc
@@ -37,7 +37,7 @@ static base_internal::ThreadIdentity* thread_identity_freelist;
 
 // A per-thread destructor for reclaiming associated ThreadIdentity objects.
 // Since we must preserve their storage we cache them for re-use.
-static void ReclaimThreadIdentity(void* v) {
+void ReclaimThreadIdentity(void* v) {
   base_internal::ThreadIdentity* identity =
       static_cast<base_internal::ThreadIdentity*>(v);
 
@@ -47,6 +47,8 @@ static void ReclaimThreadIdentity(void* v) {
     base_internal::LowLevelAlloc::Free(identity->per_thread_synch.all_locks);
   }
 
+  PerThreadSem::Destroy(identity);
+
   // We must explicitly clear the current thread's identity:
   // (a) Subsequent (unrelated) per-thread destructors may require an identity.
   //     We must guarantee a new identity is used in this case (this instructor
diff --git a/absl/synchronization/internal/create_thread_identity.h b/absl/synchronization/internal/create_thread_identity.h
index ebb16c56a7..97237a6ede 100644
--- a/absl/synchronization/internal/create_thread_identity.h
+++ b/absl/synchronization/internal/create_thread_identity.h
@@ -35,6 +35,10 @@ namespace synchronization_internal {
 // For private use only.
 base_internal::ThreadIdentity* CreateThreadIdentity();
 
+// A per-thread destructor for reclaiming associated ThreadIdentity objects.
+// For private use only.
+void ReclaimThreadIdentity(void* v);
+
 // Returns the ThreadIdentity object representing the calling thread; guaranteed
 // to be unique for its lifetime.  The returned object will remain valid for the
 // program's lifetime; although it may be re-assigned to a subsequent thread.
diff --git a/absl/synchronization/internal/per_thread_sem.cc b/absl/synchronization/internal/per_thread_sem.cc
index b7014fb2b0..2a78b20fc8 100644
--- a/absl/synchronization/internal/per_thread_sem.cc
+++ b/absl/synchronization/internal/per_thread_sem.cc
@@ -40,12 +40,16 @@ std::atomic<int> *PerThreadSem::GetThreadBlockedCounter() {
 }
 
 void PerThreadSem::Init(base_internal::ThreadIdentity *identity) {
-  Waiter::GetWaiter(identity)->Init();
+  new (Waiter::GetWaiter(identity)) Waiter();
   identity->ticker.store(0, std::memory_order_relaxed);
   identity->wait_start.store(0, std::memory_order_relaxed);
   identity->is_idle.store(false, std::memory_order_relaxed);
 }
 
+void PerThreadSem::Destroy(base_internal::ThreadIdentity *identity) {
+  Waiter::GetWaiter(identity)->~Waiter();
+}
+
 void PerThreadSem::Tick(base_internal::ThreadIdentity *identity) {
   const int ticker =
       identity->ticker.fetch_add(1, std::memory_order_relaxed) + 1;
diff --git a/absl/synchronization/internal/per_thread_sem.h b/absl/synchronization/internal/per_thread_sem.h
index e7da070ba7..113cdfb7dc 100644
--- a/absl/synchronization/internal/per_thread_sem.h
+++ b/absl/synchronization/internal/per_thread_sem.h
@@ -65,6 +65,10 @@ class PerThreadSem {
   // REQUIRES: May only be called by ThreadIdentity.
   static void Init(base_internal::ThreadIdentity* identity);
 
+  // Destroy the PerThreadSem associated with "identity".
+  // REQUIRES: May only be called by ThreadIdentity.
+  static void Destroy(base_internal::ThreadIdentity* identity);
+
   // Increments "identity"'s count.
   static inline void Post(base_internal::ThreadIdentity* identity);
 
@@ -77,6 +81,7 @@ class PerThreadSem {
   friend class PerThreadSemTest;
   friend class absl::Mutex;
   friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
+  friend void ReclaimThreadIdentity(void* v);
 };
 
 }  // namespace synchronization_internal
diff --git a/absl/synchronization/internal/waiter.cc b/absl/synchronization/internal/waiter.cc
index 2f41aa5f2b..8e71377053 100644
--- a/absl/synchronization/internal/waiter.cc
+++ b/absl/synchronization/internal/waiter.cc
@@ -122,10 +122,12 @@ class Futex {
   }
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   futex_.store(0, std::memory_order_relaxed);
 }
 
+Waiter::~Waiter() = default;
+
 bool Waiter::Wait(KernelTimeout t) {
   // Loop until we can atomically decrement futex from a positive
   // value, waiting on a futex while we believe it is zero.
@@ -199,7 +201,7 @@ class PthreadMutexHolder {
   pthread_mutex_t *mu_;
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   const int err = pthread_mutex_init(&mu_, 0);
   if (err != 0) {
     ABSL_RAW_LOG(FATAL, "pthread_mutex_init failed: %d", err);
@@ -214,6 +216,18 @@ void Waiter::Init() {
   wakeup_count_ = 0;
 }
 
+Waiter::~Waiter() {
+  const int err = pthread_mutex_destroy(&mu_);
+  if (err != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_mutex_destroy failed: %d", err);
+  }
+
+  const int err2 = pthread_cond_destroy(&cv_);
+  if (err2 != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_cond_destroy failed: %d", err2);
+  }
+}
+
 bool Waiter::Wait(KernelTimeout t) {
   struct timespec abs_timeout;
   if (t.has_timeout()) {
@@ -274,13 +288,19 @@ void Waiter::InternalCondVarPoke() {
 
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
 
-void Waiter::Init() {
+Waiter::Waiter() {
   if (sem_init(&sem_, 0, 0) != 0) {
     ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
   }
   wakeups_.store(0, std::memory_order_relaxed);
 }
 
+Waiter::~Waiter() {
+  if (sem_destroy(&sem_) != 0) {
+    ABSL_RAW_LOG(FATAL, "sem_destroy failed with errno %d\n", errno);
+  }
+}
+
 bool Waiter::Wait(KernelTimeout t) {
   struct timespec abs_timeout;
   if (t.has_timeout()) {
@@ -388,39 +408,32 @@ class LockHolder {
   SRWLOCK* mu_;
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   auto *mu = ::new (static_cast<void *>(&mu_storage_)) SRWLOCK;
   auto *cv = ::new (static_cast<void *>(&cv_storage_)) CONDITION_VARIABLE;
   InitializeSRWLock(mu);
   InitializeConditionVariable(cv);
-  waiter_count_.store(0, std::memory_order_relaxed);
-  wakeup_count_.store(0, std::memory_order_relaxed);
+  waiter_count_ = 0;
+  wakeup_count_ = 0;
 }
 
+// SRW locks and condition variables do not need to be explicitly destroyed.
+// https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
+// https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
+Waiter::~Waiter() = default;
+
 bool Waiter::Wait(KernelTimeout t) {
   SRWLOCK *mu = WinHelper::GetLock(this);
   CONDITION_VARIABLE *cv = WinHelper::GetCond(this);
 
   LockHolder h(mu);
-  waiter_count_.fetch_add(1, std::memory_order_relaxed);
+  ++waiter_count_;
 
   // Loop until we find a wakeup to consume or timeout.
   // Note that, since the thread ticker is just reset, we don't need to check
   // whether the thread is idle on the very first pass of the loop.
   bool first_pass = true;
-  while (true) {
-    int x = wakeup_count_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
-                                               std::memory_order_acquire,
-                                               std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-      return true;
-    }
-
+  while (wakeup_count_ == 0) {
     if (!first_pass) MaybeBecomeIdle();
     // No wakeups available, time to wait.
     if (!SleepConditionVariableSRW(cv, mu, t.InMillisecondsFromNow(), 0)) {
@@ -429,7 +442,7 @@ bool Waiter::Wait(KernelTimeout t) {
       // initialization guarantees this is not a narrowing conversion.
       const unsigned long err{GetLastError()};  // NOLINT(runtime/int)
       if (err == ERROR_TIMEOUT) {
-        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+        --waiter_count_;
         return false;
       } else {
         ABSL_RAW_LOG(FATAL, "SleepConditionVariableSRW failed: %lu", err);
@@ -437,23 +450,27 @@ bool Waiter::Wait(KernelTimeout t) {
     }
     first_pass = false;
   }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
 }
 
 void Waiter::Post() {
-  wakeup_count_.fetch_add(1, std::memory_order_release);
-  Poke();
+  LockHolder h(WinHelper::GetLock(this));
+  ++wakeup_count_;
+  InternalCondVarPoke();
 }
 
 void Waiter::Poke() {
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  // Potentially a waiter. Take the lock and check again.
   LockHolder h(WinHelper::GetLock(this));
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
+  InternalCondVarPoke();
+}
+
+void Waiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    WakeConditionVariable(WinHelper::GetCond(this));
   }
-  WakeConditionVariable(WinHelper::GetCond(this));
 }
 
 #else
diff --git a/absl/synchronization/internal/waiter.h b/absl/synchronization/internal/waiter.h
index c23897007e..cac81d5fd2 100644
--- a/absl/synchronization/internal/waiter.h
+++ b/absl/synchronization/internal/waiter.h
@@ -58,14 +58,15 @@ namespace synchronization_internal {
 // Waiter is an OS-specific semaphore.
 class Waiter {
  public:
-  // No constructor, instances use the reserved space in ThreadIdentity.
-  // All initialization logic belongs in `Init()`.
-  Waiter() = delete;
+  // Prepare any data to track waits.
+  Waiter();
+
+  // Not copyable or movable
   Waiter(const Waiter&) = delete;
   Waiter& operator=(const Waiter&) = delete;
 
-  // Prepare any data to track waits.
-  void Init();
+  // Destroy any data to track waits.
+  ~Waiter();
 
   // Blocks the calling thread until a matching call to `Post()` or
   // `t` has passed. Returns `true` if woken (`Post()` called),
@@ -122,13 +123,8 @@ class Waiter {
   std::atomic<int> wakeups_;
 
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
-  // The Windows API has lots of choices for synchronization
-  // primivitives.  We are using SRWLOCK and CONDITION_VARIABLE
-  // because they don't require a destructor to release system
-  // resources.
-  //
-  // However, we can't include Windows.h in our headers, so we use aligned
-  // storage buffers to define the storage.
+  // We can't include Windows.h in our headers, so we use aligned storage
+  // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
   using SRWLockStorage =
       typename std::aligned_storage<sizeof(void*), alignof(void*)>::type;
   using ConditionVariableStorage =
@@ -138,10 +134,13 @@ class Waiter {
   // condition variable storage once the types are complete.
   class WinHelper;
 
+  // REQUIRES: WinHelper::GetLock(this) must be held.
+  void InternalCondVarPoke();
+
   SRWLockStorage mu_storage_;
   ConditionVariableStorage cv_storage_;
-  std::atomic<int> waiter_count_;
-  std::atomic<int> wakeup_count_;
+  int waiter_count_;
+  int wakeup_count_;
 
 #else
   #error Unknown ABSL_WAITER_MODE