Export of internal Abseil changes

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506fa3e10b3d8399ad937c32ecea26d1ad4e62bb by Abseil Team <absl-team@google.com>:

Disable ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE when GCC < 8.2.0 is used with libc++

PiperOrigin-RevId: 399707056

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656b7c7cee87f46a4bc7953618796f82da08e62c by Derek Mauro <dmauro@google.com>:

Remove the MSVC flag implementation from flag.h to help clarify that
methods on absl::Flag<T> are not part of the public API

PiperOrigin-RevId: 399584678

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a92a9bc156303bc663b84c4b704891ec8f67e333 by Abseil Team <absl-team@google.com>:

Get rid of MemcpyIfAllowed while continuing to suppress erroneous warnings

PiperOrigin-RevId: 399468864

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5f9a66895f707ba001fb51b88c0c6025f8c872a3 by Abseil Team <absl-team@google.com>:

Use feature testing to check for availability of invoke_result.

Feature testing should be available by C++20, which removes invoke_result.
https://en.cppreference.com/w/cpp/feature_test

PiperOrigin-RevId: 399447373

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946c0a502b4499dbfcabf1ab93ddde0048288fb4 by CJ Johnson <johnsoncj@google.com>:

Add rvalue-reference qualifier to the Commit method on ConstructionTransaction

PiperOrigin-RevId: 399442206

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726a4d036eff49aeb6fd0ca2b1775699b6844395 by Greg Falcon <gfalcon@google.com>:

Internal change

PiperOrigin-RevId: 399441870

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1df6d3f659b88dbac13c3d8e13db23bb3844ece2 by Abseil Team <absl-team@google.com>:

Clang-format whitespace changes

PiperOrigin-RevId: 399281271

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4a828cde95a07421d699ebac775b37810624214f by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 399234071

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e520c72b34ba2f98668c889139001f8276243d31 by Greg Falcon <gfalcon@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 399233662
GitOrigin-RevId: 506fa3e10b3d8399ad937c32ecea26d1ad4e62bb
Change-Id: I92b9176d2387c08eb167f9268efa78b55b8e09c2

1 files changed, 116 insertions, 0 deletions

diff --git a/absl/flags/internal/flag_msvc.inc b/absl/flags/internal/flag_msvc.inc
new file mode 100644
index 00000000..c31bd27f
--- /dev/null
+++ b/absl/flags/internal/flag_msvc.inc
@@ -0,0 +1,116 @@
+//
+//  Copyright 2021 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Do not include this file directly.
+// Include absl/flags/flag.h instead.
+
+// 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();
+}  // namespace flags_internal
+
+// Public methods of `absl::Flag<T>` are NOT part of the Abseil Flags API.
+// See https://abseil.io/docs/cpp/guides/flags
+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 char* filename,
+                 const flags_internal::HelpGenFunc help_gen,
+                 const flags_internal::FlagDfltGenFunc default_value_gen)
+      : name_(name),
+        filename_(filename),
+        help_gen_(help_gen),
+        default_value_gen_(default_value_gen),
+        inited_(false),
+        impl_(nullptr) {}
+#endif
+
+  flags_internal::Flag<T>& GetImpl() const {
+    if (!inited_.load(std::memory_order_acquire)) {
+      absl::MutexLock l(flags_internal::GetGlobalConstructionGuard());
+
+      if (inited_.load(std::memory_order_acquire)) {
+        return *impl_;
+      }
+
+      impl_ = new flags_internal::Flag<T>(
+          name_, filename_,
+          {flags_internal::FlagHelpMsg(help_gen_),
+           flags_internal::FlagHelpKind::kGenFunc},
+          {flags_internal::FlagDefaultSrc(default_value_gen_),
+           flags_internal::FlagDefaultKind::kGenFunc});
+      inited_.store(true, std::memory_order_release);
+    }
+
+    return *impl_;
+  }
+
+  // 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(); }
+  absl::string_view Name() const { return GetImpl().Name(); }
+  std::string Help() const { return GetImpl().Help(); }
+  bool IsModified() const { return GetImpl().IsModified(); }
+  bool IsSpecifiedOnCommandLine() const {
+    return GetImpl().IsSpecifiedOnCommandLine();
+  }
+  std::string Filename() const { return GetImpl().Filename(); }
+  std::string DefaultValue() const { return GetImpl().DefaultValue(); }
+  std::string CurrentValue() const { return GetImpl().CurrentValue(); }
+  template <typename U>
+  inline bool IsOfType() const {
+    return GetImpl().template IsOfType<U>();
+  }
+  T Get() const {
+    return flags_internal::FlagImplPeer::InvokeGet<T>(GetImpl());
+  }
+  void Set(const T& v) {
+    flags_internal::FlagImplPeer::InvokeSet(GetImpl(), v);
+  }
+  void InvokeCallback() { GetImpl().InvokeCallback(); }
+
+  const CommandLineFlag& Reflect() const {
+    return flags_internal::FlagImplPeer::InvokeReflect(GetImpl());
+  }
+
+  // The data members are logically private, but they need to be public for
+  // this to be an aggregate type.
+  const char* name_;
+  const char* filename_;
+  const flags_internal::HelpGenFunc help_gen_;
+  const flags_internal::FlagDfltGenFunc default_value_gen_;
+
+  mutable std::atomic<bool> inited_;
+  mutable flags_internal::Flag<T>* impl_;
+};