Merge branch 'master' into master

38 files changed, 455 insertions, 185 deletions

diff --git a/.ci/abseil-cpp.json b/.ci/abseil-cpp.json
deleted file mode 100644
index a90cc1717e54..000000000000
--- a/.ci/abseil-cpp.json
+++ /dev/null
@@ -1,8 +0,0 @@
-// This is a relaxed JSON format, you can have comments in it.
-// This is a list of configuration for the job that does not specify a configuration.
-[
-    {"node": "linux-x86_64"},
-    {"node": "ubuntu_16.04-x86_64"},
-    {"node": "darwin-x86_64"},
-    {"node": "windows-x86_64"}
-]
diff --git a/.clang-format b/.clang-format
new file mode 100644
index 000000000000..06ea346a1067
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,4 @@
+---
+Language:        Cpp
+BasedOnStyle:  Google
+...
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index c9c89bbc4248..40351ddcfaa1 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -47,7 +47,8 @@ will be expected to conform to the style outlined
     made and **why** it was made. Link to a GitHub issue if it exists.
 
 *   Don't fix code style and formatting unless you are already changing that
-    line to address an issue. PRs with irrelevant changes won't be merged. If
+    line to address an issue. Formatting of modified lines may be done using
+   `git clang-format`. PRs with irrelevant changes won't be merged. If
     you do want to fix formatting or style, do that in a separate PR.
 
 *   Unless your PR is trivial, you should expect there will be reviewer comments
diff --git a/README.md b/README.md
index 592dc66b2da7..dcd01ac6926e 100644
--- a/README.md
+++ b/README.md
@@ -90,7 +90,3 @@ For more information about Abseil:
 ## Build with CMake
 
 Please check the [CMake build instructions](CMake/README.md)
-
-## Disclaimer
-
-*   This is not an official Google product.
diff --git a/absl/BUILD.bazel b/absl/BUILD.bazel
index 403a35c384ee..439addbfc828 100644
--- a/absl/BUILD.bazel
+++ b/absl/BUILD.bazel
@@ -23,6 +23,7 @@ config_setting(
     values = {
         "compiler": "llvm",
     },
+    visibility = [":__subpackages__"],
 )
 
 # following configs are based on mapping defined in: https://git.io/v5Ijz
@@ -31,6 +32,7 @@ config_setting(
     values = {
         "cpu": "darwin",
     },
+    visibility = [":__subpackages__"],
 )
 
 config_setting(
@@ -38,6 +40,7 @@ config_setting(
     values = {
         "cpu": "x64_windows",
     },
+    visibility = [":__subpackages__"],
 )
 
 config_setting(
@@ -45,4 +48,5 @@ config_setting(
     values = {
         "cpu": "ppc",
     },
+    visibility = [":__subpackages__"],
 )
diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 23439a090418..e68c4500e561 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -25,11 +25,10 @@ package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])  # Apache 2.0
 
-exports_files(["thread_annotations.h"])
-
 cc_library(
     name = "spinlock_wait",
     srcs = [
+        "internal/spinlock_akaros.inc",
         "internal/spinlock_posix.inc",
         "internal/spinlock_wait.cc",
         "internal/spinlock_win32.inc",
@@ -39,6 +38,9 @@ cc_library(
         "internal/spinlock_wait.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
+    visibility = [
+        "//absl/base:__pkg__",
+    ],
     deps = [":core_headers"],
 )
 
@@ -83,6 +85,9 @@ cc_library(
         "internal/malloc_extension_c.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
     deps = [
         ":core_headers",
         ":dynamic_annotations",
@@ -108,6 +113,9 @@ cc_library(
     textual_hdrs = [
         "internal/malloc_hook_invoke.h",
     ],
+    visibility = [
+        "//absl:__subpackages__",
+    ],
     deps = [
         ":base",
         ":config",
@@ -124,6 +132,9 @@ cc_library(
         "internal/invoke.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
 )
 
 cc_library(
@@ -183,6 +194,9 @@ cc_library(
     features = [
         "-use_header_modules",
     ],
+    visibility = [
+        "//absl:__subpackages__",
+    ],
     deps = [
         ":base",
         ":config",
@@ -205,6 +219,7 @@ cc_library(
     testonly = 1,
     hdrs = ["internal/exception_testing.h"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//absl:__subpackages__"],
     deps = [
         ":config",
         "@com_google_googletest//:gtest",
diff --git a/absl/base/config.h b/absl/base/config.h
index 5f0dd04c8e66..495811bd88c3 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -372,4 +372,19 @@
 #endif
 #endif
 
+// For MSVC, `__has_include` is supported in VS 2017 15.3, which is later than
+// the support for <optional>, <any>, <string_view>. So we use _MSC_VER to check
+// whether we have VS 2017 RTM (when <optional>, <any>, <string_view> is
+// implemented) or higher.
+// Also, `__cplusplus` is not correctly set by MSVC, so we use `_MSVC_LANG` to
+// check the language version.
+// TODO(zhangxy): fix tests before enabling aliasing for `std::any`,
+// `std::string_view`.
+#if defined(_MSC_VER) && _MSC_VER >= 1910 && \
+    ((defined(_MSVC_LANG) && _MSVC_LANG > 201402) || __cplusplus > 201402)
+// #define ABSL_HAVE_STD_ANY 1
+#define ABSL_HAVE_STD_OPTIONAL 1
+// #define ABSL_HAVE_STD_STRING_VIEW 1
+#endif
+
 #endif  // ABSL_BASE_CONFIG_H_
diff --git a/absl/base/internal/low_level_alloc.cc b/absl/base/internal/low_level_alloc.cc
index 08f89ea9ee54..8e2f9c98798b 100644
--- a/absl/base/internal/low_level_alloc.cc
+++ b/absl/base/internal/low_level_alloc.cc
@@ -30,6 +30,7 @@
 #ifndef ABSL_LOW_LEVEL_ALLOC_MISSING
 
 #ifndef _WIN32
+#include <pthread.h>
 #include <signal.h>
 #include <sys/mman.h>
 #include <unistd.h>
diff --git a/absl/base/internal/malloc_extension.cc b/absl/base/internal/malloc_extension.cc
index 3da981ce06a3..d48ec5bcf548 100644
--- a/absl/base/internal/malloc_extension.cc
+++ b/absl/base/internal/malloc_extension.cc
@@ -29,6 +29,13 @@ namespace base_internal {
 SysAllocator::~SysAllocator() {}
 void SysAllocator::GetStats(char* buffer, int) { buffer[0] = 0; }
 
+// Dummy key method to avoid weak vtable.
+void MallocExtensionWriter::UnusedKeyMethod() {}
+
+void StringMallocExtensionWriter::Write(const char* buf, int len) {
+  out_->append(buf, len);
+}
+
 // Default implementation -- does nothing
 MallocExtension::~MallocExtension() { }
 bool MallocExtension::VerifyAllMemory() { return true; }
diff --git a/absl/base/internal/malloc_extension.h b/absl/base/internal/malloc_extension.h
index 46b767ff2c3f..75a00ce9a78d 100644
--- a/absl/base/internal/malloc_extension.h
+++ b/absl/base/internal/malloc_extension.h
@@ -388,6 +388,9 @@ class MallocExtensionWriter {
   MallocExtensionWriter() {}
   MallocExtensionWriter(const MallocExtensionWriter&) = delete;
   MallocExtensionWriter& operator=(const MallocExtensionWriter&) = delete;
+
+ private:
+  virtual void UnusedKeyMethod();  // Dummy key method to avoid weak vtable.
 };
 
 // A subclass that writes to the std::string "out".  NOTE: The generated
@@ -396,9 +399,7 @@ class MallocExtensionWriter {
 class StringMallocExtensionWriter : public MallocExtensionWriter {
  public:
   explicit StringMallocExtensionWriter(std::string* out) : out_(out) {}
-  virtual void Write(const char* buf, int len) {
-    out_->append(buf, len);
-  }
+  void Write(const char* buf, int len) override;
 
  private:
   std::string* const out_;
diff --git a/absl/base/internal/spinlock_akaros.inc b/absl/base/internal/spinlock_akaros.inc
new file mode 100644
index 000000000000..051c8cf87fb6
--- /dev/null
+++ b/absl/base/internal/spinlock_akaros.inc
@@ -0,0 +1,35 @@
+// Copyright 2017 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
+//
+//      http://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.
+//
+// This file is an Akaros-specific part of spinlock_wait.cc
+
+#include <atomic>
+
+#include "absl/base/internal/scheduling_mode.h"
+
+extern "C" {
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
+    std::atomic<uint32_t>* /* lock_word */, uint32_t /* value */,
+    int /* loop */, absl::base_internal::SchedulingMode /* mode */) {
+  // In Akaros, one must take care not to call anything that could cause a
+  // malloc(), a blocking system call, or a uthread_yield() while holding a
+  // spinlock. Our callers assume will not call into libraries or other
+  // arbitrary code.
+}
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(
+    std::atomic<uint32_t>* /* lock_word */, bool /* all */) {}
+
+}  // extern "C"
diff --git a/absl/base/internal/spinlock_wait.cc b/absl/base/internal/spinlock_wait.cc
index 0fd36286629d..8f951b66dc36 100644
--- a/absl/base/internal/spinlock_wait.cc
+++ b/absl/base/internal/spinlock_wait.cc
@@ -23,6 +23,8 @@
 
 #if defined(_WIN32)
 #include "absl/base/internal/spinlock_win32.inc"
+#elif defined(__akaros__)
+#include "absl/base/internal/spinlock_akaros.inc"
 #else
 #include "absl/base/internal/spinlock_posix.inc"
 #endif
diff --git a/absl/base/internal/sysinfo.cc b/absl/base/internal/sysinfo.cc
index 9e0140fadb1f..00e98b662cc6 100644
--- a/absl/base/internal/sysinfo.cc
+++ b/absl/base/internal/sysinfo.cc
@@ -284,6 +284,30 @@ pid_t GetTID() {
   return syscall(SYS_gettid);
 }
 
+#elif defined(__akaros__)
+
+pid_t GetTID() {
+  // Akaros has a concept of "vcore context", which is the state the program
+  // is forced into when we need to make a user-level scheduling decision, or
+  // run a signal handler.  This is analogous to the interrupt context that a
+  // CPU might enter if it encounters some kind of exception.
+  //
+  // There is no current thread context in vcore context, but we need to give
+  // a reasonable answer if asked for a thread ID (e.g., in a signal handler).
+  // Thread 0 always exists, so if we are in vcore context, we return that.
+  //
+  // Otherwise, we know (since we are using pthreads) that the uthread struct
+  // current_uthread is pointing to is the first element of a
+  // struct pthread_tcb, so we extract and return the thread ID from that.
+  //
+  // TODO(dcross): Akaros anticipates moving the thread ID to the uthread
+  // structure at some point. We should modify this code to remove the cast
+  // when that happens.
+  if (in_vcore_context())
+    return 0;
+  return reinterpret_cast<struct pthread_tcb *>(current_uthread)->id;
+}
+
 #else
 
 // Fallback implementation of GetTID using pthread_getspecific.
diff --git a/absl/base/internal/tsan_mutex_interface.h b/absl/base/internal/tsan_mutex_interface.h
index a1303e67c8f5..6bb4faedb879 100644
--- a/absl/base/internal/tsan_mutex_interface.h
+++ b/absl/base/internal/tsan_mutex_interface.h
@@ -19,7 +19,22 @@
 #ifndef ABSL_BASE_INTERNAL_TSAN_MUTEX_INTERFACE_H_
 #define ABSL_BASE_INTERNAL_TSAN_MUTEX_INTERFACE_H_
 
-#ifdef THREAD_SANITIZER
+// ABSL_INTERNAL_HAVE_TSAN_INTERFACE
+// Macro intended only for internal use.
+//
+// Checks whether LLVM Thread Sanitizer interfaces are available.
+// First made available in LLVM 5.0 (Sep 2017).
+#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
+#error "ABSL_INTERNAL_HAVE_TSAN_INTERFACE cannot be directly set."
+#endif
+
+#if defined(THREAD_SANITIZER) && defined(__has_include)
+#if __has_include(<sanitizer/tsan_interface.h>)
+#define ABSL_INTERNAL_HAVE_TSAN_INTERFACE 1
+#endif
+#endif
+
+#ifdef ABSL_INTERNAL_HAVE_TSAN_INTERFACE
 #include <sanitizer/tsan_interface.h>
 
 #define ABSL_TSAN_MUTEX_CREATE __tsan_mutex_create
diff --git a/absl/base/macros.h b/absl/base/macros.h
index 31d1c02ecfa6..d41408727731 100644
--- a/absl/base/macros.h
+++ b/absl/base/macros.h
@@ -29,6 +29,7 @@
 #ifndef ABSL_BASE_MACROS_H_
 #define ABSL_BASE_MACROS_H_
 
+#include <cassert>
 #include <cstddef>
 
 #include "absl/base/port.h"
diff --git a/absl/container/BUILD.bazel b/absl/container/BUILD.bazel
index ee017431c88b..7d550cb16c5a 100644
--- a/absl/container/BUILD.bazel
+++ b/absl/container/BUILD.bazel
@@ -112,6 +112,9 @@ cc_library(
     srcs = ["internal/test_instance_tracker.cc"],
     hdrs = ["internal/test_instance_tracker.h"],
     copts = ABSL_DEFAULT_COPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
 )
 
 cc_test(
diff --git a/absl/container/inlined_vector.h b/absl/container/inlined_vector.h
index f68ca5074149..e0bb900cbb66 100644
--- a/absl/container/inlined_vector.h
+++ b/absl/container/inlined_vector.h
@@ -82,7 +82,8 @@ class InlinedVector {
   using reverse_iterator = std::reverse_iterator<iterator>;
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
-  InlinedVector() noexcept(noexcept(allocator_type()))
+  InlinedVector() noexcept(
+      std::is_nothrow_default_constructible<allocator_type>::value)
       : allocator_and_tag_(allocator_type()) {}
 
   explicit InlinedVector(const allocator_type& alloc) noexcept
@@ -148,6 +149,9 @@ class InlinedVector {
   ~InlinedVector() { clear(); }
 
   InlinedVector& operator=(const InlinedVector& v) {
+    if (this == &v) {
+      return *this;
+    }
     // Optimized to avoid reallocation.
     // Prefer reassignment to copy construction for elements.
     if (size() < v.size()) {  // grow
@@ -680,6 +684,8 @@ class InlinedVector {
   // portion and the start of the uninitialized portion of the created gap.
   // The number of initialized spots is pair.second - pair.first;
   // the number of raw spots is n - (pair.second - pair.first).
+  //
+  // Updates the size of the InlinedVector internally.
   std::pair<iterator, iterator> ShiftRight(const_iterator position,
                                            size_type n);
 
@@ -1013,28 +1019,19 @@ typename InlinedVector<T, N, A>::iterator InlinedVector<T, N, A>::emplace(
     emplace_back(std::forward<Args>(args)...);
     return end() - 1;
   }
-  size_type s = size();
-  size_type idx = std::distance(cbegin(), position);
-  if (s == capacity()) {
-    EnlargeBy(1);
-  }
-  assert(s < capacity());
-  iterator pos = begin() + idx;  // Set 'pos' to a post-enlarge iterator.
 
-  pointer space;
-  if (allocated()) {
-    tag().set_allocated_size(s + 1);
-    space = allocated_space();
+  T new_t = T(std::forward<Args>(args)...);
+
+  auto range = ShiftRight(position, 1);
+  if (range.first == range.second) {
+    // constructing into uninitialized memory
+    Construct(range.first, std::move(new_t));
   } else {
-    tag().set_inline_size(s + 1);
-    space = inlined_space();
+    // assigning into moved-from object
+    *range.first = T(std::move(new_t));
   }
-  Construct(space + s, std::move(space[s - 1]));
-  std::move_backward(pos, space + s - 1, space + s);
-  Destroy(pos, pos + 1);
-  Construct(pos, std::forward<Args>(args)...);
 
-  return pos;
+  return range.first;
 }
 
 template <typename T, size_t N, typename A>
@@ -1219,6 +1216,7 @@ auto InlinedVector<T, N, A>::ShiftRight(const_iterator position, size_type n)
     start_used = pos;
     start_raw = pos + new_elements_in_used_space;
   }
+  tag().add_size(n);
   return std::make_pair(start_used, start_raw);
 }
 
@@ -1297,10 +1295,12 @@ auto InlinedVector<T, N, A>::InsertWithCount(const_iterator position,
     -> iterator {
   assert(position >= begin() && position <= end());
   if (n == 0) return const_cast<iterator>(position);
+
+  value_type copy = v;
   std::pair<iterator, iterator> it_pair = ShiftRight(position, n);
-  std::fill(it_pair.first, it_pair.second, v);
-  UninitializedFill(it_pair.second, it_pair.first + n, v);
-  tag().add_size(n);
+  std::fill(it_pair.first, it_pair.second, copy);
+  UninitializedFill(it_pair.second, it_pair.first + n, copy);
+
   return it_pair.first;
 }
 
@@ -1336,7 +1336,6 @@ auto InlinedVector<T, N, A>::InsertWithRange(const_iterator position,
   ForwardIter open_spot = std::next(first, used_spots);
   std::copy(first, open_spot, it_pair.first);
   UninitializedCopy(open_spot, last, it_pair.second);
-  tag().add_size(n);
   return it_pair.first;
 }
 
diff --git a/absl/container/inlined_vector_test.cc b/absl/container/inlined_vector_test.cc
index c559a9a1fcba..055bca983cac 100644
--- a/absl/container/inlined_vector_test.cc
+++ b/absl/container/inlined_vector_test.cc
@@ -14,6 +14,7 @@
 
 #include "absl/container/inlined_vector.h"
 
+#include <algorithm>
 #include <forward_list>
 #include <list>
 #include <memory>
@@ -569,6 +570,16 @@ TEST(IntVec, CopyConstructorAndAssignment) {
   }
 }
 
+TEST(IntVec, AliasingCopyAssignment) {
+  for (int len = 0; len < 20; ++len) {
+    IntVec original;
+    Fill(&original, len);
+    IntVec dup = original;
+    dup = dup;
+    EXPECT_EQ(dup, original);
+  }
+}
+
 TEST(IntVec, MoveConstructorAndAssignment) {
   for (int len = 0; len < 20; len++) {
     IntVec v_in;
@@ -606,6 +617,78 @@ TEST(IntVec, MoveConstructorAndAssignment) {
   }
 }
 
+class NotTriviallyDestructible {
+ public:
+  NotTriviallyDestructible() : p_(new int(1)) {}
+  explicit NotTriviallyDestructible(int i) : p_(new int(i)) {}
+
+  NotTriviallyDestructible(const NotTriviallyDestructible& other)
+      : p_(new int(*other.p_)) {}
+
+  NotTriviallyDestructible& operator=(const NotTriviallyDestructible& other) {
+    p_ = absl::make_unique<int>(*other.p_);
+    return *this;
+  }
+
+  bool operator==(const NotTriviallyDestructible& other) const {
+    return *p_ == *other.p_;
+  }
+
+ private:
+  std::unique_ptr<int> p_;
+};
+
+TEST(AliasingTest, Emplace) {
+  for (int i = 2; i < 20; ++i) {
+    absl::InlinedVector<NotTriviallyDestructible, 10> vec;
+    for (int j = 0; j < i; ++j) {
+      vec.push_back(NotTriviallyDestructible(j));
+    }
+    vec.emplace(vec.begin(), vec[0]);
+    EXPECT_EQ(vec[0], vec[1]);
+    vec.emplace(vec.begin() + i / 2, vec[i / 2]);
+    EXPECT_EQ(vec[i / 2], vec[i / 2 + 1]);
+    vec.emplace(vec.end() - 1, vec.back());
+    EXPECT_EQ(vec[vec.size() - 2], vec.back());
+  }
+}
+
+TEST(AliasingTest, InsertWithCount) {
+  for (int i = 1; i < 20; ++i) {
+    absl::InlinedVector<NotTriviallyDestructible, 10> vec;
+    for (int j = 0; j < i; ++j) {
+      vec.push_back(NotTriviallyDestructible(j));
+    }
+    for (int n = 0; n < 5; ++n) {
+      // We use back where we can because it's guaranteed to become invalidated
+      vec.insert(vec.begin(), n, vec.back());
+      auto b = vec.begin();
+      EXPECT_TRUE(
+          std::all_of(b, b + n, [&vec](const NotTriviallyDestructible& x) {
+            return x == vec.back();
+          }));
+
+      auto m_idx = vec.size() / 2;
+      vec.insert(vec.begin() + m_idx, n, vec.back());
+      auto m = vec.begin() + m_idx;
+      EXPECT_TRUE(
+          std::all_of(m, m + n, [&vec](const NotTriviallyDestructible& x) {
+            return x == vec.back();
+          }));
+
+      // We want distinct values so the equality test is meaningful,
+      // vec[vec.size() - 1] is also almost always invalidated.
+      auto old_e = vec.size() - 1;
+      auto val = vec[old_e];
+      vec.insert(vec.end(), n, vec[old_e]);
+      auto e = vec.begin() + old_e;
+      EXPECT_TRUE(std::all_of(
+          e, e + n,
+          [&val](const NotTriviallyDestructible& x) { return x == val; }));
+    }
+  }
+}
+
 TEST(OverheadTest, Storage) {
   // Check for size overhead.
   // In particular, ensure that std::allocator doesn't cost anything to store.
diff --git a/absl/debugging/internal/elf_mem_image.cc b/absl/debugging/internal/elf_mem_image.cc
index f6c6bc07d511..3dfef5e83e57 100644
--- a/absl/debugging/internal/elf_mem_image.cc
+++ b/absl/debugging/internal/elf_mem_image.cc
@@ -75,8 +75,9 @@ const T *GetTableElement(const ElfW(Ehdr) * ehdr, ElfW(Off) table_offset,
 
 }  // namespace
 
-const void *const ElfMemImage::kInvalidBase =
-    reinterpret_cast<const void *>(~0L);
+// The value of this variable doesn't matter; it's used only for its
+// unique address.
+const int ElfMemImage::kInvalidBaseSentinel = 0;
 
 ElfMemImage::ElfMemImage(const void *base) {
   ABSL_RAW_CHECK(base != kInvalidBase, "bad pointer");
diff --git a/absl/debugging/internal/elf_mem_image.h b/absl/debugging/internal/elf_mem_image.h
index 7f3dbb971bd6..20a32a490cb2 100644
--- a/absl/debugging/internal/elf_mem_image.h
+++ b/absl/debugging/internal/elf_mem_image.h
@@ -43,9 +43,14 @@ namespace debug_internal {
 
 // An in-memory ELF image (may not exist on disk).
 class ElfMemImage {
+ private:
+  // Sentinel: there could never be an elf image at &kInvalidBaseSentinel.
+  static const int kInvalidBaseSentinel;
+
  public:
   // Sentinel: there could never be an elf image at this address.
-  static const void *const kInvalidBase;
+  static constexpr const void *const kInvalidBase =
+    static_cast<const void*>(&kInvalidBaseSentinel);
 
   // Information about a single vdso symbol.
   // All pointers are into .dynsym, .dynstr, or .text of the VDSO.
diff --git a/absl/debugging/internal/stacktrace_x86-inl.inc b/absl/debugging/internal/stacktrace_x86-inl.inc
index 6e1af017878e..9bdaa542ce27 100644
--- a/absl/debugging/internal/stacktrace_x86-inl.inc
+++ b/absl/debugging/internal/stacktrace_x86-inl.inc
@@ -114,7 +114,9 @@ static const int kMaxFrameBytes = 100000;
 // vuc is a ucontext_t *.  We use void* to avoid the use
 // of ucontext_t on non-POSIX systems.
 static uintptr_t GetFP(const void *vuc) {
-#if defined(__linux__)
+#if !defined(__linux__)
+  static_cast<void>(vuc);  // Avoid an unused argument compiler warning.
+#else
   if (vuc != nullptr) {
     auto *uc = reinterpret_cast<const ucontext_t *>(vuc);
 #if defined(__i386__)
diff --git a/absl/debugging/internal/vdso_support.cc b/absl/debugging/internal/vdso_support.cc
index 5026e1c1a9c9..815e702f5699 100644
--- a/absl/debugging/internal/vdso_support.cc
+++ b/absl/debugging/internal/vdso_support.cc
@@ -20,10 +20,15 @@
 
 #ifdef ABSL_HAVE_VDSO_SUPPORT     // defined in vdso_support.h
 
+#include <errno.h>
 #include <fcntl.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
+#if __GLIBC_PREREQ(2, 16)  // GLIBC-2.16 implements getauxval.
+#include <sys/auxv.h>
+#endif
+
 #include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/port.h"
@@ -35,8 +40,10 @@
 namespace absl {
 namespace debug_internal {
 
+ABSL_CONST_INIT
 std::atomic<const void *> VDSOSupport::vdso_base_(
     debug_internal::ElfMemImage::kInvalidBase);
+
 std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(&InitAndGetCPU);
 VDSOSupport::VDSOSupport()
     // If vdso_base_ is still set to kInvalidBase, we got here
@@ -56,37 +63,44 @@ VDSOSupport::VDSOSupport()
 // Finally, even if there is a race here, it is harmless, because
 // the operation should be idempotent.
 const void *VDSOSupport::Init() {
-  if (vdso_base_.load(std::memory_order_relaxed) ==
-      debug_internal::ElfMemImage::kInvalidBase) {
-    {
-      // Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
-      // on stack, and so glibc works as if VDSO was not present.
-      // But going directly to kernel via /proc/self/auxv below bypasses
-      // Valgrind zapping. So we check for Valgrind separately.
-      if (RunningOnValgrind()) {
-        vdso_base_.store(nullptr, std::memory_order_relaxed);
-        getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
-        return nullptr;
-      }
-      int fd = open("/proc/self/auxv", O_RDONLY);
-      if (fd == -1) {
-        // Kernel too old to have a VDSO.
-        vdso_base_.store(nullptr, std::memory_order_relaxed);
-        getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
-        return nullptr;
-      }
-      ElfW(auxv_t) aux;
-      while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
-        if (aux.a_type == AT_SYSINFO_EHDR) {
-          vdso_base_.store(reinterpret_cast<void *>(aux.a_un.a_val),
-                           std::memory_order_relaxed);
-          break;
-        }
+  const auto kInvalidBase = debug_internal::ElfMemImage::kInvalidBase;
+#if __GLIBC_PREREQ(2, 16)
+  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
+    errno = 0;
+    const void *const sysinfo_ehdr =
+        reinterpret_cast<const void *>(getauxval(AT_SYSINFO_EHDR));
+    if (errno == 0) {
+      vdso_base_.store(sysinfo_ehdr, std::memory_order_relaxed);
+    }
+  }
+#endif  // __GLIBC_PREREQ(2, 16)
+  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
+    // Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
+    // on stack, and so glibc works as if VDSO was not present.
+    // But going directly to kernel via /proc/self/auxv below bypasses
+    // Valgrind zapping. So we check for Valgrind separately.
+    if (RunningOnValgrind()) {
+      vdso_base_.store(nullptr, std::memory_order_relaxed);
+      getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
+      return nullptr;
+    }
+    int fd = open("/proc/self/auxv", O_RDONLY);
+    if (fd == -1) {
+      // Kernel too old to have a VDSO.
+      vdso_base_.store(nullptr, std::memory_order_relaxed);
+      getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
+      return nullptr;
+    }
+    ElfW(auxv_t) aux;
+    while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
+      if (aux.a_type == AT_SYSINFO_EHDR) {
+        vdso_base_.store(reinterpret_cast<void *>(aux.a_un.a_val),
+                         std::memory_order_relaxed);
+        break;
       }
-      close(fd);
     }
-    if (vdso_base_.load(std::memory_order_relaxed) ==
-        debug_internal::ElfMemImage::kInvalidBase) {
+    close(fd);
+    if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
       // Didn't find AT_SYSINFO_EHDR in auxv[].
       vdso_base_.store(nullptr, std::memory_order_relaxed);
     }
@@ -135,6 +149,7 @@ long VDSOSupport::GetCPUViaSyscall(unsigned *cpu,  // NOLINT(runtime/int)
   return syscall(SYS_getcpu, cpu, nullptr, nullptr);
 #else
   // x86_64 never implemented sys_getcpu(), except as a VDSO call.
+  static_cast<void>(cpu);  // Avoid an unused argument compiler warning.
   errno = ENOSYS;
   return -1;
 #endif
diff --git a/absl/memory/memory.h b/absl/memory/memory.h
index c67996084ef6..15cd85f4e1ed 100644
--- a/absl/memory/memory.h
+++ b/absl/memory/memory.h
@@ -38,8 +38,8 @@ namespace absl {
 // Function Template: WrapUnique()
 // -----------------------------------------------------------------------------
 //
-// Transfers ownership of a raw pointer to a `std::unique_ptr`. The returned
-// value is a `std::unique_ptr` of deduced type.
+//  Adopts ownership from a raw pointer and transfers it to the returned
+//  `std::unique_ptr`, whose type is deduced.
 //
 // Example:
 //   X* NewX(int, int);
@@ -81,6 +81,9 @@ struct MakeUniqueResult<T[N]> {
 
 }  // namespace memory_internal
 
+#if __cplusplus >= 201402L || defined(_MSC_VER)
+using std::make_unique;
+#else
 // -----------------------------------------------------------------------------
 // Function Template: make_unique<T>()
 // -----------------------------------------------------------------------------
@@ -164,13 +167,14 @@ typename memory_internal::MakeUniqueResult<T>::array make_unique(size_t n) {
 template <typename T, typename... Args>
 typename memory_internal::MakeUniqueResult<T>::invalid make_unique(
     Args&&... /* args */) = delete;
+#endif
 
 // -----------------------------------------------------------------------------
 // Function Template: RawPtr()
 // -----------------------------------------------------------------------------
 //
-// Extracts the raw pointer from a pointer-like 'ptr'. `absl::RawPtr` is useful
-// within templates that need to handle a complement of raw pointers,
+// Extracts the raw pointer from a pointer-like value `ptr`. `absl::RawPtr` is
+// useful within templates that need to handle a complement of raw pointers,
 // `std::nullptr_t`, and smart pointers.
 template <typename T>
 auto RawPtr(T&& ptr) -> decltype(&*ptr) {
@@ -183,9 +187,9 @@ inline std::nullptr_t RawPtr(std::nullptr_t) { return nullptr; }
 // Function Template: ShareUniquePtr()
 // -----------------------------------------------------------------------------
 //
-// Transforms a `std::unique_ptr` rvalue into a `std::shared_ptr`. The returned
-// value is a `std::shared_ptr` of deduced type and ownership is transferred to
-// the shared pointer.
+// Adopts a `std::unique_ptr` rvalue and returns a `std::shared_ptr` of deduced
+// type. Ownership (if any) of the held value is transferred to the returned
+// shared pointer.
 //
 // Example:
 //
@@ -194,8 +198,11 @@ inline std::nullptr_t RawPtr(std::nullptr_t) { return nullptr; }
 //     CHECK_EQ(*sp, 10);
 //     CHECK(up == nullptr);
 //
-// Note that this conversion is correct even when T is an array type, although
-// the resulting shared pointer may not be very useful.
+// Note that this conversion is correct even when T is an array type, and more
+// generally it works for *any* deleter of the `unique_ptr` (single-object
+// deleter, array deleter, or any custom deleter), since the deleter is adopted
+// by the shared pointer as well. The deleter is copied (unless it is a
+// reference).
 //
 // Implements the resolution of [LWG 2415](http://wg21.link/lwg2415), by which a
 // null shared pointer does not attempt to call the deleter.
diff --git a/absl/memory/memory_test.cc b/absl/memory/memory_test.cc
index 8a5f5522a089..7d047ca0c726 100644
--- a/absl/memory/memory_test.cc
+++ b/absl/memory/memory_test.cc
@@ -138,6 +138,16 @@ TEST(Make_UniqueTest, Array) {
   EXPECT_THAT(ArrayWatch::allocs(), ElementsAre(5 * sizeof(ArrayWatch)));
 }
 
+TEST(Make_UniqueTest, NotAmbiguousWithStdMakeUnique) {
+  // Ensure that absl::make_unique is not ambiguous with std::make_unique.
+  // In C++14 mode, the below call to make_unique has both types as candidates.
+  struct TakesStdType {
+    explicit TakesStdType(const std::vector<int> &vec) {}
+  };
+  using absl::make_unique;
+  make_unique<TakesStdType>(std::vector<int>());
+}
+
 #if 0
 // TODO(billydonahue): Make a proper NC test.
 // These tests shouldn't compile.
diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index b26106631f03..49f49abd8d78 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -103,6 +103,7 @@ cc_test(
     size = "small",
     srcs = ["match_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "@com_google_googletest//:gtest_main",
@@ -117,6 +118,7 @@ cc_test(
         "internal/escaping_test_common.inc",
     ],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:core_headers",
@@ -130,6 +132,7 @@ cc_test(
     size = "small",
     srcs = ["ascii_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:core_headers",
@@ -145,6 +148,7 @@ cc_test(
         "internal/memutil_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:core_headers",
@@ -159,6 +163,7 @@ cc_test(
         "internal/utf8_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":internal",
         ":strings",
@@ -172,6 +177,7 @@ cc_test(
     size = "small",
     srcs = ["string_view_test.cc"],
     copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:config",
@@ -186,6 +192,7 @@ cc_test(
     size = "small",
     srcs = ["substitute_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:core_headers",
@@ -198,6 +205,7 @@ cc_test(
     size = "small",
     srcs = ["str_replace_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "@com_google_googletest//:gtest_main",
@@ -208,6 +216,7 @@ cc_test(
     name = "str_split_test",
     srcs = ["str_split_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:core_headers",
@@ -221,6 +230,7 @@ cc_test(
     size = "small",
     srcs = ["internal/ostringstream_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":internal",
         "@com_google_googletest//:gtest_main",
@@ -235,6 +245,7 @@ cc_test(
         "internal/resize_uninitialized_test.cc",
     ],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         "//absl/base:core_headers",
         "//absl/meta:type_traits",
@@ -247,6 +258,7 @@ cc_test(
     size = "small",
     srcs = ["str_join_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:core_headers",
@@ -260,6 +272,7 @@ cc_test(
     size = "small",
     srcs = ["str_cat_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base:core_headers",
@@ -278,6 +291,7 @@ cc_test(
     tags = [
         "no_test_loonix",
     ],
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "//absl/base",
@@ -291,6 +305,7 @@ cc_test(
     size = "small",
     srcs = ["strip_test.cc"],
     copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
     deps = [
         ":strings",
         "@com_google_googletest//:gtest_main",
diff --git a/absl/strings/internal/utf8.h b/absl/strings/internal/utf8.h
index 705eea7f16ab..5bd82e8427a6 100644
--- a/absl/strings/internal/utf8.h
+++ b/absl/strings/internal/utf8.h
@@ -25,7 +25,6 @@
 #include <cstddef>
 #include <cstdint>
 
-
 namespace absl {
 namespace strings_internal {
 
diff --git a/absl/strings/match.h b/absl/strings/match.h
index 4ac35f192e00..3d54da81d5f0 100644
--- a/absl/strings/match.h
+++ b/absl/strings/match.h
@@ -53,7 +53,7 @@ inline bool StrContains(absl::string_view haystack, absl::string_view needle) {
 inline bool StartsWith(absl::string_view text, absl::string_view prefix) {
   return prefix.empty() ||
          (text.size() >= prefix.size() &&
-         memcmp(text.data(), prefix.data(), prefix.size()) == 0);
+          memcmp(text.data(), prefix.data(), prefix.size()) == 0);
 }
 
 // EndsWith()
@@ -63,7 +63,8 @@ inline bool EndsWith(absl::string_view text, absl::string_view suffix) {
   return suffix.empty() ||
          (text.size() >= suffix.size() &&
           memcmp(text.data() + (text.size() - suffix.size()), suffix.data(),
-  suffix.size()) == 0);
+                 suffix.size()) == 0
+         );
 }
 
 // StartsWithIgnoreCase()
diff --git a/absl/strings/numbers.h b/absl/strings/numbers.h
index 74aebc8028ea..1f3bbcfae596 100644
--- a/absl/strings/numbers.h
+++ b/absl/strings/numbers.h
@@ -62,9 +62,9 @@ ABSL_MUST_USE_RESULT bool SimpleAtod(absl::string_view str, double* value);
 
 // SimpleAtob()
 //
-// Converts the given std::string into into a boolean, returning `true` if
-// successful. The following case-insensitive strings are interpreted as boolean
-// `true`: "true", "t", "yes", "y", "1". The following case-insensitive strings
+// Converts the given std::string into a boolean, returning `true` if successful.
+// The following case-insensitive strings are interpreted as boolean `true`:
+// "true", "t", "yes", "y", "1". The following case-insensitive strings
 // are interpreted as boolean `false`: "false", "f", "no", "n", "0".
 ABSL_MUST_USE_RESULT bool SimpleAtob(absl::string_view str, bool* value);
 
diff --git a/absl/strings/string_view.h b/absl/strings/string_view.h
index 951e9cbc039c..c3acd7290993 100644
--- a/absl/strings/string_view.h
+++ b/absl/strings/string_view.h
@@ -295,9 +295,8 @@ class string_view {
 
   // string_view::remove_prefix()
   //
-  // Removes the first `n` characters from the `string_view`, returning a
-  // pointer to the new first character. Note that the underlying std::string is not
-  // changed, only the view.
+  // Removes the first `n` characters from the `string_view`. Note that the
+  // underlying std::string is not changed, only the view.
   void remove_prefix(size_type n) {
     assert(n <= length_);
     ptr_ += n;
diff --git a/absl/strings/string_view_test.cc b/absl/strings/string_view_test.cc
index 6be6f3b82b11..13fc214b841c 100644
--- a/absl/strings/string_view_test.cc
+++ b/absl/strings/string_view_test.cc
@@ -922,6 +922,10 @@ TEST(StringViewTest, ConstexprCompiles) {
   constexpr absl::string_view::iterator const_begin_empty = sp.begin();
   constexpr absl::string_view::iterator const_end_empty = sp.end();
   EXPECT_EQ(const_begin_empty, const_end_empty);
+
+  constexpr absl::string_view::iterator const_begin_nullptr = cstr.begin();
+  constexpr absl::string_view::iterator const_end_nullptr = cstr.end();
+  EXPECT_EQ(const_begin_nullptr, const_end_nullptr);
 #endif
 
   constexpr absl::string_view::iterator const_begin = cstr_len.begin();
diff --git a/absl/synchronization/BUILD.bazel b/absl/synchronization/BUILD.bazel
index cc8cecf97991..4faf62de5f5e 100644
--- a/absl/synchronization/BUILD.bazel
+++ b/absl/synchronization/BUILD.bazel
@@ -34,6 +34,9 @@ cc_library(
         "internal/graphcycles.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
     deps = [
         "//absl/base",
         "//absl/base:core_headers",
diff --git a/absl/synchronization/mutex_test.cc b/absl/synchronization/mutex_test.cc
index cfe81096dc98..5a5874def72d 100644
--- a/absl/synchronization/mutex_test.cc
+++ b/absl/synchronization/mutex_test.cc
@@ -89,8 +89,6 @@ static void CheckSumG0G1(void *v) {
 }
 
 static void TestMu(TestContext *cxt, int c) {
-  SetInvariantChecked(false);
-  cxt->mu.EnableInvariantDebugging(CheckSumG0G1, cxt);
   for (int i = 0; i != cxt->iterations; i++) {
     absl::MutexLock l(&cxt->mu);
     int a = cxt->g0 + 1;
@@ -100,8 +98,6 @@ static void TestMu(TestContext *cxt, int c) {
 }
 
 static void TestTry(TestContext *cxt, int c) {
-  SetInvariantChecked(false);
-  cxt->mu.EnableInvariantDebugging(CheckSumG0G1, cxt);
   for (int i = 0; i != cxt->iterations; i++) {
     do {
       std::this_thread::yield();
@@ -122,8 +118,6 @@ static void TestR20ms(TestContext *cxt, int c) {
 }
 
 static void TestRW(TestContext *cxt, int c) {
-  SetInvariantChecked(false);
-  cxt->mu.EnableInvariantDebugging(CheckSumG0G1, cxt);
   if ((c & 1) == 0) {
     for (int i = 0; i != cxt->iterations; i++) {
       absl::WriterMutexLock l(&cxt->mu);
@@ -356,67 +350,57 @@ static void EndTest(int *c0, int *c1, absl::Mutex *mu, absl::CondVar *cv,
   cv->Signal();
 }
 
-// Basis for the parameterized tests configured below.
-static int RunTest(void (*test)(TestContext *cxt, int), int threads,
-                   int iterations, int operations) {
-  TestContext cxt;
+// Code common to RunTest() and RunTestWithInvariantDebugging().
+static int RunTestCommon(TestContext *cxt, void (*test)(TestContext *cxt, int),
+                         int threads, int iterations, int operations) {
   absl::Mutex mu2;
   absl::CondVar cv2;
-  int c0;
-  int c1;
-
-  // run with large thread count for full test and to get timing
-
-#if !defined(ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED)
-  absl::EnableMutexInvariantDebugging(false);
-#endif
-  c0 = 0;
-  c1 = 0;
-  cxt.g0 = 0;
-  cxt.g1 = 0;
-  cxt.iterations = iterations;
-  cxt.threads = threads;
+  int c0 = 0;
+  int c1 = 0;
+  cxt->g0 = 0;
+  cxt->g1 = 0;
+  cxt->iterations = iterations;
+  cxt->threads = threads;
   absl::synchronization_internal::ThreadPool tp(threads);
   for (int i = 0; i != threads; i++) {
     tp.Schedule(std::bind(&EndTest, &c0, &c1, &mu2, &cv2,
                           std::function<void(int)>(
-                              std::bind(test, &cxt, std::placeholders::_1))));
+                              std::bind(test, cxt, std::placeholders::_1))));
   }
   mu2.Lock();
   while (c1 != threads) {
     cv2.Wait(&mu2);
   }
   mu2.Unlock();
-  int saved_g0 = cxt.g0;
+  return cxt->g0;
+}
 
-  // run again with small number of iterations to test invariant checking
+// Basis for the parameterized tests configured below.
+static int RunTest(void (*test)(TestContext *cxt, int), int threads,
+                   int iterations, int operations) {
+  TestContext cxt;
+  return RunTestCommon(&cxt, test, threads, iterations, operations);
+}
 
+// Like RunTest(), but sets an invariant on the tested Mutex and
+// verifies that the invariant check happened. The invariant function
+// will be passed the TestContext* as its arg and must call
+// SetInvariantChecked(true);
 #if !defined(ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED)
+static int RunTestWithInvariantDebugging(void (*test)(TestContext *cxt, int),
+                                         int threads, int iterations,
+                                         int operations,
+                                         void (*invariant)(void *)) {
   absl::EnableMutexInvariantDebugging(true);
-#endif
-  SetInvariantChecked(true);
-  c0 = 0;
-  c1 = 0;
-  cxt.g0 = 0;
-  cxt.g1 = 0;
-  cxt.iterations = (iterations > 10 ? 10 : iterations);
-  cxt.threads = threads;
-  for (int i = 0; i != threads; i++) {
-    tp.Schedule(std::bind(&EndTest, &c0, &c1, &mu2, &cv2,
-                          std::function<void(int)>(
-                              std::bind(test, &cxt, std::placeholders::_1))));
-  }
-  mu2.Lock();
-  while (c1 != threads) {
-    cv2.Wait(&mu2);
-  }
-  mu2.Unlock();
-#if !defined(ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED)
+  SetInvariantChecked(false);
+  TestContext cxt;
+  cxt.mu.EnableInvariantDebugging(invariant, &cxt);
+  int ret = RunTestCommon(&cxt, test, threads, iterations, operations);
   ABSL_RAW_CHECK(GetInvariantChecked(), "Invariant not checked");
-#endif
-
-  return saved_g0;
+  absl::EnableMutexInvariantDebugging(false);  // Restore.
+  return ret;
 }
+#endif
 
 // --------------------------------------------------------
 // Test for fix of bug in TryRemove()
@@ -1463,6 +1447,13 @@ TEST_P(MutexVariableThreadCountTest, Mutex) {
   int iterations = ScaleIterations(10000000) / threads;
   int operations = threads * iterations;
   EXPECT_EQ(RunTest(&TestMu, threads, iterations, operations), operations);
+#if !defined(ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED)
+  iterations = std::min(iterations, 10);
+  operations = threads * iterations;
+  EXPECT_EQ(RunTestWithInvariantDebugging(&TestMu, threads, iterations,
+                                          operations, CheckSumG0G1),
+            operations);
+#endif
 }
 
 TEST_P(MutexVariableThreadCountTest, Try) {
@@ -1470,6 +1461,13 @@ TEST_P(MutexVariableThreadCountTest, Try) {
   int iterations = 1000000 / threads;
   int operations = iterations * threads;
   EXPECT_EQ(RunTest(&TestTry, threads, iterations, operations), operations);
+#if !defined(ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED)
+  iterations = std::min(iterations, 10);
+  operations = threads * iterations;
+  EXPECT_EQ(RunTestWithInvariantDebugging(&TestTry, threads, iterations,
+                                          operations, CheckSumG0G1),
+            operations);
+#endif
 }
 
 TEST_P(MutexVariableThreadCountTest, R20ms) {
@@ -1484,6 +1482,13 @@ TEST_P(MutexVariableThreadCountTest, RW) {
   int iterations = ScaleIterations(20000000) / threads;
   int operations = iterations * threads;
   EXPECT_EQ(RunTest(&TestRW, threads, iterations, operations), operations / 2);
+#if !defined(ABSL_MUTEX_ENABLE_INVARIANT_DEBUGGING_NOT_IMPLEMENTED)
+  iterations = std::min(iterations, 10);
+  operations = threads * iterations;
+  EXPECT_EQ(RunTestWithInvariantDebugging(&TestRW, threads, iterations,
+                                          operations, CheckSumG0G1),
+            operations / 2);
+#endif
 }
 
 TEST_P(MutexVariableThreadCountTest, Await) {
diff --git a/absl/time/BUILD.bazel b/absl/time/BUILD.bazel
index c34f52489c0d..3d1d2df563ba 100644
--- a/absl/time/BUILD.bazel
+++ b/absl/time/BUILD.bazel
@@ -57,6 +57,9 @@ cc_library(
     ],
     hdrs = ["internal/test_util.h"],
     copts = ABSL_DEFAULT_COPTS,
+    visibility = [
+        "//absl/time:__pkg__",
+    ],
     deps = [
         ":time",
         "//absl/base",
diff --git a/absl/time/internal/test_util.cc b/absl/time/internal/test_util.cc
index 1a415f89469e..8bb27a8f3ef2 100644
--- a/absl/time/internal/test_util.cc
+++ b/absl/time/internal/test_util.cc
@@ -63,7 +63,7 @@ const struct ZoneInfo {
     {"US/Pacific",  //
      reinterpret_cast<char*>(America_Los_Angeles), America_Los_Angeles_len},
 
-    // Allows use of the local time zone from a common system-specific location.
+    // Allows use of the local time zone from a system-specific location.
 #ifdef _MSC_VER
     {"localtime",  //
      reinterpret_cast<char*>(America_Los_Angeles), America_Los_Angeles_len},
diff --git a/absl/time/time.h b/absl/time/time.h
index 093f168df449..c01977b003eb 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -1126,8 +1126,10 @@ constexpr Duration OppositeInfinity(Duration d) {
              : MakeDuration(std::numeric_limits<int64_t>::min(), ~0U);
 }
 
-// Returns (-n)-1 (equivalently -(n+1)) without overflowing on any input value.
+// Returns (-n)-1 (equivalently -(n+1)) without avoidable overflow.
 constexpr int64_t NegateAndSubtractOne(int64_t n) {
+  // Note: Good compilers will optimize this expression to ~n when using
+  // a two's-complement representation (which is required for int64_t).
   return (n < 0) ? -(n + 1) : (-n) - 1;
 }
 
@@ -1232,31 +1234,26 @@ constexpr bool operator==(Duration lhs, Duration rhs) {
 constexpr Duration operator-(Duration d) {
   // This is a little interesting because of the special cases.
   //
-  // Infinities stay infinite, and just change direction.
+  // If rep_lo_ is zero, we have it easy; it's safe to negate rep_hi_, we're
+  // dealing with an integral number of seconds, and the only special case is
+  // the maximum negative finite duration, which can't be negated.
   //
-  // The maximum negative finite duration can't be negated (at least, not
-  // on a two's complement machine), so we return infinity for that case.
-  // Next we dispatch the case where rep_lo_ is zero, observing that it's
-  // safe to negate rep_hi_ in this case because it's not int64_t-min (or
-  // else we'd have handled it above, returning InfiniteDuration()).
+  // Infinities stay infinite, and just change direction.
   //
   // Finally we're in the case where rep_lo_ is non-zero, and we can borrow
   // a second's worth of ticks and avoid overflow (as negating int64_t-min + 1
   // is safe).
-  return time_internal::IsInfiniteDuration(d)
-             ? time_internal::OppositeInfinity(d)
-             : (time_internal::GetRepHi(d) ==
-                    std::numeric_limits<int64_t>::min() &&
-                time_internal::GetRepLo(d) == 0)
+  return time_internal::GetRepLo(d) == 0
+             ? time_internal::GetRepHi(d) == std::numeric_limits<int64_t>::min()
                    ? InfiniteDuration()
-                   : (time_internal::GetRepLo(d) == 0)
-                         ? time_internal::MakeDuration(
-                               -time_internal::GetRepHi(d))
-                         : time_internal::MakeDuration(
-                               time_internal::NegateAndSubtractOne(
-                                   time_internal::GetRepHi(d)),
-                               time_internal::kTicksPerSecond -
-                                   time_internal::GetRepLo(d));
+                   : time_internal::MakeDuration(-time_internal::GetRepHi(d))
+             : time_internal::IsInfiniteDuration(d)
+                   ? time_internal::OppositeInfinity(d)
+                   : time_internal::MakeDuration(
+                         time_internal::NegateAndSubtractOne(
+                             time_internal::GetRepHi(d)),
+                         time_internal::kTicksPerSecond -
+                             time_internal::GetRepLo(d));
 }
 
 constexpr Duration Nanoseconds(int64_t n) {
diff --git a/absl/types/any.h b/absl/types/any.h
index a51dea110d0f..2e7bf21f55f6 100644
--- a/absl/types/any.h
+++ b/absl/types/any.h
@@ -94,23 +94,20 @@ namespace absl {
 
 namespace any_internal {
 
-// FastTypeId<Type>() evaluates at compile/link-time to a unique integer for the
-// passed in type. Their values are neither contiguous nor small, making them
-// unfit for using as an index into a vector, but a good match for keys into
-// maps or straight up comparisons.
-// Note that on 64-bit (unix) systems size_t is 64-bit while int is 32-bit and
-// the compiler will happily and quietly assign such a 64-bit value to a
-// 32-bit integer. While a client should never do that it SHOULD still be safe,
-// assuming the BSS segment doesn't span more than 4GiB.
+template <typename Type>
+struct TypeTag {
+  constexpr static char dummy_var = 0;
+};
+
+template <typename Type>
+constexpr char TypeTag<Type>::dummy_var;
+
+// FastTypeId<Type>() evaluates at compile/link-time to a unique pointer for the
+// passed in type. These are meant to be good match for keys into maps or
+// straight up comparisons.
 template<typename Type>
-inline size_t FastTypeId() {
-  static_assert(sizeof(char*) <= sizeof(size_t),
-                "ptr size too large for size_t");
-
-  // This static variable isn't actually used, only its address, so there are
-  // no concurrency issues.
-  static char dummy_var;
-  return reinterpret_cast<size_t>(&dummy_var);
+constexpr inline const void* FastTypeId() {
+  return &TypeTag<Type>::dummy_var;
 }
 
 }  // namespace any_internal
@@ -382,7 +379,7 @@ class any {
    public:
     virtual ~ObjInterface() = default;
     virtual std::unique_ptr<ObjInterface> Clone() const = 0;
-    virtual size_t type_id() const noexcept = 0;
+    virtual const void* ObjTypeId() const noexcept = 0;
 #if ABSL_ANY_DETAIL_HAS_RTTI
     virtual const std::type_info& Type() const noexcept = 0;
 #endif  // ABSL_ANY_DETAIL_HAS_RTTI
@@ -400,7 +397,7 @@ class any {
       return std::unique_ptr<ObjInterface>(new Obj(in_place, value));
     }
 
-    size_t type_id() const noexcept final { return IdForType<T>(); }
+    const void* ObjTypeId() const noexcept final { return IdForType<T>(); }
 
 #if ABSL_ANY_DETAIL_HAS_RTTI
     const std::type_info& Type() const noexcept final { return typeid(T); }
@@ -415,7 +412,7 @@ class any {
   }
 
   template <typename T>
-  static size_t IdForType() {
+  constexpr static const void* IdForType() {
     // Note: This type dance is to make the behavior consistent with typeid.
     using NormalizedType =
         typename std::remove_cv<typename std::remove_reference<T>::type>::type;
@@ -423,8 +420,8 @@ class any {
     return any_internal::FastTypeId<NormalizedType>();
   }
 
-  size_t GetObjTypeId() const {
-    return obj_ == nullptr ? any_internal::FastTypeId<void>() : obj_->type_id();
+  const void* GetObjTypeId() const {
+    return obj_ ? obj_->ObjTypeId() : any_internal::FastTypeId<void>();
   }
 
   // `absl::any` nonmember functions //
diff --git a/absl/types/optional_test.cc b/absl/types/optional_test.cc
index 645f5b93cf89..5eedfcfd5223 100644
--- a/absl/types/optional_test.cc
+++ b/absl/types/optional_test.cc
@@ -270,8 +270,17 @@ TEST(optionalTest, CopyConstructor) {
     EXPECT_TRUE(absl::is_trivially_copy_constructible<
                 absl::optional<const TrivialCopyable>>::value);
 #endif
+    // When testing with VS 2017 15.3, there seems to be a bug in MSVC
+    // std::optional when T is volatile-qualified. So skipping this test.
+    // Bug report:
+    // https://connect.microsoft.com/VisualStudio/feedback/details/3142534
+#if defined(ABSL_HAVE_STD_OPTIONAL) && defined(_MSC_VER) && _MSC_VER >= 1911
+#define ABSL_MSVC_OPTIONAL_VOLATILE_COPY_BUG 1
+#endif
+#ifndef ABSL_MSVC_OPTIONAL_VOLATILE_COPY_BUG
     EXPECT_FALSE(std::is_copy_constructible<
                  absl::optional<volatile TrivialCopyable>>::value);
+#endif
   }
 }
 
diff --git a/absl/types/span.h b/absl/types/span.h
index e1f006ad9f7e..f4738153116e 100644
--- a/absl/types/span.h
+++ b/absl/types/span.h
@@ -378,7 +378,7 @@ class Span {
   //
   // Returns a reference to the i'th element of this span.
   constexpr reference at(size_type i) const {
-    return ABSL_PREDICT_FALSE(i < size())
+    return ABSL_PREDICT_TRUE(i < size())
                ? ptr_[i]
                : (base_internal::ThrowStdOutOfRange(
                       "Span::at failed bounds check"),