Changes imported from Abseil "staging" branch:

  - 5e874e644191fbf99f5636d6303de2b28b23392c Adds a absl::apply function, similar to c++17's std::apply. by Abseil Team <absl-team@google.com>
  - 16373c438d16a09725dace03ab7ba0f7c2337279 Add debugging_internal::StackTraceWorksForTest by Abseil Team <absl-team@google.com>
  - a623257aaaff8a5fba3377f34f92f319a104e444 Update absl::CondVar documentation in response to GitHub ... by Derek Mauro <dmauro@google.com>
  - 87d58a25bc4ecd46165dd1c417121c86cbc07be0 Add assert against uint128 negative bit shift undefined b... by Alex Strelnikov <strel@google.com>
  - af155c0d2a3556b56a9bcd6f9ee7416277185df8 Fix comment typos. by Abseil Team <absl-team@google.com>
  - 1824ae832eb75d447dea730b5968d952897e135a Rollback of: Add debugging_internal::StackTraceWorksForTest by Abseil Team <absl-team@google.com>
  - 97318f087ce63dd5acf1e0d3d697cd90a7d6ebfd Add debugging_internal::StackTraceWorksForTest by Abseil Team <absl-team@google.com>
  - 9dd1d17dca17f0ded3bda336b7521fd57d08a5cc Move log_severity.h out of internal. by Abseil Team <absl-team@google.com>
  - 2212bb56b1a8365d2303ff0983441298d08444e5 Internal change. by Alex Strelnikov <strel@google.com>

GitOrigin-RevId: 5e874e644191fbf99f5636d6303de2b28b23392c
Change-Id: Ic270393ac1f15866afb64617d28269cd829030f6

19 files changed, 326 insertions, 58 deletions

diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 609d78ad6287..1778f4f8453f 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -151,7 +151,6 @@ cc_library(
         "casts.h",
         "internal/atomic_hook.h",
         "internal/cycleclock.h",
-        "internal/log_severity.h",
         "internal/low_level_scheduling.h",
         "internal/per_thread_tls.h",
         "internal/raw_logging.h",
@@ -160,6 +159,7 @@ cc_library(
         "internal/thread_identity.h",
         "internal/tsan_mutex_interface.h",
         "internal/unscaledcycleclock.h",
+        "log_severity.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     deps = [
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index cb0daa69ffdc..d306616305dd 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -20,6 +20,7 @@ list(APPEND BASE_PUBLIC_HEADERS
   "casts.h"
   "config.h"
   "dynamic_annotations.h"
+  "log_severity.h"
   "macros.h"
   "optimization.h"
   "policy_checks.h"
@@ -36,7 +37,6 @@ list(APPEND BASE_INTERNAL_HEADERS
   "internal/exception_safety_testing.h"
   "internal/identity.h"
   "internal/invoke.h"
-  "internal/log_severity.h"
   "internal/low_level_alloc.h"
   "internal/low_level_scheduling.h"
   "internal/malloc_extension.h"
diff --git a/absl/base/internal/raw_logging.cc b/absl/base/internal/raw_logging.cc
index 301b108c07e9..86e34d458f4e 100644
--- a/absl/base/internal/raw_logging.cc
+++ b/absl/base/internal/raw_logging.cc
@@ -22,7 +22,7 @@
 
 #include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
-#include "absl/base/internal/log_severity.h"
+#include "absl/base/log_severity.h"
 
 // We know how to perform low-level writes to stderr in POSIX and Windows.  For
 // these platforms, we define the token ABSL_LOW_LEVEL_WRITE_SUPPORTED.
diff --git a/absl/base/internal/raw_logging.h b/absl/base/internal/raw_logging.h
index 568d2afce10a..1b2a44b7a7da 100644
--- a/absl/base/internal/raw_logging.h
+++ b/absl/base/internal/raw_logging.h
@@ -20,7 +20,7 @@
 #define ABSL_BASE_INTERNAL_RAW_LOGGING_H_
 
 #include "absl/base/attributes.h"
-#include "absl/base/internal/log_severity.h"
+#include "absl/base/log_severity.h"
 #include "absl/base/macros.h"
 #include "absl/base/port.h"
 
diff --git a/absl/base/internal/log_severity.h b/absl/base/log_severity.h
index deaf6a578925..deaf6a578925 100644
--- a/absl/base/internal/log_severity.h
+++ b/absl/base/log_severity.h
diff --git a/absl/debugging/internal/stacktrace_aarch64-inl.inc b/absl/debugging/internal/stacktrace_aarch64-inl.inc
index c125ea29064f..45802e7383bb 100644
--- a/absl/debugging/internal/stacktrace_aarch64-inl.inc
+++ b/absl/debugging/internal/stacktrace_aarch64-inl.inc
@@ -178,4 +178,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return true;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_AARCH64_INL_H_
diff --git a/absl/debugging/internal/stacktrace_arm-inl.inc b/absl/debugging/internal/stacktrace_arm-inl.inc
index 566b6d34189b..c84083379bb2 100644
--- a/absl/debugging/internal/stacktrace_arm-inl.inc
+++ b/absl/debugging/internal/stacktrace_arm-inl.inc
@@ -112,4 +112,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return false;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_ARM_INL_H_
diff --git a/absl/debugging/internal/stacktrace_generic-inl.inc b/absl/debugging/internal/stacktrace_generic-inl.inc
index de4881e36067..2c9ca410c608 100644
--- a/absl/debugging/internal/stacktrace_generic-inl.inc
+++ b/absl/debugging/internal/stacktrace_generic-inl.inc
@@ -48,4 +48,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return result_count;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return true;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_GENERIC_INL_H_
diff --git a/absl/debugging/internal/stacktrace_powerpc-inl.inc b/absl/debugging/internal/stacktrace_powerpc-inl.inc
index 9193e2dba60e..4b113269a8e0 100644
--- a/absl/debugging/internal/stacktrace_powerpc-inl.inc
+++ b/absl/debugging/internal/stacktrace_powerpc-inl.inc
@@ -232,4 +232,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return true;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_POWERPC_INL_H_
diff --git a/absl/debugging/internal/stacktrace_unimplemented-inl.inc b/absl/debugging/internal/stacktrace_unimplemented-inl.inc
index a66be7797d94..e256fdd4ae76 100644
--- a/absl/debugging/internal/stacktrace_unimplemented-inl.inc
+++ b/absl/debugging/internal/stacktrace_unimplemented-inl.inc
@@ -11,4 +11,12 @@ static int UnwindImpl(void** /* result */, int* /* sizes */,
   return 0;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return false;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_UNIMPLEMENTED_INL_H_
diff --git a/absl/debugging/internal/stacktrace_win32-inl.inc b/absl/debugging/internal/stacktrace_win32-inl.inc
index 4c7f855bbd38..a8f8a56afb37 100644
--- a/absl/debugging/internal/stacktrace_win32-inl.inc
+++ b/absl/debugging/internal/stacktrace_win32-inl.inc
@@ -72,4 +72,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return false;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_WIN32_INL_H_
diff --git a/absl/debugging/internal/stacktrace_x86-inl.inc b/absl/debugging/internal/stacktrace_x86-inl.inc
index 9bdaa542ce27..7c146ad3221f 100644
--- a/absl/debugging/internal/stacktrace_x86-inl.inc
+++ b/absl/debugging/internal/stacktrace_x86-inl.inc
@@ -326,4 +326,12 @@ static int UnwindImpl(void **result, int *sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return true;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_X86_INL_INC_
diff --git a/absl/debugging/stacktrace.h b/absl/debugging/stacktrace.h
index 0aab5749b952..82da3f15fa7d 100644
--- a/absl/debugging/stacktrace.h
+++ b/absl/debugging/stacktrace.h
@@ -155,6 +155,13 @@ extern int DefaultStackUnwinder(void** pcs, int* sizes, int max_depth,
                                 int skip_count, const void* uc,
                                 int* min_dropped_frames);
 
+namespace debugging_internal {
+// Returns true for platforms which are expected to have functioning stack trace
+// implementations. Intended to be used for tests which want to exclude
+// verification of logic known to be broken because stack traces are not
+// working.
+extern bool StackTraceWorksForTest();
+}  // namespace debugging_internal
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_STACKTRACE_H_
diff --git a/absl/numeric/int128.h b/absl/numeric/int128.h
index bd1b553ab2dc..94d2f7ce2985 100644
--- a/absl/numeric/int128.h
+++ b/absl/numeric/int128.h
@@ -511,8 +511,8 @@ inline uint128& uint128::operator^=(uint128 other) {
 // Shift and arithmetic assign operators.
 
 inline uint128& uint128::operator<<=(int amount) {
-  // Shifts of >= 128 are undefined.
-  assert(amount < 128);
+  assert(amount >= 0);   // Negative shifts are undefined.
+  assert(amount < 128);  // Shifts of >= 128 are undefined.
 
   // uint64_t shifts of >= 64 are undefined, so we will need some
   // special-casing.
@@ -529,8 +529,8 @@ inline uint128& uint128::operator<<=(int amount) {
 }
 
 inline uint128& uint128::operator>>=(int amount) {
-  // Shifts of >= 128 are undefined.
-  assert(amount < 128);
+  assert(amount >= 0);   // Negative shifts are undefined.
+  assert(amount < 128);  // Shifts of >= 128 are undefined.
 
   // uint64_t shifts of >= 64 are undefined, so we will need some
   // special-casing.
diff --git a/absl/synchronization/mutex.h b/absl/synchronization/mutex.h
index 26ac7f619bff..b09802b523bc 100644
--- a/absl/synchronization/mutex.h
+++ b/absl/synchronization/mutex.h
@@ -713,7 +713,7 @@ class Condition {
 // The implementation may deliver signals to any condition variable at
 // any time, even when no call to `Signal()` or `SignalAll()` is made; as a
 // result, upon being awoken, you must check the logical condition you have
-// been waiting upon. The implementation wakes waiters in the FIFO order.
+// been waiting upon.
 //
 // Examples:
 //
@@ -742,29 +742,19 @@ class CondVar {
 
   // CondVar::Wait()
   //
-  // Atomically releases a `Mutex` and blocks on this condition variable. After
-  // blocking, the thread will unblock, reacquire the `Mutex`, and return if
-  // either:
-  //  - this condition variable is signalled with `SignalAll()`, or
-  //  - this condition variable is signalled in any manner and this thread
-  //    was the most recently blocked thread that has not yet woken.
+  // Atomically releases a `Mutex` and blocks on this condition variable.
+  // Waits until awakened by a call to `Signal()` or `SignalAll()` (or a
+  // spurious wakeup), then reacquires the `Mutex` and returns.
+  //
   // Requires and ensures that the current thread holds the `Mutex`.
   void Wait(Mutex *mu);
 
   // CondVar::WaitWithTimeout()
   //
-  // Atomically releases a `Mutex`, blocks on this condition variable, and
-  // attempts to reacquire the mutex upon being signalled, or upon reaching the
-  // timeout.
-  //
-  // After blocking, the thread will unblock, reacquire the `Mutex`, and return
-  // for any of the following:
-  //  - this condition variable is signalled with `SignalAll()`
-  //  - the timeout has expired
-  //  - this condition variable is signalled in any manner and this thread
-  //    was the most recently blocked thread that has not yet woken.
-  //
-  // Negative timeouts are equivalent to a zero timeout.
+  // Atomically releases a `Mutex` and blocks on this condition variable.
+  // Waits until awakened by a call to `Signal()` or `SignalAll()` (or a
+  // spurious wakeup), or until the timeout has expired, then reacquires
+  // the `Mutex` and returns.
   //
   // Returns true if the timeout has expired without this `CondVar`
   // being signalled in any manner. If both the timeout has expired
@@ -776,15 +766,10 @@ class CondVar {
 
   // CondVar::WaitWithDeadline()
   //
-  // Atomically releases a `Mutex`, blocks on this condition variable, and
-  // attempts to reacquire the mutex within the provided deadline.
-  //
-  // After blocking, the thread will unblock, reacquire the `Mutex`, and return
-  // for any of the following:
-  //  - this condition variable is signalled with `SignalAll()`
-  //  - the deadline has passed
-  //  - this condition variable is signalled in any manner and this thread
-  //    was the most recently blocked thread that has not yet woken.
+  // Atomically releases a `Mutex` and blocks on this condition variable.
+  // Waits until awakened by a call to `Signal()` or `SignalAll()` (or a
+  // spurious wakeup), or until the deadline has passed, then reacquires
+  // the `Mutex` and returns.
   //
   // Deadlines in the past are equivalent to an immediate deadline.
   //
diff --git a/absl/types/optional.h b/absl/types/optional.h
index 353e61836515..ef8255915746 100644
--- a/absl/types/optional.h
+++ b/absl/types/optional.h
@@ -496,7 +496,7 @@ class optional : private optional_internal::optional_data<T>,
   // default constructed `T`.
   constexpr optional() noexcept {}
 
-  // Construct an` optional` initialized with `nullopt` to hold an empty value.
+  // Construct an `optional` initialized with `nullopt` to hold an empty value.
   constexpr optional(nullopt_t) noexcept {}  // NOLINT(runtime/explicit)
 
   // Copy constructor, standard semantics
@@ -515,7 +515,7 @@ class optional : private optional_internal::optional_data<T>,
   constexpr explicit optional(in_place_t, Args&&... args)
       : data_base(in_place_t(), absl::forward<Args>(args)...) {}
 
-  // Constructs a non-empty `optional' direct-initialized value of type `T` from
+  // Constructs a non-empty `optional` direct-initialized value of type `T` from
   // the arguments of an initializer_list and `std::forward<Args>(args)...`.
   // (The `in_place_t` is a tag used to indicate that the contained object
   // should be constructed in-place.)
diff --git a/absl/utility/BUILD.bazel b/absl/utility/BUILD.bazel
index 396b05f85a75..6298462643f4 100644
--- a/absl/utility/BUILD.bazel
+++ b/absl/utility/BUILD.bazel
@@ -14,6 +14,7 @@ cc_library(
     hdrs = ["utility.h"],
     copts = ABSL_DEFAULT_COPTS,
     deps = [
+        "//absl/base:base_internal",
         "//absl/base:config",
         "//absl/meta:type_traits",
     ],
@@ -26,6 +27,8 @@ cc_test(
     deps = [
         ":utility",
         "//absl/base:core_headers",
+        "//absl/memory",
+        "//absl/strings",
         "@com_google_googletest//:gtest_main",
     ],
 )
diff --git a/absl/utility/utility.h b/absl/utility/utility.h
index 732cd4c59245..d37f4c301a44 100644
--- a/absl/utility/utility.h
+++ b/absl/utility/utility.h
@@ -23,6 +23,7 @@
 //   * make_integer_sequence<T, N>   == std::make_integer_sequence<T, N>
 //   * make_index_sequence<N>        == std::make_index_sequence<N>
 //   * index_sequence_for<Ts...>     == std::index_sequence_for<Ts...>
+//   * apply<Functor, Tuple>         == std::apply<Functor, Tuple>
 //
 // This header file also provides the tag types `in_place_t`, `in_place_type_t`,
 // and `in_place_index_t`, as well as the constant `in_place`, and
@@ -31,34 +32,20 @@
 // References:
 //
 //  http://en.cppreference.com/w/cpp/utility/integer_sequence
+//  http://en.cppreference.com/w/cpp/utility/apply
 //  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3658.html
 //
-//
-// Example:
-//   // Unpack a tuple for use as a function call's argument list.
-//
-//   template <typename F, typename Tup, size_t... Is>
-//   auto Impl(F f, const Tup& tup, absl::index_sequence<Is...>)
-//       -> decltype(f(std::get<Is>(tup) ...)) {
-//     return f(std::get<Is>(tup) ...);
-//   }
-//
-//   template <typename Tup>
-//   using TupIdxSeq = absl::make_index_sequence<std::tuple_size<Tup>::value>;
-//
-//   template <typename F, typename Tup>
-//   auto ApplyFromTuple(F f, const Tup& tup)
-//       -> decltype(Impl(f, tup, TupIdxSeq<Tup>{})) {
-//     return Impl(f, tup, TupIdxSeq<Tup>{});
-//   }
+
 #ifndef ABSL_UTILITY_UTILITY_H_
 #define ABSL_UTILITY_UTILITY_H_
 
 #include <cstddef>
 #include <cstdlib>
+#include <tuple>
 #include <utility>
 
 #include "absl/base/config.h"
+#include "absl/base/internal/invoke.h"
 #include "absl/meta/type_traits.h"
 
 namespace absl {
@@ -214,6 +201,62 @@ constexpr T&& forward(
   return static_cast<T&&>(t);
 }
 
+namespace utility_internal {
+// Helper method for expanding tuple into a called method.
+template <typename Functor, typename Tuple, std::size_t... Indexes>
+auto apply_helper(Functor&& functor, Tuple&& t, index_sequence<Indexes...>)
+    -> decltype(absl::base_internal::Invoke(
+        absl::forward<Functor>(functor),
+        std::get<Indexes>(absl::forward<Tuple>(t))...)) {
+  return absl::base_internal::Invoke(
+      absl::forward<Functor>(functor),
+      std::get<Indexes>(absl::forward<Tuple>(t))...);
+}
+
+}  // namespace utility_internal
+
+// apply
+//
+// Invokes a Callable using elements of a tuple as its arguments.
+// Each element of the tuple corresponds to an argument of the call (in order).
+// Both the Callable argument and the tuple argument are perfect-forwarded.
+// For member-function Callables, the first tuple element acts as the `this`
+// pointer. `absl::apply` is designed to be a drop-in replacement for C++17's
+// `std::apply`. Unlike C++17's `std::apply`, this is not currently `constexpr`.
+//
+// Example:
+//
+//   class Foo{void Bar(int);};
+//   void user_function(int, std::string);
+//   void user_function(std::unique_ptr<Foo>);
+//
+//   int main()
+//   {
+//       std::tuple<int, std::string> tuple1(42, "bar");
+//       // Invokes the user function overload on int, std::string.
+//       absl::apply(&user_function, tuple1);
+//
+//       auto foo = absl::make_unique<Foo>();
+//       std::tuple<Foo*, int> tuple2(foo.get(), 42);
+//       // Invokes the method Bar on foo with one argument 42.
+//       absl::apply(&Foo::Bar, foo.get(), 42);
+//
+//       std::tuple<std::unique_ptr<Foo>> tuple3(absl::make_unique<Foo>());
+//       // Invokes the user function that takes ownership of the unique
+//       // pointer.
+//       absl::apply(&user_function, std::move(tuple));
+//   }
+template <typename Functor, typename Tuple>
+auto apply(Functor&& functor, Tuple&& t)
+    -> decltype(utility_internal::apply_helper(
+        absl::forward<Functor>(functor), absl::forward<Tuple>(t),
+        absl::make_index_sequence<std::tuple_size<
+            typename std::remove_reference<Tuple>::type>::value>{})) {
+  return utility_internal::apply_helper(
+      absl::forward<Functor>(functor), absl::forward<Tuple>(t),
+      absl::make_index_sequence<std::tuple_size<
+          typename std::remove_reference<Tuple>::type>::value>{});
+}
 }  // namespace absl
 
 #endif  // ABSL_UTILITY_UTILITY_H_
diff --git a/absl/utility/utility_test.cc b/absl/utility/utility_test.cc
index 62cb46f4d160..342165ed5240 100644
--- a/absl/utility/utility_test.cc
+++ b/absl/utility/utility_test.cc
@@ -23,6 +23,8 @@
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/str_cat.h"
 
 namespace {
 
@@ -38,8 +40,9 @@ namespace {
 #pragma warning( disable : 4101 )  // unreferenced local variable
 #endif  // _MSC_VER
 
-using testing::StaticAssertTypeEq;
-using testing::ElementsAre;
+using ::testing::ElementsAre;
+using ::testing::Pointee;
+using ::testing::StaticAssertTypeEq;
 
 TEST(IntegerSequenceTest, ValueType) {
   StaticAssertTypeEq<int, absl::integer_sequence<int>::value_type>();
@@ -159,5 +162,176 @@ TEST(IndexSequenceForTest, Example) {
               ElementsAre("12", "abc", "3.14"));
 }
 
+int Function(int a, int b) { return a - b; }
+
+int Sink(std::unique_ptr<int> p) { return *p; }
+
+std::unique_ptr<int> Factory(int n) { return absl::make_unique<int>(n); }
+
+void NoOp() {}
+
+struct ConstFunctor {
+  int operator()(int a, int b) const { return a - b; }
+};
+
+struct MutableFunctor {
+  int operator()(int a, int b) { return a - b; }
+};
+
+struct EphemeralFunctor {
+  EphemeralFunctor() {}
+  EphemeralFunctor(const EphemeralFunctor&) {}
+  EphemeralFunctor(EphemeralFunctor&&) {}
+  int operator()(int a, int b) && { return a - b; }
+};
+
+struct OverloadedFunctor {
+  OverloadedFunctor() {}
+  OverloadedFunctor(const OverloadedFunctor&) {}
+  OverloadedFunctor(OverloadedFunctor&&) {}
+  template <typename... Args>
+  std::string operator()(const Args&... args) & {
+    return absl::StrCat("&", args...);
+  }
+  template <typename... Args>
+  std::string operator()(const Args&... args) const& {
+    return absl::StrCat("const&", args...);
+  }
+  template <typename... Args>
+  std::string operator()(const Args&... args) && {
+    return absl::StrCat("&&", args...);
+  }
+};
+
+struct Class {
+  int Method(int a, int b) { return a - b; }
+  int ConstMethod(int a, int b) const { return a - b; }
+
+  int member;
+};
+
+struct FlipFlop {
+  int ConstMethod() const { return member; }
+  FlipFlop operator*() const { return {-member}; }
+
+  int member;
+};
+
+TEST(ApplyTest, Function) {
+  EXPECT_EQ(1, absl::apply(Function, std::make_tuple(3, 2)));
+  EXPECT_EQ(1, absl::apply(&Function, std::make_tuple(3, 2)));
+}
+
+TEST(ApplyTest, NonCopyableArgument) {
+  EXPECT_EQ(42, absl::apply(Sink, std::make_tuple(absl::make_unique<int>(42))));
+}
+
+TEST(ApplyTest, NonCopyableResult) {
+  EXPECT_THAT(absl::apply(Factory, std::make_tuple(42)),
+              ::testing::Pointee(42));
+}
+
+TEST(ApplyTest, VoidResult) { absl::apply(NoOp, std::tuple<>()); }
+
+TEST(ApplyTest, ConstFunctor) {
+  EXPECT_EQ(1, absl::apply(ConstFunctor(), std::make_tuple(3, 2)));
+}
+
+TEST(ApplyTest, MutableFunctor) {
+  MutableFunctor f;
+  EXPECT_EQ(1, absl::apply(f, std::make_tuple(3, 2)));
+  EXPECT_EQ(1, absl::apply(MutableFunctor(), std::make_tuple(3, 2)));
+}
+TEST(ApplyTest, EphemeralFunctor) {
+  EphemeralFunctor f;
+  EXPECT_EQ(1, absl::apply(std::move(f), std::make_tuple(3, 2)));
+  EXPECT_EQ(1, absl::apply(EphemeralFunctor(), std::make_tuple(3, 2)));
+}
+TEST(ApplyTest, OverloadedFunctor) {
+  OverloadedFunctor f;
+  const OverloadedFunctor& cf = f;
+
+  EXPECT_EQ("&", absl::apply(f, std::tuple<>{}));
+  EXPECT_EQ("& 42", absl::apply(f, std::make_tuple(" 42")));
+
+  EXPECT_EQ("const&", absl::apply(cf, std::tuple<>{}));
+  EXPECT_EQ("const& 42", absl::apply(cf, std::make_tuple(" 42")));
+
+  EXPECT_EQ("&&", absl::apply(std::move(f), std::tuple<>{}));
+  OverloadedFunctor f2;
+  EXPECT_EQ("&& 42", absl::apply(std::move(f2), std::make_tuple(" 42")));
+}
+
+TEST(ApplyTest, ReferenceWrapper) {
+  ConstFunctor cf;
+  MutableFunctor mf;
+  EXPECT_EQ(1, absl::apply(std::cref(cf), std::make_tuple(3, 2)));
+  EXPECT_EQ(1, absl::apply(std::ref(cf), std::make_tuple(3, 2)));
+  EXPECT_EQ(1, absl::apply(std::ref(mf), std::make_tuple(3, 2)));
+}
+
+TEST(ApplyTest, MemberFunction) {
+  std::unique_ptr<Class> p(new Class);
+  std::unique_ptr<const Class> cp(new Class);
+  EXPECT_EQ(
+      1, absl::apply(&Class::Method,
+                     std::tuple<std::unique_ptr<Class>&, int, int>(p, 3, 2)));
+  EXPECT_EQ(1, absl::apply(&Class::Method,
+                           std::tuple<Class*, int, int>(p.get(), 3, 2)));
+  EXPECT_EQ(
+      1, absl::apply(&Class::Method, std::tuple<Class&, int, int>(*p, 3, 2)));
+
+  EXPECT_EQ(
+      1, absl::apply(&Class::ConstMethod,
+                     std::tuple<std::unique_ptr<Class>&, int, int>(p, 3, 2)));
+  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
+                           std::tuple<Class*, int, int>(p.get(), 3, 2)));
+  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
+                           std::tuple<Class&, int, int>(*p, 3, 2)));
+
+  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
+                           std::tuple<std::unique_ptr<const Class>&, int, int>(
+                               cp, 3, 2)));
+  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
+                           std::tuple<const Class*, int, int>(cp.get(), 3, 2)));
+  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
+                           std::tuple<const Class&, int, int>(*cp, 3, 2)));
+
+  EXPECT_EQ(1, absl::apply(&Class::Method,
+                           std::make_tuple(absl::make_unique<Class>(), 3, 2)));
+  EXPECT_EQ(1, absl::apply(&Class::ConstMethod,
+                           std::make_tuple(absl::make_unique<Class>(), 3, 2)));
+  EXPECT_EQ(
+      1, absl::apply(&Class::ConstMethod,
+                     std::make_tuple(absl::make_unique<const Class>(), 3, 2)));
+}
+
+TEST(ApplyTest, DataMember) {
+  std::unique_ptr<Class> p(new Class{42});
+  std::unique_ptr<const Class> cp(new Class{42});
+  EXPECT_EQ(
+      42, absl::apply(&Class::member, std::tuple<std::unique_ptr<Class>&>(p)));
+  EXPECT_EQ(42, absl::apply(&Class::member, std::tuple<Class&>(*p)));
+  EXPECT_EQ(42, absl::apply(&Class::member, std::tuple<Class*>(p.get())));
+
+  absl::apply(&Class::member, std::tuple<std::unique_ptr<Class>&>(p)) = 42;
+  absl::apply(&Class::member, std::tuple<Class*>(p.get())) = 42;
+  absl::apply(&Class::member, std::tuple<Class&>(*p)) = 42;
+
+  EXPECT_EQ(42, absl::apply(&Class::member,
+                            std::tuple<std::unique_ptr<const Class>&>(cp)));
+  EXPECT_EQ(42, absl::apply(&Class::member, std::tuple<const Class&>(*cp)));
+  EXPECT_EQ(42,
+            absl::apply(&Class::member, std::tuple<const Class*>(cp.get())));
+}
+
+TEST(ApplyTest, FlipFlop) {
+  FlipFlop obj = {42};
+  // This call could resolve to (obj.*&FlipFlop::ConstMethod)() or
+  // ((*obj).*&FlipFlop::ConstMethod)(). We verify that it's the former.
+  EXPECT_EQ(42, absl::apply(&FlipFlop::ConstMethod, std::make_tuple(obj)));
+  EXPECT_EQ(42, absl::apply(&FlipFlop::member, std::make_tuple(obj)));
+}
+
 }  // namespace