Export of internal Abseil changes.

--
bdce7e57e9e886eff1114d0266781b443f7ec639 by Derek Mauro <dmauro@google.com>:

Change {Get|Set}EnvironmentVariable to {Get|Set}EnvironmentVariableA for
compatibility with /DUNICODE.

PiperOrigin-RevId: 239229514

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2276ed502326a044a84060d34eb19d499e3a3be2 by Derek Mauro <dmauro@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 239228622

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a462efb970ff43b08a362ef2343fb75ac1295a50 by Derek Mauro <dmauro@google.com>:

Adding linking of CoreFoundation to CMakeLists in absl/time.
Import https://github.com/abseil/abseil-cpp/pull/280.

Fix #283

PiperOrigin-RevId: 239220785

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fc23327b97f940c682aae1956cf7a1bf87f88c06 by Derek Mauro <dmauro@google.com>:

Add hermetic test script that uses Docker to build with a very recent
version of gcc (8.3.0 today) with libstdc++ and bazel.

PiperOrigin-RevId: 239220448

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418c08a8f6a53e63b84e39473035774417ca3aa7 by Derek Mauro <dmauro@google.com>:

Disable part of the variant exeception safety test on move assignment
when using versions of libstd++ that contain a bug.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87431#c7

PiperOrigin-RevId: 239062455

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799722217aeda79679577843c91d5be62cbcbb42 by Matt Calabrese <calabrese@google.com>:

Add internal-only IsSwappable traits corresponding to std::is_swappable and std::is_nothrow_swappable, which are used with the swap implementations of optional and variant.

PiperOrigin-RevId: 239049448

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aa46a036038a3de5c68ac5e5d3b4bf76f818d2ea by CJ Johnson <johnsoncj@google.com>:

Make InlinedVectorStorage constructor explicit

PiperOrigin-RevId: 239044361

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17949715b3aa21c794701f69f2154e91b6acabc3 by CJ Johnson <johnsoncj@google.com>:

Add absl namesapce to internal/inlined_vector.h

PiperOrigin-RevId: 239030789

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834628325953078cc08ed10d23bb8890e5bec897 by Derek Mauro <dmauro@google.com>:

Add test script that uses Docker to build Abseil with gcc-4.8,
libstdc++, and cmake.

PiperOrigin-RevId: 239028433

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80fe24149ed73ed2ced995ad1e372fb060c60427 by CJ Johnson <johnsoncj@google.com>:

Factors data members of InlinedVector into an impl type called InlinedVectorStorage so that (in future changes) the contents of a vector can be grouped together with a single pointer.

PiperOrigin-RevId: 239021086

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585331436d5d4d79f845e45dcf79d918a0dc6169 by Derek Mauro <dmauro@google.com>:

Add -Wno-missing-field-initializers to gcc compiler flags.
gcc-4.x has spurious missing field initializer warnings.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36750

PiperOrigin-RevId: 239017217

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94602fe4e33ee3a552a7f2939c0f57a992f55075 by Abseil Team <absl-team@google.com>:

Formatting fixes.

PiperOrigin-RevId: 238983038

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a1c1b63c08505574e0a8c491561840cecb2bb93e by Derek Mauro <dmauro@google.com>:

Add hermetic test script that uses Docker to build with a very recent
version of clang with libc++ and bazel.

PiperOrigin-RevId: 238669118

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e525f8d20bc2f79a0d69336b902f63858f3bff9d by Derek Mauro <dmauro@google.com>:

Disable the test optionalTest.InPlaceTSFINAEBug until libc++ is updated.

PiperOrigin-RevId: 238661703

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f99a2a0b5ec424a059678f7f226600f137b4c74e by Derek Mauro <dmauro@google.com>:

Correct the check for the FlatHashMap-Any test bug (list conditions
instead of platforms when possible)

PiperOrigin-RevId: 238653344

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777928035dbcbf39f361eb7d10dc3696822f692f by Jon Cohen <cohenjon@google.com>:

Add install rules for Abseil CMake.

These are attempted to be limited to in-project installation.  This serves two purposes -- first it's morally the same as using Abseil in-source, except you don't have to rebuild us every time.  Second, the presence of an install rule makes life massively simpler for package manager maintainers.

Currently this doesn't install absl tests or testonly libraries.  This can be added in a follow-up patch.

Fixes #38, Fixes #80, Closes #182

PiperOrigin-RevId: 238645836

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ded1c6ce697c191b7a6ff14572b3e6d183117b2c by Derek Mauro <dmauro@google.com>:

Add hermetic test script that uses Docker to build with a very recent
version of clang with libstdc++ and bazel.

PiperOrigin-RevId: 238517815
GitOrigin-RevId: bdce7e57e9e886eff1114d0266781b443f7ec639
Change-Id: I6f745869cb8ef63851891ccac05ae9a7dd241c4f

2 files changed, 142 insertions, 0 deletions

diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index 8a788dea..b1b14914 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -485,4 +485,65 @@ inline void AssertHashEnabled() {
 
 }  // namespace absl
 
+// An internal namespace that is required to implement the C++17 swap traits.
+//
+// NOTE: This is its own top-level namespace to avoid subtleties due to
+//       functions named "swap" that may appear in the absl namespace.
+namespace absl_internal_swap {
+
+using std::swap;
+
+template <class T>
+using IsSwappableImpl = decltype(swap(std::declval<T&>(), std::declval<T&>()));
+
+// NOTE: This dance with the default template parameter is for MSVC.
+template <class T,
+          class IsNoexcept = std::integral_constant<
+              bool, noexcept(swap(std::declval<T&>(), std::declval<T&>()))>>
+using IsNothrowSwappableImpl = typename std::enable_if<IsNoexcept::value>::type;
+
+// IsSwappable
+//
+// Determines whether the standard swap idiom is a valid expression for
+// arguments of type `T`.
+template <class T>
+struct IsSwappable
+    : absl::type_traits_internal::is_detected<IsSwappableImpl, T> {};
+
+// IsNothrowSwappable
+//
+// Determines whether the standard swap idiom is a valid expression for
+// arguments of type `T` and is noexcept.
+template <class T>
+struct IsNothrowSwappable
+    : absl::type_traits_internal::is_detected<IsNothrowSwappableImpl, T> {};
+
+// Swap()
+//
+// Performs the swap idiom from a namespace with no additional `swap` overloads.
+template <class T, absl::enable_if_t<IsSwappable<T>::value, int> = 0>
+void Swap(T& lhs, T& rhs) noexcept(IsNothrowSwappable<T>::value) {
+  swap(lhs, rhs);
+}
+
+}  // namespace absl_internal_swap
+
+namespace absl {
+namespace type_traits_internal {
+
+// Make the swap-related traits/function accessible from this namespace.
+using absl_internal_swap::IsNothrowSwappable;
+using absl_internal_swap::IsSwappable;
+using absl_internal_swap::Swap;
+
+// StdSwapIsUnconstrained
+//
+// Some standard library implementations are broken in that they do not
+// constrain `std::swap`. This will effectively tell us if we are dealing with
+// one of those implementations.
+using StdSwapIsUnconstrained = IsSwappable<void()>;
+
+}  // namespace type_traits_internal
+}  // namespace absl
+
 #endif  // ABSL_META_TYPE_TRAITS_H_
diff --git a/absl/meta/type_traits_test.cc b/absl/meta/type_traits_test.cc
index 29a6db69..912336e9 100644
--- a/absl/meta/type_traits_test.cc
+++ b/absl/meta/type_traits_test.cc
@@ -953,4 +953,85 @@ TEST(TypeTraitsTest, IsMoveAssignable) {
 #endif  // _LIBCPP_VERSION
 }
 
+namespace adl_namespace {
+
+struct DeletedSwap {
+};
+
+void swap(DeletedSwap&, DeletedSwap&) = delete;
+
+struct SpecialNoexceptSwap {
+  SpecialNoexceptSwap(SpecialNoexceptSwap&&) {}
+  SpecialNoexceptSwap& operator=(SpecialNoexceptSwap&&) { return *this; }
+  ~SpecialNoexceptSwap() = default;
+};
+
+void swap(SpecialNoexceptSwap&, SpecialNoexceptSwap&) noexcept {}
+
+}  // namespace adl_namespace
+
+TEST(TypeTraitsTest, IsSwappable) {
+  using absl::type_traits_internal::IsSwappable;
+  using absl::type_traits_internal::StdSwapIsUnconstrained;
+
+  EXPECT_TRUE(IsSwappable<int>::value);
+
+  struct S {};
+  EXPECT_TRUE(IsSwappable<S>::value);
+
+  struct NoConstruct {
+    NoConstruct(NoConstruct&&) = delete;
+    NoConstruct& operator=(NoConstruct&&) { return *this; }
+    ~NoConstruct() = default;
+  };
+
+  EXPECT_EQ(IsSwappable<NoConstruct>::value, StdSwapIsUnconstrained::value);
+  struct NoAssign {
+    NoAssign(NoAssign&&) {}
+    NoAssign& operator=(NoAssign&&) = delete;
+    ~NoAssign() = default;
+  };
+
+  EXPECT_EQ(IsSwappable<NoAssign>::value, StdSwapIsUnconstrained::value);
+
+  EXPECT_FALSE(IsSwappable<adl_namespace::DeletedSwap>::value);
+
+  EXPECT_TRUE(IsSwappable<adl_namespace::SpecialNoexceptSwap>::value);
+}
+
+TEST(TypeTraitsTest, IsNothrowSwappable) {
+  using absl::type_traits_internal::IsNothrowSwappable;
+  using absl::type_traits_internal::StdSwapIsUnconstrained;
+
+  EXPECT_TRUE(IsNothrowSwappable<int>::value);
+
+  struct NonNoexceptMoves {
+    NonNoexceptMoves(NonNoexceptMoves&&) {}
+    NonNoexceptMoves& operator=(NonNoexceptMoves&&) { return *this; }
+    ~NonNoexceptMoves() = default;
+  };
+
+  EXPECT_FALSE(IsNothrowSwappable<NonNoexceptMoves>::value);
+
+  struct NoConstruct {
+    NoConstruct(NoConstruct&&) = delete;
+    NoConstruct& operator=(NoConstruct&&) { return *this; }
+    ~NoConstruct() = default;
+  };
+
+  EXPECT_FALSE(IsNothrowSwappable<NoConstruct>::value);
+
+  struct NoAssign {
+    NoAssign(NoAssign&&) {}
+    NoAssign& operator=(NoAssign&&) = delete;
+    ~NoAssign() = default;
+  };
+
+  EXPECT_FALSE(IsNothrowSwappable<NoAssign>::value);
+
+  EXPECT_FALSE(IsNothrowSwappable<adl_namespace::DeletedSwap>::value);
+
+  EXPECT_TRUE(IsNothrowSwappable<adl_namespace::SpecialNoexceptSwap>::value);
+}
+
 }  // namespace