Creation of LTS branch "lts_2019_08_08"20190808

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  - 27c30ec671cb7b5ba84c4e79feff7fd0b0ac6338 Avoid undefined behavior when nullptr is passed to memcpy... by Roman Gershman <romange@gmail.com>
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  - 253eb7416421661873afbaa33828a850db978541 [CMake] Set correct flags for clang-cl (#278) by Loo Rong Jie <loorongjie@gmail.com>
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  - c1cecb25a94c075725e9d2640f6b978a8f61957b Implement Span::first and Span::last from C++20 (#274) by Girts <girtsf@users.noreply.github.com>
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GitOrigin-RevId: 9ee91d3e430fb33a4590486573792eb0fa146c2d
Change-Id: Ia06e548bc106cc9d136f6c65714be6645317aced

1 files changed, 82 insertions, 123 deletions

diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index a51ca954..81f1edf8 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -4,7 +4,7 @@
 // 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
+//      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,
@@ -50,7 +50,7 @@
 #include "absl/hash/internal/city.h"
 
 namespace absl {
-inline namespace lts_2018_12_18 {
+inline namespace lts_2019_08_08 {
 namespace hash_internal {
 
 // HashStateBase
@@ -222,7 +222,9 @@ typename std::enable_if<std::is_enum<Enum>::value, H>::type AbslHashValue(
 }
 // AbslHashValue() for hashing floating-point values
 template <typename H, typename Float>
-typename std::enable_if<std::is_floating_point<Float>::value, H>::type
+typename std::enable_if<std::is_same<Float, float>::value ||
+                            std::is_same<Float, double>::value,
+                        H>::type
 AbslHashValue(H hash_state, Float value) {
   return hash_internal::hash_bytes(std::move(hash_state),
                                    value == 0 ? 0 : value);
@@ -232,8 +234,9 @@ AbslHashValue(H hash_state, Float value) {
 // For example, in x86 sizeof(long double)==16 but it only really uses 80-bits
 // of it. This means we can't use hash_bytes on a long double and have to
 // convert it to something else first.
-template <typename H>
-H AbslHashValue(H hash_state, long double value) {
+template <typename H, typename LongDouble>
+typename std::enable_if<std::is_same<LongDouble, long double>::value, H>::type
+AbslHashValue(H hash_state, LongDouble value) {
   const int category = std::fpclassify(value);
   switch (category) {
     case FP_INFINITE:
@@ -265,7 +268,12 @@ H AbslHashValue(H hash_state, long double value) {
 // AbslHashValue() for hashing pointers
 template <typename H, typename T>
 H AbslHashValue(H hash_state, T* ptr) {
-  return hash_internal::hash_bytes(std::move(hash_state), ptr);
+  auto v = reinterpret_cast<uintptr_t>(ptr);
+  // Due to alignment, pointers tend to have low bits as zero, and the next few
+  // bits follow a pattern since they are also multiples of some base value.
+  // Mixing the pointer twice helps prevent stuck low bits for certain alignment
+  // values.
+  return H::combine(std::move(hash_state), v, v);
 }
 
 // AbslHashValue() for hashing nullptr_t
@@ -528,53 +536,22 @@ hash_range_or_bytes(H hash_state, const T* data, size_t size) {
   return hash_state;
 }
 
-// InvokeHashTag
-//
-// InvokeHash(H, const T&) invokes the appropriate hash implementation for a
-// hasher of type `H` and a value of type `T`. If `T` is not hashable, there
-// will be no matching overload of InvokeHash().
-// Note: Some platforms (eg MSVC) do not support the detect idiom on
-// std::hash. In those platforms the last fallback will be std::hash and
-// InvokeHash() will always have a valid overload even if std::hash<T> is not
-// valid.
-//
-// We try the following options in order:
-//   * If is_uniquely_represented, hash bytes directly.
-//   * ADL AbslHashValue(H, const T&) call.
-//   * std::hash<T>
-
-// In MSVC we can't probe std::hash or stdext::hash because it triggers a
-// static_assert instead of failing substitution.
-#if defined(_MSC_VER)
-#define ABSL_HASH_INTERNAL_CAN_POISON_ 0
-#else   // _MSC_VER
-#define ABSL_HASH_INTERNAL_CAN_POISON_ 1
-#endif  // _MSC_VER
-
 #if defined(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE) && \
-    ABSL_HASH_INTERNAL_CAN_POISON_
+    ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
 #define ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_ 1
 #else
 #define ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_ 0
 #endif
 
-enum class InvokeHashTag {
-  kUniquelyRepresented,
-  kHashValue,
-#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
-  kLegacyHash,
-#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
-  kStdHash,
-  kNone
-};
-
 // HashSelect
 //
 // Type trait to select the appropriate hash implementation to use.
-// HashSelect<T>::value is an instance of InvokeHashTag that indicates the best
-// available hashing mechanism.
-// See `Note` above about MSVC.
-template <typename T>
+// HashSelect::type<T> will give the proper hash implementation, to be invoked
+// as:
+//   HashSelect::type<T>::Invoke(state, value)
+// Also, HashSelect::type<T>::value is a boolean equal to `true` if there is a
+// valid `Invoke` function. Types that are not hashable will have a ::value of
+// `false`.
 struct HashSelect {
  private:
   struct State : HashStateBase<State> {
@@ -583,95 +560,75 @@ struct HashSelect {
     using State::HashStateBase::combine_contiguous;
   };
 
-  // `Probe<V, Tag>::value` evaluates to `V<T>::value` if it is a valid
-  // expression, and `false` otherwise.
-  // `Probe<V, Tag>::tag` always evaluates to `Tag`.
-  template <template <typename> class V, InvokeHashTag Tag>
-  struct Probe {
-   private:
-    template <typename U, typename std::enable_if<V<U>::value, int>::type = 0>
-    static std::true_type Test(int);
-    template <typename U>
-    static std::false_type Test(char);
-
-   public:
-    static constexpr InvokeHashTag kTag = Tag;
-    static constexpr bool value = decltype(
-        Test<absl::remove_const_t<absl::remove_reference_t<T>>>(0))::value;
+  struct UniquelyRepresentedProbe {
+    template <typename H, typename T>
+    static auto Invoke(H state, const T& value)
+        -> absl::enable_if_t<is_uniquely_represented<T>::value, H> {
+      return hash_internal::hash_bytes(std::move(state), value);
+    }
   };
 
-  template <typename U>
-  using ProbeUniquelyRepresented = is_uniquely_represented<U>;
-
-  template <typename U>
-  using ProbeHashValue =
-      std::is_same<State, decltype(AbslHashValue(std::declval<State>(),
-                                                 std::declval<const U&>()))>;
+  struct HashValueProbe {
+    template <typename H, typename T>
+    static auto Invoke(H state, const T& value) -> absl::enable_if_t<
+        std::is_same<H,
+                     decltype(AbslHashValue(std::move(state), value))>::value,
+        H> {
+      return AbslHashValue(std::move(state), value);
+    }
+  };
 
+  struct LegacyHashProbe {
 #if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
-  template <typename U>
-  using ProbeLegacyHash =
-      std::is_convertible<decltype(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE::hash<
-                                   U>()(std::declval<const U&>())),
-                          size_t>;
+    template <typename H, typename T>
+    static auto Invoke(H state, const T& value) -> absl::enable_if_t<
+        std::is_convertible<
+            decltype(ABSL_INTERNAL_LEGACY_HASH_NAMESPACE::hash<T>()(value)),
+            size_t>::value,
+        H> {
+      return hash_internal::hash_bytes(
+          std::move(state),
+          ABSL_INTERNAL_LEGACY_HASH_NAMESPACE::hash<T>{}(value));
+    }
 #endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
+  };
 
-  template <typename U>
-  using ProbeStdHash =
-#if ABSL_HASH_INTERNAL_CAN_POISON_
-      std::is_convertible<decltype(std::hash<U>()(std::declval<const U&>())),
-                          size_t>;
-#else   // ABSL_HASH_INTERNAL_CAN_POISON_
-      std::true_type;
-#endif  // ABSL_HASH_INTERNAL_CAN_POISON_
+  struct StdHashProbe {
+    template <typename H, typename T>
+    static auto Invoke(H state, const T& value)
+        -> absl::enable_if_t<type_traits_internal::IsHashable<T>::value, H> {
+      return hash_internal::hash_bytes(std::move(state), std::hash<T>{}(value));
+    }
+  };
 
-  template <typename U>
-  using ProbeNone = std::true_type;
+  template <typename Hash, typename T>
+  struct Probe : Hash {
+   private:
+    template <typename H, typename = decltype(H::Invoke(
+                              std::declval<State>(), std::declval<const T&>()))>
+    static std::true_type Test(int);
+    template <typename U>
+    static std::false_type Test(char);
+
+   public:
+    static constexpr bool value = decltype(Test<Hash>(0))::value;
+  };
 
  public:
   // Probe each implementation in order.
-  // disjunction provides short circuting wrt instantiation.
-  static constexpr InvokeHashTag value = absl::disjunction<
-      Probe<ProbeUniquelyRepresented, InvokeHashTag::kUniquelyRepresented>,
-      Probe<ProbeHashValue, InvokeHashTag::kHashValue>,
-#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
-      Probe<ProbeLegacyHash, InvokeHashTag::kLegacyHash>,
-#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
-      Probe<ProbeStdHash, InvokeHashTag::kStdHash>,
-      Probe<ProbeNone, InvokeHashTag::kNone>>::kTag;
+  // disjunction provides short circuiting wrt instantiation.
+  template <typename T>
+  using Apply = absl::disjunction<         //
+      Probe<UniquelyRepresentedProbe, T>,  //
+      Probe<HashValueProbe, T>,            //
+      Probe<LegacyHashProbe, T>,           //
+      Probe<StdHashProbe, T>,              //
+      std::false_type>;
 };
 
 template <typename T>
-struct is_hashable : std::integral_constant<bool, HashSelect<T>::value !=
-                                                      InvokeHashTag::kNone> {};
-
-template <typename H, typename T>
-absl::enable_if_t<HashSelect<T>::value == InvokeHashTag::kUniquelyRepresented,
-                  H>
-InvokeHash(H state, const T& value) {
-  return hash_internal::hash_bytes(std::move(state), value);
-}
-
-template <typename H, typename T>
-absl::enable_if_t<HashSelect<T>::value == InvokeHashTag::kHashValue, H>
-InvokeHash(H state, const T& value) {
-  return AbslHashValue(std::move(state), value);
-}
-
-#if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
-template <typename H, typename T>
-absl::enable_if_t<HashSelect<T>::value == InvokeHashTag::kLegacyHash, H>
-InvokeHash(H state, const T& value) {
-  return hash_internal::hash_bytes(
-      std::move(state), ABSL_INTERNAL_LEGACY_HASH_NAMESPACE::hash<T>{}(value));
-}
-#endif  // ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
-
-template <typename H, typename T>
-absl::enable_if_t<HashSelect<T>::value == InvokeHashTag::kStdHash, H>
-InvokeHash(H state, const T& value) {
-  return hash_internal::hash_bytes(std::move(state), std::hash<T>{}(value));
-}
+struct is_hashable
+    : std::integral_constant<bool, HashSelect::template Apply<T>::value> {};
 
 // CityHashState
 class CityHashState : public HashStateBase<CityHashState> {
@@ -684,7 +641,8 @@ class CityHashState : public HashStateBase<CityHashState> {
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
   static constexpr uint64_t kMul =
-      sizeof(size_t) == 4 ? uint64_t{0xcc9e2d51} : uint64_t{0x9ddfea08eb382d69};
+      sizeof(size_t) == 4 ? uint64_t{0xcc9e2d51}
+                          : uint64_t{0x9ddfea08eb382d69};
 
   template <typename T>
   using IntegralFastPath =
@@ -881,7 +839,8 @@ struct Hash
 template <typename H>
 template <typename T, typename... Ts>
 H HashStateBase<H>::combine(H state, const T& value, const Ts&... values) {
-  return H::combine(hash_internal::InvokeHash(std::move(state), value),
+  return H::combine(hash_internal::HashSelect::template Apply<T>::Invoke(
+                        std::move(state), value),
                     values...);
 }
 
@@ -892,7 +851,7 @@ H HashStateBase<H>::combine_contiguous(H state, const T* data, size_t size) {
   return hash_internal::hash_range_or_bytes(std::move(state), data, size);
 }
 }  // namespace hash_internal
-}  // inline namespace lts_2018_12_18
+}  // inline namespace lts_2019_08_08
 }  // namespace absl
 
 #endif  // ABSL_HASH_INTERNAL_HASH_H_