Fix compilation errors with gcc 4.8.5

Thanks to @aengusjiang for the initial PR, which I updated to simplify and fix compilation issues on windows.

parallel_hashmap/btree.h

@@ -108,7 +108,7 @@ namespace phmap {
             bool, std::is_copy_constructible<
                       type_traits_internal::SingleMemberUnion<T>>::value &&
             std::is_trivially_destructible<T>::value> {};
-
+#if 0
         template <class T>
         struct IsTriviallyMoveAssignableReference : std::false_type {};
 
@@ -119,7 +119,7 @@ namespace phmap {
         template <class T>
         struct IsTriviallyMoveAssignableReference<T&&>
             : std::is_trivially_move_assignable<T>::type {};
-
+#endif
     }  // namespace type_traits_internal
 
 
@@ -148,8 +148,8 @@ namespace phmap {
 
         public:
             static constexpr bool kValue =
-                (std::is_trivially_copyable<ExtentsRemoved>::value || !kIsCopyOrMoveConstructible) &&
-                (std::is_trivially_copy_assignable<ExtentsRemoved>::value || !kIsCopyOrMoveAssignable) &&
+                (phmap::is_trivially_copyable<ExtentsRemoved>::value || !kIsCopyOrMoveConstructible) &&
+                (phmap::is_trivially_copy_assignable<ExtentsRemoved>::value || !kIsCopyOrMoveAssignable) &&
                 (kIsCopyOrMoveConstructible || kIsCopyOrMoveAssignable) &&
                 std::is_trivially_destructible<ExtentsRemoved>::value &&
                 // We need to check for this explicitly because otherwise we'll say
@@ -3321,8 +3321,8 @@ namespace priv {
         // ----------------
         template <typename K = key_type>
         size_type count(const key_arg<K> &key) const {
-            auto equal_range = this->equal_range(key);
-            return std::distance(equal_range.first, equal_range.second);
+            auto er = this->equal_range(key);
+            return std::distance(er.first, er.second);
         }
         template <typename K = key_type>
         iterator find(const key_arg<K> &key) {
@@ -3362,8 +3362,8 @@ namespace priv {
         }
         template <typename K = key_type>
         size_type erase(const key_arg<K> &key) {
-            auto equal_range = this->equal_range(key);
-            return tree_.erase_range(equal_range.first, equal_range.second).first;
+            auto er = this->equal_range(key);
+            return tree_.erase_range(er.first, er.second).first;
         }
         node_type extract(iterator position) {
             // Use Move instead of Transfer, because the rebalancing code expects to

parallel_hashmap/phmap_base.h

@@ -223,6 +223,36 @@ struct disjunction<> : std::false_type {};
 template <typename T>
 struct negation : std::integral_constant<bool, !T::value> {};
 
+#if defined(__GNUC__) && __GNUC__ < 5 && !defined(__clang__) && !defined(_MSC_VER) && !defined(__INTEL_COMPILER)
+    #define PHMAP_OLD_GCC 1
+#else
+    #define PHMAP_OLD_GCC 0
+#endif
+
+#if PHMAP_OLD_GCC
+  template <typename T>
+  struct is_trivially_copy_constructible
+     : std::integral_constant<bool,
+                              __has_trivial_copy(typename std::remove_reference<T>::type) &&
+                              std::is_copy_constructible<T>::value &&
+                              std::is_trivially_destructible<T>::value> {};
+ 
+  template <typename T>
+  struct is_trivially_copy_assignable :
+     std::integral_constant<bool,
+                            __has_trivial_assign(typename std::remove_reference<T>::type) &&
+                            phmap::is_copy_assignable<T>::value> {};
+
+  template <typename T>
+  struct is_trivially_copyable :
+     std::integral_constant<bool, __has_trivial_copy(typename std::remove_reference<T>::type)> {};
+
+#else
+  template <typename T> using is_trivially_copy_constructible = std::is_trivially_copy_constructible<T>;
+  template <typename T> using is_trivially_copy_assignable = std::is_trivially_copy_assignable<T>;
+  template <typename T> using is_trivially_copyable = std::is_trivially_copyable<T>;
+#endif
+
 // -----------------------------------------------------------------------------
 // C++14 "_t" trait aliases
 // -----------------------------------------------------------------------------
@@ -1788,8 +1818,8 @@ protected:
 // supported now, so we use is_trivially_* traits instead.
 template <typename T,
           bool unused =
-          std::is_trivially_copy_constructible<T>::value &&
-          std::is_trivially_copy_assignable<typename std::remove_cv<T>::type>::value &&
+          phmap::is_trivially_copy_constructible<T>::value &&
+          phmap::is_trivially_copy_assignable<typename std::remove_cv<T>::type>::value &&
           std::is_trivially_destructible<T>::value>
 class optional_data;
 
@@ -2021,6 +2051,11 @@ struct optional_hash_base<T, decltype(std::hash<phmap::remove_const_t<T> >()(
 // -----------------------------------------------------------------------------
 // phmap::optional class definition
 // -----------------------------------------------------------------------------
+#if PHMAP_OLD_GCC
+    #define PHMAP_OPTIONAL_NOEXCEPT
+#else
+    #define PHMAP_OPTIONAL_NOEXCEPT noexcept
+#endif
 
 template <typename T>
 class optional : private optional_internal::optional_data<T>,
@@ -2047,7 +2082,7 @@ public:
     optional(const optional& src) = default;
 
     // Move constructor, standard semantics
-    optional(optional&& src) noexcept = default;
+    optional(optional&& src) PHMAP_OPTIONAL_NOEXCEPT = default;
 
     // Constructs a non-empty `optional` direct-initialized value of type `T` from
     // the arguments `std::forward<Args>(args)...`  within the `optional`.
@@ -2187,7 +2222,7 @@ public:
     optional& operator=(const optional& src) = default;
 
     // Move assignment operator, standard semantics
-    optional& operator=(optional&& src) noexcept = default;
+    optional& operator=(optional&& src) PHMAP_OPTIONAL_NOEXCEPT = default;
 
     // Value assignment operators
     template <

parallel_hashmap/phmap_config.h

@@ -120,7 +120,7 @@
     #define PHMAP_HAVE_BUILTIN(x) 0
 #endif
 
-#if (defined(_MSVC_LANG) && _MSVC_LANG >= 201703) || __cplusplus >= 201703
+#if (!defined(__GNUC__) || __GNUC__ >= 5) && ((defined(_MSVC_LANG) && _MSVC_LANG >= 201703L) || __cplusplus >= 201703L)
     #define PHMAP_HAVE_CC17 1
 #else
     #define PHMAP_HAVE_CC17 0
@@ -313,8 +313,12 @@
     #endif
 #endif
 
-#if PHMAP_HAVE_CC17 && (!defined(__has_include) || __has_include(<shared_mutex>))
-    #define PHMAP_HAVE_SHARED_MUTEX 1
+#if PHMAP_HAVE_CC17
+    #ifdef __has_include
+       #if __has_include(<shared_mutex>)
+           #define PHMAP_HAVE_SHARED_MUTEX 1
+       #endif
+    #endif
 #endif
 
 #ifndef PHMAP_HAVE_STD_STRING_VIEW