[libc++] Implement LWG 2911 - add an is_aggregate type-trait

[EricWF]

Summary

This patch implements http://cplusplus.github.io/LWG/lwg-defects.html#2911.

I'm putting this up for review until __is_aggregate is added to clang (See D31513)

[EricWF]

GCC implemented __is_aggregate hours ago as part of 7.0.1. Update the feature checks to reflect that.

[EricWF]

• Fix accidental change.

[EricWF]

• Fix typo in // UNSUPPORTED directive.

[mclow.lists]

LGTM

[EricWF]

• Add comment explaining _GNUC_VER_NEW.

include/__config

 #ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #pragma GCC system_header
 #endif
 
 #ifdef __cplusplus
 
 #ifdef __GNUC__
 #define _GNUC_VER (__GNUC__ * 100 + __GNUC_MINOR__)
+#define _GNUC_VER_NEW (_GNUC_VER * 10 + __GNUC_PATCHLEVEL__)
 #else
 #define _GNUC_VER 0
+#define _GNUC_VER_NEW 0
 #endif
 
 #define _LIBCPP_VERSION 5000
 
 #ifndef _LIBCPP_ABI_VERSION
 #define _LIBCPP_ABI_VERSION 1
 #endif
 

 
 #if __has_attribute(require_constant_initialization)
 #define _LIBCPP_SAFE_STATIC __attribute__((__require_constant_initialization__))
 #else
 #define _LIBCPP_SAFE_STATIC
 #endif
 
 #if !__has_builtin(__builtin_addressof) && _GNUC_VER < 700
-#define _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
+# define _LIBCPP_HAS_NO_BUILTIN_ADDRESSOF
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_OFF_T_FUNCTIONS)
 #if defined(_LIBCPP_MSVCRT) || defined(_NEWLIB_VERSION)
 #define _LIBCPP_HAS_NO_OFF_T_FUNCTIONS
 #endif
 #endif
 

 # define _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS
 # define _LIBCPP_ENABLE_CXX17_REMOVED_RANDOM_SHUFFLE
 #endif // _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES
 
 #if !defined(__cpp_deduction_guides) || __cpp_deduction_guides < 201611
 # define _LIBCPP_HAS_NO_DEDUCTION_GUIDES
 #endif
 
+#if !__has_keyword(__is_aggregate) && (_GNUC_VER_NEW < 7001)
+# define _LIBCPP_HAS_NO_IS_AGGREGATE
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG

include/type_traits

namespace std
     template <class T> struct is_trivial;
     template <class T> struct is_trivially_copyable;
     template <class T> struct is_standard_layout;
     template <class T> struct is_literal_type;
     template <class T> struct is_empty;
     template <class T> struct is_polymorphic;
     template <class T> struct is_abstract;
     template <class T> struct is_final; // C++14
+    template <class T> struct is_aggregate; // C++17
 
     template <class T, class... Args> struct is_constructible;
     template <class T>                struct is_default_constructible;
     template <class T>                struct is_copy_constructible;
     template <class T>                struct is_move_constructible;
     template <class T, class U>       struct is_assignable;
     template <class T>                struct is_copy_assignable;
     template <class T>                struct is_move_assignable;
template <class...>
       template <class T> constexpr bool is_empty_v
         = is_empty<T>::value;                                            // C++17
       template <class T> constexpr bool is_polymorphic_v
         = is_polymorphic<T>::value;                                      // C++17
       template <class T> constexpr bool is_abstract_v
         = is_abstract<T>::value;                                         // C++17
       template <class T> constexpr bool is_final_v
         = is_final<T>::value;                                            // C++17
+      template <class T> constexpr bool is_aggregate_v
+        = is_aggregate<T>::value;                                        // C++17
       template <class T> constexpr bool is_signed_v
         = is_signed<T>::value;                                           // C++17
       template <class T> constexpr bool is_unsigned_v
         = is_unsigned<T>::value;                                         // C++17
       template <class T, class... Args> constexpr bool is_constructible_v
         = is_constructible<T, Args...>::value;                           // C++17
       template <class T> constexpr bool is_default_constructible_v
         = is_default_constructible<T>::value;                            // C++17

 is_final : public integral_constant<bool, __is_final(_Tp)> {};
 #endif
 
 #if _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_VARIABLE_TEMPLATES)
 template <class _Tp> _LIBCPP_CONSTEXPR bool is_final_v
     = is_final<_Tp>::value;
 #endif
 
+// is_aggregate
+#if _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_IS_AGGREGATE)
+
+template <class _Tp> struct _LIBCPP_TEMPLATE_VIS
+is_aggregate : public integral_constant<bool, __is_aggregate(_Tp)> {};
+
+#if !defined(_LIBCPP_HAS_NO_VARIABLE_TEMPLATES)
+template <class _Tp>
+constexpr bool is_aggregate_v = is_aggregate<_Tp>::value;
+#endif
+
+#endif // _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_IS_AGGREGATE)
+
 // is_base_of
 
 #ifdef _LIBCPP_HAS_IS_BASE_OF
 
 template <class _Bp, class _Dp>
 struct _LIBCPP_TEMPLATE_VIS is_base_of
     : public integral_constant<bool, __is_base_of(_Bp, _Dp)> {};
 

test/libcxx/utilities/meta/meta.unary/meta.unary.prop/missing_is_aggregate_trait.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, C++14
+
+// <type_traits>
+
+// template <class T> struct is_aggregate;
+// template <class T> constexpr bool is_aggregate_v = is_aggregate<T>::value;
+
+#include <type_traits>
+
+int main ()
+{
+#ifdef _LIBCPP_HAS_NO_IS_AGGREGATE
+  // This should not compile when _LIBCPP_HAS_NO_IS_AGGREGATE is defined.
+  bool b = __is_aggregate(void);
+  ((void)b);
+#else
+#error Forcing failure...
+#endif
+}

test/std/utilities/meta/meta.unary/meta.unary.prop/is_aggregate.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, C++14
+
+// <type_traits>
+
+// template <class T> struct is_aggregate;
+// template <class T> constexpr bool is_aggregate_v = is_aggregate<T>::value;
+
+#include <type_traits>
+#include "test_macros.h"
+
+template <class T>
+void test_true()
+{
+#if !defined(_LIBCPP_HAS_NO_IS_AGGREGATE)
+    static_assert( std::is_aggregate<T>::value, "");
+    static_assert( std::is_aggregate<const T>::value, "");
+    static_assert( std::is_aggregate<volatile T>::value, "");
+    static_assert( std::is_aggregate<const volatile T>::value, "");
+    static_assert( std::is_aggregate_v<T>, "");
+    static_assert( std::is_aggregate_v<const T>, "");
+    static_assert( std::is_aggregate_v<volatile T>, "");
+    static_assert( std::is_aggregate_v<const volatile T>, "");
+#endif
+}
+
+template <class T>
+void test_false()
+{
+#if !defined(_LIBCPP_HAS_NO_IS_AGGREGATE)
+    static_assert(!std::is_aggregate<T>::value, "");
+    static_assert(!std::is_aggregate<const T>::value, "");
+    static_assert(!std::is_aggregate<volatile T>::value, "");
+    static_assert(!std::is_aggregate<const volatile T>::value, "");
+    static_assert(!std::is_aggregate_v<T>, "");
+    static_assert(!std::is_aggregate_v<const T>, "");
+    static_assert(!std::is_aggregate_v<volatile T>, "");
+    static_assert(!std::is_aggregate_v<const volatile T>, "");
+#endif
+}
+
+struct Aggregate {};
+struct HasCons { HasCons(int); };
+struct HasPriv {
+  void PreventUnusedPrivateMemberWarning();
+private:
+  int x;
+};
+struct Union { int x; void* y; };
+
+
+int main ()
+{
+  {
+    test_false<void>();
+    test_false<int>();
+    test_false<void*>();
+    test_false<void()>();
+    test_false<void() const>();
+    test_false<void(Aggregate::*)(int) const>();
+    test_false<Aggregate&>();
+    test_false<HasCons>();
+    test_false<HasPriv>();
+  }
+  {
+    test_true<Aggregate>();
+    test_true<Aggregate[]>();
+    test_true<Aggregate[42][101]>();
+    test_true<Union>();
+  }
+}

www/cxx1z_status.html

<!--   <I>Note: "NAD" means that the issue was deemed "Not a defect"</I> -->
   <tr><td><a href="http://wg21.link/LWG2876">2876</a></td><td>shared_ptr::shared_ptr(const weak_ptr&lt;Y&gt;&amp;) constructor should be constrained</td><td>Kona</td><td></td></tr>
   <tr><td><a href="http://wg21.link/LWG2878">2878</a></td><td>Missing DefaultConstructible requirement for istream_iterator default constructor</td><td>Kona</td><td></td></tr>
   <tr><td><a href="http://wg21.link/LWG2890">2890</a></td><td>The definition of 'object state' applies only to class types</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="http://wg21.link/LWG2900">2900</a></td><td>The copy and move constructors of optional are not constexpr</td><td>Kona</td><td></td></tr>
   <tr><td><a href="http://wg21.link/LWG2903">2903</a></td><td>The form of initialization for the emplace-constructors is not specified</td><td>Kona</td><td></td></tr>
   <tr><td><a href="http://wg21.link/LWG2904">2904</a></td><td>Make variant move-assignment more exception safe</td><td>Kona</td><td></td></tr>
   <tr><td><a href="http://wg21.link/LWG2905">2905</a></td><td>is_constructible_v&lt;unique_ptr&lt;P, D&gt;, P, D const &amp;&gt; should be false when D is not copy constructible</td><td>Kona</td><td></td></tr>
   <tr><td><a href="http://wg21.link/LWG2908">2908</a></td><td>The less-than operator for shared pointers could do more</td><td>Kona</td><td></td></tr>
-  <tr><td><a href="http://wg21.link/LWG2911">2911</a></td><td>An is_aggregate type trait is needed</td><td>Kona</td><td></td></tr>
+  <tr><td><a href="http://wg21.link/LWG2911">2911</a></td><td>An is_aggregate type trait is needed</td><td>Kona</td><td>Complete</td></tr>
   <tr><td><a href="http://wg21.link/LWG2921">2921</a></td><td>packaged_task and type-erased allocators</td><td>Kona</td><td></td></tr>
   <tr><td><a href="http://wg21.link/LWG2934">2934</a></td><td>optional&lt;const T&gt; doesn't compare with T</td><td>Kona</td><td>Complete</td></tr>
 <!--
   <tr><td><a href="http://wg21.link/LWG1214">1214</a></td><td>Insufficient/inconsistent key immutability requirements for associative containers</td><td>Urbana</td><td></td></tr>
 -->
 <!--   <tr><td></td><td></td><td></td><td></td></tr> -->
   </table>
 
   <p>Last Updated: 23-Mar-2017</p>
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