Make <functional> placeholders constexpr in C++17

[EricWF]

Summary

Implement LWG issue #2488 (http://cplusplus.github.io/LWG/lwg-defects.html#2488).

[EricWF]

Already committed.

include/functional

 
 template <int _Np> struct __ph {};
 
+#if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_BUILDING_BIND)
 _LIBCPP_FUNC_VIS extern __ph<1>   _1;
 _LIBCPP_FUNC_VIS extern __ph<2>   _2;
 _LIBCPP_FUNC_VIS extern __ph<3>   _3;

 _LIBCPP_FUNC_VIS extern __ph<8>   _8;
 _LIBCPP_FUNC_VIS extern __ph<9>   _9;
 _LIBCPP_FUNC_VIS extern __ph<10> _10;
+#else
+constexpr __ph<1>   _1;
+constexpr __ph<2>   _2;
+constexpr __ph<3>   _3;
+constexpr __ph<4>   _4;
+constexpr __ph<5>   _5;
+constexpr __ph<6>   _6;
+constexpr __ph<7>   _7;
+constexpr __ph<8>   _8;
+constexpr __ph<9>   _9;
+constexpr __ph<10> _10;
+#endif
 
 }  // placeholders
 

src/bind.cpp

 //
 //===----------------------------------------------------------------------===//
 
+#define _LIBCPP_BUILDING_BIND
 #include "functional"
 
 _LIBCPP_BEGIN_NAMESPACE_STD

test/std/utilities/function.objects/bind/func.bind/func.bind.place/placeholders.pass.cpp

 // <functional>
 
 // placeholders
+// The placeholders are constexpr in C++17 and beyond
 
 #include <functional>
 #include <type_traits>
 
+#include "test_macros.h"
+
+#if TEST_STD_VER > 14
+template <class T>
+constexpr bool constexpr_test(const T& t)
+{
+    // Test default constructible.
+    T t2;
+    ((void)t2);
+    // Test copy constructible.
+    T t3 = t;
+    ((void)t3);
+    static_assert(std::is_nothrow_copy_constructible<T>::value, "");
+    static_assert(std::is_nothrow_move_constructible<T>::value, "");
+    return true;
+}
+#endif
+
 template <class T>
 void
 test(const T& t)

     test(std::placeholders::_8);
     test(std::placeholders::_9);
     test(std::placeholders::_10);
+#if TEST_STD_VER > 14
+    static_assert(constexpr_test(std::placeholders::_1));
+    static_assert(constexpr_test(std::placeholders::_2));
+    static_assert(constexpr_test(std::placeholders::_3));
+    static_assert(constexpr_test(std::placeholders::_4));
+    static_assert(constexpr_test(std::placeholders::_5));
+    static_assert(constexpr_test(std::placeholders::_6));
+    static_assert(constexpr_test(std::placeholders::_7));
+    static_assert(constexpr_test(std::placeholders::_8));
+    static_assert(constexpr_test(std::placeholders::_9));
+    static_assert(constexpr_test(std::placeholders::_10));
+#endif
 }

www/cxx1z_status.html

   <tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2467">2467</td><td>is_always_equal has slightly inconsistent default</td><td>Lenexa</td><td>Complete</td></tr>
   <tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2470">2470</td><td>Allocator's destroy function should be allowed to fail to instantiate</td><td>Lenexa</td><td>Complete</td></tr>
   <tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2482">2482</td><td>[c.strings] Table 73 mentions nonexistent functions</td><td>Lenexa</td><td>Complete</td></tr>
-  <tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2488">2488</td><td>Placeholders should be allowed and encouraged to be constexpr</td><td>Lenexa</td><td></td></tr>
+  <tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2488">2488</td><td>Placeholders should be allowed and encouraged to be constexpr</td><td>Lenexa</td><td>Complete</td></tr>
    <tr><td></td><td></td><td></td><td></td></tr>
   <tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#1169">1169</a></td><td><tt>num_get</tt> not fully compatible with <tt>strto*</tt></td><td>Kona</td><td></td></tr>
   <tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2072">2072</a></td><td>Unclear wording about capacity of temporary buffers</td><td>Kona</td><td>Complete</td></tr>