Implement LWG 2488 - Make the placeholders constexpr.

This patch makes the bind placeholders in std::placeholders both (1) const and
(2) constexpr (See below).

This is technically a breaking change for any code using the placeholders
outside of std::bind and depending on them being non-const. However I don't
think this will break any real world code.

(1) Previously the placeholders were non-const extern globals in all
dialects. This patch changes these extern globals to be const in all dialects.
Since the cv-qualifiers don't participate in name mangling for globals this
is an ABI compatible change.

(2) Make the placeholders constexpr in C++11 and beyond. Although LWG 2488 only
applies to C++17 I don't see any reason not to backport this change.

llvm-svn: 273824

libcxx/include/functional

@@ -1980,16 +1980,29 @@ namespace placeholders
 
 template <int _Np> struct __ph {};
 
-_LIBCPP_FUNC_VIS extern __ph<1>   _1;
+#if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_BIND)
-_LIBCPP_FUNC_VIS extern __ph<2>   _2;
+_LIBCPP_FUNC_VIS extern const __ph<1>   _1;
-_LIBCPP_FUNC_VIS extern __ph<3>   _3;
+_LIBCPP_FUNC_VIS extern const __ph<2>   _2;
-_LIBCPP_FUNC_VIS extern __ph<4>   _4;
+_LIBCPP_FUNC_VIS extern const __ph<3>   _3;
-_LIBCPP_FUNC_VIS extern __ph<5>   _5;
+_LIBCPP_FUNC_VIS extern const __ph<4>   _4;
-_LIBCPP_FUNC_VIS extern __ph<6>   _6;
+_LIBCPP_FUNC_VIS extern const __ph<5>   _5;
-_LIBCPP_FUNC_VIS extern __ph<7>   _7;
+_LIBCPP_FUNC_VIS extern const __ph<6>   _6;
-_LIBCPP_FUNC_VIS extern __ph<8>   _8;
+_LIBCPP_FUNC_VIS extern const __ph<7>   _7;
-_LIBCPP_FUNC_VIS extern __ph<9>   _9;
+_LIBCPP_FUNC_VIS extern const __ph<8>   _8;
-_LIBCPP_FUNC_VIS extern __ph<10> _10;
+_LIBCPP_FUNC_VIS extern const __ph<9>   _9;
+_LIBCPP_FUNC_VIS extern const __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 // defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_BIND)
 
 }  // placeholders
 

libcxx/src/bind.cpp

@@ -7,6 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#define _LIBCPP_BUILDING_BIND
 #include "functional"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -14,16 +15,16 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 namespace placeholders
 {
 
-__ph<1>   _1;
+const __ph<1>   _1{};
-__ph<2>   _2;
+const __ph<2>   _2{};
-__ph<3>   _3;
+const __ph<3>   _3{};
-__ph<4>   _4;
+const __ph<4>   _4{};
-__ph<5>   _5;
+const __ph<5>   _5{};
-__ph<6>   _6;
+const __ph<6>   _6{};
-__ph<7>   _7;
+const __ph<7>   _7{};
-__ph<8>   _8;
+const __ph<8>   _8{};
-__ph<9>   _9;
+const __ph<9>   _9{};
-__ph<10> _10;
+const __ph<10> _10{};
 
 }  // placeholders
 

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

@@ -10,10 +10,14 @@
 // <functional>
 
 // placeholders
+// The placeholders are constexpr in C++17 and beyond.
+// libc++ provides constexpr placeholders in C++11 and beyond.
 
 #include <functional>
 #include <type_traits>
 
+#include "test_macros.h"
+
 template <class T>
 void
 test(const T& t)
@@ -28,6 +32,30 @@ test(const T& t)
     static_assert(std::is_nothrow_move_constructible<T>::value, "");
 }
 
+#if TEST_STD_VER >= 11
+constexpr decltype(std::placeholders::_1)  default1{};
+constexpr decltype(std::placeholders::_2)  default2{};
+constexpr decltype(std::placeholders::_3)  default3{};
+constexpr decltype(std::placeholders::_4)  default4{};
+constexpr decltype(std::placeholders::_5)  default5{};
+constexpr decltype(std::placeholders::_6)  default6{};
+constexpr decltype(std::placeholders::_7)  default7{};
+constexpr decltype(std::placeholders::_8)  default8{};
+constexpr decltype(std::placeholders::_9)  default9{};
+constexpr decltype(std::placeholders::_10) default10{};
+
+constexpr decltype(std::placeholders::_1)  cp1 = std::placeholders::_1;
+constexpr decltype(std::placeholders::_2)  cp2 = std::placeholders::_2;
+constexpr decltype(std::placeholders::_3)  cp3 = std::placeholders::_3;
+constexpr decltype(std::placeholders::_4)  cp4 = std::placeholders::_4;
+constexpr decltype(std::placeholders::_5)  cp5 = std::placeholders::_5;
+constexpr decltype(std::placeholders::_6)  cp6 = std::placeholders::_6;
+constexpr decltype(std::placeholders::_7)  cp7 = std::placeholders::_7;
+constexpr decltype(std::placeholders::_8)  cp8 = std::placeholders::_8;
+constexpr decltype(std::placeholders::_9)  cp9 = std::placeholders::_9;
+constexpr decltype(std::placeholders::_10) cp10 = std::placeholders::_10;
+#endif // TEST_STD_VER >= 11
+
 int main()
 {
     test(std::placeholders::_1);

libcxx/www/cxx1z_status.html

@@ -161,7 +161,7 @@
 	<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>Complete</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>