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

1 files changed, 28 insertions, 0 deletions

diff --git a/libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.place/placeholders.pass.cpp b/libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.place/placeholders.pass.cpp
index 246186040c5..68986ac1aeb 100644
--- a/libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.place/placeholders.pass.cpp
+++ b/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);