[libcxx] Change _LIBCXX_CONSTEXPR_AFTER_CXX11 to check for relaxed constexpr rules

[EricWF]

Summary

Currently libc++ will not compile in c++1y mode with GCC since GCC does not yet support relaxed constexpr rules. This patch only defines _LIBCXX_CONSTEXPR_AFTER_CXX11 only if relaxed constexpr rules are available.

This patch introduces the _LIBCXX_HAS_NO_RELAXED_CONSTEXPR macro that should be defined when a compiler does not support the relaxed constexpr rules. Currently this macro is defined:

• On clang when __has_feature(cxx_relaxed_constexpr) is false.
• Unconditionally on GCC.
• Unconditionally on MSVC.

Although not all uses of _LIBCXX_CONSTEXPR_AFTER_CXX11 require the relaxed rules it would be very messy to selectively enable usages that do.

Are there any opposing viewpoints to this method of handling _LIBCPP_CONSTEXPR_AFTER_CXX11 ?

[EricWF]

Adding cfe-commits.

[EricWF]

Pinging this.

[mclow.lists]

I don't like the name "relaxed constexpr". What if (as seems likely) the restrictions on constexpr get loosened (again) for C++17?

How about _LIBCXX_IMPLEMENTS_CXX14_CONSTEXPR instead?

P.S. Any idea on when gcc will implement this part of C++14? Might it be in 4.9.1? [ I will ping someone on the gcc side and try to find out ]

[EricWF]

How about _LIBCPP_HAS_CXX14_CONSTEXPR since its more inline with current names?

[EricWF]

Sorry that should be _LIBCPP_HAS_NO_CXX14_CONSTEXPR since most existing names are of the negative form.

[mclow.lists]

I like _LIBCPP_HAS_NO_CXX14_CONSTEXPR.

[EricWF]

Changed the name _LIBCPP_HAS_NO_RELAXED_CONSTEXPR -> _LIBCPP_HAS_NO_CXX14_CONSTEXPR

[mclow.lists]

LGTM. Thanks!

[EricWF]

Landed as r213225

include/__config

 #define _LIBCPP_HAS_NO_CONSTEXPR
 #endif
 
+#if !(__has_feature(cxx_relaxed_constexpr))
+#define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
+#endif
+
 #if __ISO_C_VISIBLE >= 2011 || __cplusplus >= 201103L
 #if defined(__FreeBSD__)
 #define _LIBCPP_HAS_QUICK_EXIT

 #define _LIBCPP_HAS_NO_CONSTEXPR
 #endif
 
+// No version of GCC supports relaxed constexpr rules
+#define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
+
 #define _NOEXCEPT throw()
 #define _NOEXCEPT_(x)
 #define _NOEXCEPT_OR_FALSE(x) false

 #define _LIBCPP_HAS_NO_TEMPLATE_ALIASES
 #define _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #define _LIBCPP_HAS_NO_CONSTEXPR
+#define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
 #define _LIBCPP_HAS_NO_UNICODE_CHARS
 #define _LIBCPP_HAS_NO_DELETED_FUNCTIONS
 #define _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS

 #endif
 
 #if _LIBCPP_STD_VER <= 11
-#define _LIBCPP_CONSTEXPR_AFTER_CXX11
 #define _LIBCPP_EXPLICIT_AFTER_CXX11
 #define _LIBCPP_DEPRECATED_AFTER_CXX11
 #else
-#define _LIBCPP_CONSTEXPR_AFTER_CXX11 constexpr
 #define _LIBCPP_EXPLICIT_AFTER_CXX11 explicit
 #define _LIBCPP_DEPRECATED_AFTER_CXX11 [[deprecated]]
 #endif
 
+#if _LIBCPP_STD_VER > 11 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
+#define _LIBCPP_CONSTEXPR_AFTER_CXX11 constexpr
+#else
+#define _LIBCPP_CONSTEXPR_AFTER_CXX11
+#endif
+
 #ifndef _LIBCPP_HAS_NO_ASAN
 extern "C" void __sanitizer_annotate_contiguous_container(
   const void *, const void *, const void *, const void *);