[libcxx] Prefer __nullptr to the current fallback implementation in C++03 to prevent ABI issues.

[EricWF]

Summary

The fallback version of nullptr provided by libc++ has a different mangled name than the real thing. This can cause ABI issues between code compiled in C++03 and C++11. This problem can be fixed when using Clang as the compiler. Clang provides 'nullptr' as an alternate keyword that is available in all C++ dialects. We should prefer using 'nullptr' instead of our own fallback type.

However this change is ABI breaking for any user who already depends the C++03 nullptr_t mangled name. Should we hold this change back until we better understand how to manage ABI changes?

[mclow.lists]

However this change is ABI breaking for any user who already depends the C++03 nullptr_t mangled name.

Also, this is a breaking change for anyone who mixes clang and gcc.

[EricWF]

After discussion with @mclow.lists we agree that this patch should not be applied right now.

include/__config

 #endif
 
 #if !(__has_feature(cxx_nullptr))
+#if !(__is_identifier(__nullptr))
+#define nullptr __nullptr
+#else
 #define _LIBCPP_HAS_NO_NULLPTR
 #endif
+#endif
 
 #if !(__has_feature(cxx_rvalue_references))
 #define _LIBCPP_HAS_NO_RVALUE_REFERENCES

test/std/language.support/support.types/nullptr_t_integral_cast.pass.cpp

 //
 //===----------------------------------------------------------------------===//
 
-// NOTE: nullptr_t emulation cannot handle a reinterpret_cast to an
-// integral type
-// XFAIL: c++98, c++03
-
 // typedef decltype(nullptr) nullptr_t;
 
+// C++ standard:
+//  5.2.10 Reinterpret cast [expr.reinterpret.cast]
+//    p4. A pointer can be explicitly converted to any integral type large enough
+//    to hold it. The mapping function is implementation-defined. [ Note: It is
+//    intended to be unsurprising to those who know the addressing structure of
+//    the underlying machine. — end note ] A value of type std::nullptr_t can be
+//    converted to an integral type; the conversion has the same meaning and
+//    validity as a conversion of (void*)0 to the integral type.
+//    [ Note: A reinterpret_cast cannot be used to convert a value of any type
+//      to the type std::nullptr_t.— end note ]
+
+// NOTE: libc++ provides nullptr_t emulation when 'nullptr' is not provided by
+// the compiler. However there is no way to emulate the reinterpret_cast.
+// UNSUPPORTED: C++98, C++03
 
 #include <cstddef>
 #include <cassert>