add a user-provided constructor to emulated c++11 NullPtr

Source/wtf/NullPtr.h

 /*
 
 Copyright (C) 2010 Apple Inc. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:
 1.  Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 31 matching lines @@
 namespace std {
 typedef decltype(nullptr) nullptr_t;
 }
 #endif
 
 #else
 
 namespace std {
 class nullptr_t {
 public:
+    // Required in order to create const nullptr_t objects without an
+    // explicit initializer, a la:

[Mikhail, 2014/06/10 07:15:24]

As I see at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42844 there is a
condition that "the object is of (possibly cv-qualified) non-POD class type",
nullptr_t should not be the case, right?
This is more like a GCC 4.5 particularity, could you pls add to the comment that
this is actually a build fix for GCC 4.5?

[Mostyn Bramley-Moore, 2014/06/10 07:33:58]

I think the second clause matches here (not the first that you quoted):
"if the object is of const-qualified type, the underlying class type shall have
a user-declared default constructor."
Done.

+    //
+    // const std::nullptr_t nullptr;
+    nullptr_t() { }
+
     // Make nullptr convertible to any pointer type.
     template<typename T> operator T*() const { return 0; }
     // Make nullptr convertible to any member pointer type.
     template<typename C, typename T> operator T C::*() { return 0; }
 private:
     // Do not allow taking the address of nullptr.
     void operator&();
 };
 }
 WTF_EXPORT extern const std::nullptr_t nullptr;
@@ skipping 10 matching lines @@
 #else
 #define WTF_DISALLOW_CONSTRUCTION_FROM_ZERO(ClassName) \
     private: \
         ClassName(int)
 #define WTF_DISALLOW_ZERO_ASSIGNMENT(ClassName) \
     private: \
         ClassName& operator=(int)
 #endif
 
 #endif