Split <stddef.h> out of <cstddef>.

There are a bunch of macros (__need_size_t etc) that request just one piece of
<stddef.h>; if any one of these is defined, we just directly include the
underlying header.

Note that <stddef.h> provides a ::nullptr_t. We don't want that available to
includers of <cstddef>, so instead of following the usual pattern where <cfoo>
includes <foo.h> then pulls things from :: into std:: with using-declarations,
we implement <stddef.h> and <cstddef> separately; both include <__nullptr> for
the definition of std::nullptr_t.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@249761 91177308-0d34-0410-b5e6-96231b3b80d8

include/cstddef

@@ -34,10 +34,10 @@ Types:
 */
 
 #include <__config>
+// Don't include our own <stddef.h>; we don't want to declare ::nullptr_t.
+#include_next <stddef.h>
 #include <__nullptr>
 
-#include <stddef.h>
-
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif

include/stddef.h

@@ -0,0 +1,56 @@
+// -*- C++ -*-
+//===--------------------------- stddef.h ---------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#if defined(__need_ptrdiff_t) || defined(__need_size_t) || \
+    defined(__need_wchar_t) || defined(__need_NULL) || defined(__need_wint_t)
+#include_next <stddef.h>
+
+#elif !defined(_LIBCPP_STDDEF_H)
+#define _LIBCPP_STDDEF_H
+
+/*
+    stddef.h synopsis
+
+Macros:
+
+    offsetof(type,member-designator)
+    NULL
+
+Types:
+
+    ptrdiff_t
+    size_t
+    max_align_t
+    nullptr_t
+
+*/
+
+#include <__config>
+#include_next <stddef.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#ifdef __cplusplus
+
+extern "C++" {
+#include <__nullptr>
+using std::nullptr_t;
+}
+
+// Re-use the compiler's <stddef.h> max_align_t where possible.
+#if !defined(__CLANG_MAX_ALIGN_T_DEFINED) && !defined(_GCC_MAX_ALIGN_T)
+typedef long double max_align_t;
+#endif
+
+#endif
+
+#endif  // _LIBCPP_STDDEF_H

test/std/depr/depr.c.headers/stddef_h.pass.cpp

@@ -10,6 +10,7 @@
 // <stddef.h>
 
 #include <stddef.h>
+#include <cassert>
 #include <type_traits>
 
 #ifndef NULL
@@ -22,6 +23,9 @@
 
 int main()
 {
+    void *p = NULL;
+    assert(!p);
+
     static_assert(sizeof(size_t) == sizeof(void*),
                   "sizeof(size_t) == sizeof(void*)");
     static_assert(std::is_unsigned<size_t>::value,
@@ -34,4 +38,22 @@ int main()
                   "std::is_signed<ptrdiff_t>::value");
     static_assert(std::is_integral<ptrdiff_t>::value,
                   "std::is_integral<ptrdiff_t>::value");
+    static_assert(std::is_same<decltype(nullptr), nullptr_t>::value,
+                  "decltype(nullptr) == nullptr_t");
+    static_assert(sizeof(nullptr_t) == sizeof(void*),
+                  "sizeof(nullptr_t) == sizeof(void*)");
+    static_assert(std::is_pod<max_align_t>::value,
+                  "std::is_pod<max_align_t>::value");
+    static_assert((std::alignment_of<max_align_t>::value >=
+                  std::alignment_of<long long>::value),
+                  "std::alignment_of<max_align_t>::value >= "
+                  "std::alignment_of<long long>::value");
+    static_assert(std::alignment_of<max_align_t>::value >=
+                  std::alignment_of<long double>::value,
+                  "std::alignment_of<max_align_t>::value >= "
+                  "std::alignment_of<long double>::value");
+    static_assert(std::alignment_of<max_align_t>::value >=
+                  std::alignment_of<void*>::value,
+                  "std::alignment_of<max_align_t>::value >= "
+                  "std::alignment_of<void*>::value");
 }