[C++] Expose nullptr_t from stddef.h in C++ mode (#154599)

The C++ standard requires stddef.h to declare all of its contents in the
global namespace. We were only doing it when trying to be compatible
with Microsoft extensions. Now we expose in C++11 or later, in addition
to exposing it in Microsoft extensions mode.

Fixes #154577

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -229,6 +229,7 @@ Bug Fixes in This Version
   cast chain. (#GH149967).
 - Fixed a crash with incompatible pointer to integer conversions in designated
   initializers involving string literals. (#GH154046)
+- Clang's ``<stddef.h>`` now properly declares ``nullptr_t`` in C++ mode. (#GH154577).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Headers/__stddef_nullptr_t.h

@@ -16,7 +16,8 @@
 #define _NULLPTR_T
 
 #ifdef __cplusplus
-#if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
+#if __cplusplus >= 201103L ||                                                  \
+    (defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED))
 namespace std {
 typedef decltype(nullptr) nullptr_t;
 }

clang/test/Headers/stddefneeds.cpp

@@ -1,10 +1,12 @@
 // RUN: %clang_cc1 -fsyntax-only -triple x86_64-apple-macosx10.9.0 -verify -Wsentinel -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -triple x86_64-apple-macosx10.9.0 -verify=old,expected -Wsentinel -std=c++98 %s
 
 ptrdiff_t p0; // expected-error{{unknown}}
 size_t s0; // expected-error{{unknown}}
 void* v0 = NULL; // expected-error{{undeclared}}
 wint_t w0; // expected-error{{unknown}}
 max_align_t m0; // expected-error{{unknown}}
+nullptr_t n0; // expected-error {{unknown}}
 
 #define __need_ptrdiff_t
 #include <stddef.h>
@@ -14,6 +16,7 @@ size_t s1; // expected-error{{unknown}}
 void* v1 = NULL; // expected-error{{undeclared}}
 wint_t w1; // expected-error{{unknown}}
 max_align_t m1; // expected-error{{unknown}}
+nullptr_t n1; // expected-error{{unknown}}
 
 #define __need_size_t
 #include <stddef.h>
@@ -23,6 +26,16 @@ size_t s2;
 void* v2 = NULL; // expected-error{{undeclared}}
 wint_t w2; // expected-error{{unknown}}
 max_align_t m2; // expected-error{{unknown}}
+nullptr_t n2; // expected-error{{unknown}}
+
+#define __need_nullptr_t
+#include <stddef.h>
+ptrdiff_t p6;
+size_t s6;
+void* v6 = NULL; // expected-error{{undeclared}}
+wint_t w6; // expected-error{{unknown}}
+max_align_t m6; // expected-error{{unknown}}
+nullptr_t n6; // old-error{{unknown}}
 
 #define __need_NULL
 #include <stddef.h>
@@ -32,6 +45,16 @@ size_t s3;
 void* v3 = NULL;
 wint_t w3; // expected-error{{unknown}}
 max_align_t m3; // expected-error{{unknown}}
+nullptr_t n3; // old-error{{unknown}}
+
+#define __need_max_align_t
+#include <stddef.h>
+ptrdiff_t p7;
+size_t s7;
+void* v7 = NULL;
+wint_t w7; // expected-error{{unknown}}
+max_align_t m7;
+nullptr_t n7; // old-error{{unknown}}
 
 // Shouldn't bring in wint_t by default:
 #include <stddef.h>
@@ -41,6 +64,7 @@ size_t s4;
 void* v4 = NULL;
 wint_t w4; // expected-error{{unknown}}
 max_align_t m4;
+nullptr_t n4; // old-error{{unknown}}
 
 #define __need_wint_t
 #include <stddef.h>
@@ -50,7 +74,7 @@ size_t s5;
 void* v5 = NULL;
 wint_t w5;
 max_align_t m5;
-
+nullptr_t n5; // old-error{{unknown}}
 
 // linux/stddef.h does something like this for cpp files:
 #undef NULL