[clang] Add __nullptr as a keyword to C (#123119)

This PR resolves #121503. 

---------

Co-authored-by: Aidan <[email protected]>
Co-authored-by: Erich Keane <[email protected]>
Co-authored-by: Aaron Ballman <[email protected]>

Author: 4 people

clang/docs/LanguageExtensions.rst

@@ -514,9 +514,7 @@ available in all language modes.
 __nullptr
 ---------
 
-``__nullptr`` is an alternate spelling for ``nullptr``, but is also available in
-C++ modes prior to C++11. Note that it's currently not availbale in C despite
-C23 having support for ``nullptr``.
+``__nullptr`` is an alternate spelling for ``nullptr``. It is available in all C and C++ language modes.
 
 __signed, __signed__
 --------------------

clang/docs/ReleaseNotes.rst

@@ -388,6 +388,7 @@ C Language Changes
 
 - Extend clang's ``<limits.h>`` to define ``LONG_LONG_*`` macros for Android's bionic.
 - Macro ``__STDC_NO_THREADS__`` is no longer necessary for MSVC 2022 1939 and later.
+- Exposed the the ``__nullptr`` keyword as an alias for ``nullptr`` in all C language modes.
 
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/TokenKinds.def

@@ -707,7 +707,7 @@ ALIAS("__decltype"       , decltype     , KEYCXX)
 ALIAS("__imag__"         , __imag       , KEYALL)
 ALIAS("__inline"         , inline       , KEYALL)
 ALIAS("__inline__"       , inline       , KEYALL)
-ALIAS("__nullptr"        , nullptr      , KEYCXX)
+ALIAS("__nullptr"        , nullptr      , KEYALL)
 ALIAS("__real__"         , __real       , KEYALL)
 ALIAS("__restrict"       , restrict     , KEYALL)
 ALIAS("__restrict__"     , restrict     , KEYALL)

clang/test/Sema/nullptr-prec2x.c

@@ -6,3 +6,7 @@ int nullptr; // expected-warning {{'nullptr' is a keyword in C23}}
 
 nullptr_t val; // expected-error {{unknown type name 'nullptr_t'}}
 
+void foo(void *);
+void bar() { foo(__nullptr); } // Test that it converts properly to an arbitrary pointer type without warning
+_Static_assert(__nullptr == 0, "value of __nullptr"); // Test that its value matches that of NULL
+_Static_assert(_Generic(__typeof(__nullptr), int : 0, void * : 0, default : 1), "type of __nullptr"); // Test that it's type is not the same as what NULL would generally have.

clang/test/Sema/nullptr.c

@@ -108,3 +108,10 @@ void test_f1() {
   int ir = (f1)(nullptr);
 }
 
+// __nullptr keyword in C
+void foo(void *);
+void bar() { foo(__nullptr); }
+static_assert(nullptr == __nullptr);
+static_assert(__nullptr == 0);  // Test that its value matches that of NULL
+static_assert(_Generic(typeof(__nullptr), nullptr_t: true, default: false));
+static_assert(_Generic(__typeof(__nullptr), int : 0, void * : 0, default : 1)); // Test that it's type is not the same as what NULL would generally have.