[PATCH] c-family: Allow arguments with NULLPTR_TYPE as sentinels [PR114780]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

While in C++ the ellipsis argument conversions include
"An argument that has type cv std::nullptr_t is converted to type void*"
in C23 a nullptr_t argument is not promoted in any way, but va_arg
description says:
"the type of the next argument is nullptr_t and type is a pointer type that has the same
representation and alignment requirements as a pointer to a character type."
So, while in C++ check_function_sentinel will never see NULLPTR_TYPE, for
C23 it can see that and currently we incorrectly warn about those.

The only question is whether we should warn on any argument with
nullptr_t type or just about nullptr (nullptr_t argument with integer_zerop
value).  Through undefined behavior guess one could pass non-NULL pointer
that way, say by union { void *p; nullptr_t q; } u; u.p = &whatever;
and pass u.q to ..., but valid code should always pass something that will
read as (char *) 0 when read using va_arg (ap, char *), so I think it is
better not to warn rather than warn in those cases.

Note, clang seems to pass (void *)0 rather than expression of nullptr_t
type to ellipsis in C23 mode as if it did the C++ ellipsis argument
conversions, in that case guess not warning about that would be even safer,
but what GCC does I think follows the spec more closely, even when in a
valid program one shouldn't be able to observe the difference.

Ok for trunk and later 13.3 if it passes bootstrap/regtest (so far just
checked on the sentinel related C/C++ tests)?

2024-04-19  Jakub Jelinek  <jakub@redhat.com>

	PR c/114780
	* c-common.cc (check_function_sentinel): Allow as sentinel any
	argument of NULLPTR_TYPE.

	* gcc.dg/format/sentinel-2.c: New test.

--- gcc/c-family/c-common.cc.jj	2024-03-27 19:39:17.968676626 +0100
+++ gcc/c-family/c-common.cc	2024-04-19 12:58:01.577985800 +0200
@@ -5783,6 +5783,7 @@ check_function_sentinel (const_tree fnty
       sentinel = fold_for_warn (argarray[nargs - 1 - pos]);
       if ((!POINTER_TYPE_P (TREE_TYPE (sentinel))
 	   || !integer_zerop (sentinel))
+	  && TREE_CODE (TREE_TYPE (sentinel)) != NULLPTR_TYPE
 	  /* Although __null (in C++) is only an integer we allow it
 	     nevertheless, as we are guaranteed that it's exactly
 	     as wide as a pointer, and we don't want to force
--- gcc/testsuite/gcc.dg/format/sentinel-2.c.jj	2024-04-19 12:57:57.431043948 +0200
+++ gcc/testsuite/gcc.dg/format/sentinel-2.c	2024-04-19 12:58:39.020460785 +0200
@@ -0,0 +1,21 @@
+/* PR c/114780 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -Wformat" } */
+
+#include <stddef.h>
+
+[[gnu::sentinel]] void foo (int, ...);
+[[gnu::sentinel]] void bar (...);
+
+void
+baz (nullptr_t p)
+{
+  foo (1, 2, nullptr);
+  foo (3, 4, 5, p);
+  bar (nullptr);
+  bar (p);
+  foo (6, 7, 0);	// { dg-warning "missing sentinel in function call" }
+  bar (0);		// { dg-warning "missing sentinel in function call" }
+  foo (8, 9, NULL);
+  bar (NULL);
+}

	Jakub