Support assert as a variadic macro for C23

C23 makes assert into a variadic macro to handle cases of an argument
that would be interpreted as a single function argument but more than
one macro argument (in particular, compound literals with an
unparenthesized comma in an initializer list); this change was made by
N2829.  Note that this only applies to assert, not to other macros
specified in the C standard with particular numbers of arguments.

Implement this support in glibc.  This change is only for C; C++ would
need a separate change to its separate assert implementations.  It's
also applied only in C23 mode.  It depends on support for (C99)
variadic macros, and also (in order to detect calls where more than
one expression is passed, via an unevaluated function call) a C99
boolean type.  These requirements are encapsulated in the definition
of __ASSERT_VARIADIC.  Tests with -std=c99 and -std=gnu99 (using
implementations continue to work.

I don't think we have a way in the glibc testsuite to validate that
passing more than one expression as an argument does produce the
desired error.

Tested for x86_64.

NEWS

@@ -13,6 +13,10 @@ Major new features:
 
 * The ISO C23 memalignment function has been added.
 
+* As specified in ISO C23, the assert macro is defined to take variable
+  arguments to support expressions with a comma inside a compound
+  literal initializer not surrounded by parentheses.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * Support for dumped heaps has been removed - malloc_set_state() now always

assert/Makefile

@@ -36,12 +36,18 @@ routines := \
 tests := \
   test-assert \
   test-assert-2 \
+  test-assert-c99 \
+  test-assert-gnu99 \
   test-assert-perr \
+  test-assert-variadic \
   tst-assert-c++ \
   tst-assert-g++ \
   tst-assert-sa-2025-0001 \
   # tests
 
+CFLAGS-test-assert-c99.c += -std=c99
+CFLAGS-test-assert-gnu99.c += -std=gnu99
+
 ifeq ($(have-cxx-thread_local),yes)
 CFLAGS-tst-assert-c++.o = -std=c++11
 LDLIBS-tst-assert-c++ = -lstdc++

assert/assert.h

@@ -40,6 +40,24 @@
 # define __ASSERT_VOID_CAST (void)
 #endif
 
+/* C23 makes assert a variadic macro so that expressions with a comma
+   not between parentheses, but that would still be valid as a single
+   function argument, such as those involving compound literals with a
+   comma in the initializer list, can be passed to assert.  This
+   depends on support for variadic macros (added in C99 and GCC 2.95),
+   and on support for _Bool (added in C99 and GCC 3.0) in order to
+   validate that only a single expression is passed as an argument,
+   and is currently implemented only for C.  */
+#if (__GLIBC_USE (ISOC23)						\
+     && (defined __GNUC__						\
+	 ? __GNUC_PREREQ (3, 0)						\
+	 : defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L)	\
+     && !defined __cplusplus)
+# define __ASSERT_VARIADIC 1
+#else
+# define __ASSERT_VARIADIC 0
+#endif
+
 /* void assert (int expression);
 
    If NDEBUG is defined, do nothing.
@@ -47,7 +65,11 @@
 
 #ifdef	NDEBUG
 
+# if __ASSERT_VARIADIC
+#  define assert(...)		(__ASSERT_VOID_CAST (0))
+# else
 #  define assert(expr)		(__ASSERT_VOID_CAST (0))
+# endif
 
 /* void assert_perror (int errnum);
 
@@ -80,6 +102,13 @@ extern void __assert (const char *__assertion, const char *__file, int __line)
      __THROW __attribute__ ((__noreturn__)) __COLD;
 
 
+# if __ASSERT_VARIADIC
+/* This function is not defined and is not called outside of an
+   unevaluated sizeof, but serves to verify that the argument to
+   assert is a single expression.  */
+extern _Bool __assert_single_arg (_Bool);
+# endif
+
 __END_DECLS
 
 /* When possible, define assert so that it does not add extra
@@ -102,10 +131,26 @@ __END_DECLS
       : __assert_fail (#expr, __ASSERT_FILE, __ASSERT_LINE,             \
                        __ASSERT_FUNCTION))
 # elif !defined __GNUC__ || defined __STRICT_ANSI__
+#  if __ASSERT_VARIADIC
+#   define assert(...)							\
+    (((void) sizeof (__assert_single_arg (__VA_ARGS__)), __VA_ARGS__)	\
+     ? __ASSERT_VOID_CAST (0)						\
+     : __assert_fail (#__VA_ARGS__, __FILE__, __LINE__, __ASSERT_FUNCTION))
+#  else
 #   define assert(expr)							\
     ((expr)								\
      ? __ASSERT_VOID_CAST (0)						\
      : __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION))
+#  endif
+# else
+#  if __ASSERT_VARIADIC
+#   define assert(...)							\
+    ((void) sizeof (__assert_single_arg (__VA_ARGS__)), __extension__ ({ \
+      if (__VA_ARGS__)							\
+        ; /* empty */							\
+      else								\
+        __assert_fail (#__VA_ARGS__, __FILE__, __LINE__, __ASSERT_FUNCTION); \
+    }))
 #  else
 /* The first occurrence of EXPR is not evaluated due to the sizeof,
    but will trigger any pedantic warnings masked by the __extension__
@@ -120,6 +165,7 @@ __END_DECLS
         __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION);	\
     }))
 #  endif
+# endif
 
 # ifdef	__USE_GNU
 #  define assert_perror(errnum)						\

assert/test-assert-c99.c

@@ -0,0 +1,3 @@
+#undef _GNU_SOURCE
+
+#include <test-assert.c>

assert/test-assert-gnu99.c

@@ -0,0 +1,3 @@
+#undef _GNU_SOURCE
+
+#include <test-assert.c>

assert/test-assert-variadic.c

@@ -0,0 +1,50 @@
+/* Test assert as a variadic macro.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#undef NDEBUG
+#include <assert.h>
+
+static void
+test1 (void)
+{
+  /* This is two macro arguments, but one function argument, so must
+     be accepted for C23.  */
+  assert ((int [2]) { 0, 1 }[1]);
+}
+
+#define NDEBUG
+#include <assert.h>
+
+static void
+test2 (void)
+{
+  /* With NDEBUG, arbitrary sequences of tokens are valid as assert
+     arguments; they do not need to form a single expression.  */
+  assert (1, 2, 3, *);
+  assert ();
+}
+
+static int
+do_test (void)
+{
+  test1 ();
+  test2 ();
+  return 0;
+}
+
+#include <support/test-driver.c>