assert-h: static_assert is a keyword in C23

* m4/assert_h.m4 (gl_ASSERT_H): Also test for static_assert
keyword a la C23, and define HAVE_C_STATIC_ASSERT if so.
If not, arrange for config.h to #define static_assert
by including <assert.h>, and then do "#undef assert"
so that the assert macro still needs an explicit include.
This should be safe even on very old hosts, as assert.h
has been re-includable for decades.
* tests/tests-assert.c: New test.
* modules/assert-h-tests (Files, Makefile.am): Add it.

1 files changed, 71 insertions, 0 deletions

diff --git a/tests/test-assert.c b/tests/test-assert.c
new file mode 100644
index 0000000000..45b0c0f457
--- /dev/null
+++ b/tests/test-assert.c
@@ -0,0 +1,71 @@
+/* Test assert.h and static_assert.
+   Copyright 2022 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Paul Eggert.  */
+
+#include <config.h>
+
+#define STATIC_ASSERT_TESTS \
+  static_assert (2 + 2 == 4, "arithmetic does not work"); \
+  static_assert (2 + 2 == 4); \
+  static_assert (sizeof (char) == 1, "sizeof does not work"); \
+  static_assert (sizeof (char) == 1)
+
+STATIC_ASSERT_TESTS;
+
+static char const *
+assert (char const *p, int i)
+{
+  return p + i;
+}
+
+static char const *
+f (char const *p)
+{
+  return assert (p, 0);
+}
+
+#include <assert.h>
+
+STATIC_ASSERT_TESTS;
+
+static int
+g (void)
+{
+  assert (f ("this should work"));
+  return 0;
+}
+
+#define NDEBUG 1
+#include <assert.h>
+
+STATIC_ASSERT_TESTS;
+
+static int
+h (void)
+{
+  assert (f ("this should work"));
+  return 0;
+}
+
+int
+main (void)
+{
+  STATIC_ASSERT_TESTS;
+  g ();
+  h ();
+  return 0;
+}