stddef-h tests: Add more C++ tests

[Bruno Haible via Gnulib discussion list]

This patch makes it clear what one can expect from the <cstddef> header
in C++ mode.


2025-05-25  Bruno Haible  <br...@clisp.org>

        stddef-h tests: Add more C++ tests.
        * tests/test-stddef-h-c++2.cc: Test the expected contents of <cstddef>.

diff --git a/tests/test-stddef-h-c++2.cc b/tests/test-stddef-h-c++2.cc
index d2588e1227..e207976d13 100644
--- a/tests/test-stddef-h-c++2.cc
+++ b/tests/test-stddef-h-c++2.cc
@@ -18,3 +18,31 @@
 #include <config.h>
 
 #include <cstddef>
+
+/* Test the expected contents of <cstddef>, cf.
+   <https://en.cppreference.com/w/cpp/header/cstddef>.  */
+
+/* Check that appropriate types are defined.  */
+ptrdiff_t b = 1;
+size_t c = 2;
+
+#if !defined __cplusplus || defined __GNUC__ || defined __clang__
+/* Check that NULL can be passed through varargs as a pointer type,
+   per POSIX 2008.  */
+static_assert (sizeof NULL == sizeof (void *));
+#endif
+
+/* Check that offsetof produces integer constants with correct type.  */
+struct d
+{
+  char e;
+  char f;
+};
+static_assert (sizeof (offsetof (struct d, e)) == sizeof (size_t));
+static_assert (offsetof (struct d, f) == 1);
+
+#include <limits.h> /* INT_MAX */
+
+/* offsetof promotes to an unsigned integer if and only if sizes do
+   not fit in int.  */
+static_assert ((offsetof (struct d, e) < -1) == (INT_MAX < (size_t) -1));