std-aux: rework NM_STATIC_ASSERT_EXPR() macro

Replace NM_STATIC_ASSERT_EXPR() by NM_STATIC_ASSERT_EXPR_1() and
NM_STATIC_ASSERT_EXPR_VOID(). NM_STATIC_ASSERT_EXPR_VOID() can be
used as an expression that returns void (that is, a simple statement).
On the other hand, NM_STATIC_ASSERT_EXPR_1() itself retuns
a compile time constant of int value 1. The latter is useful, because
we can use the 1 to combine static assertions in expressions that
are themself compile time constants, like

  #define STATIC_CHECK_AND_GET(cond, value) \
      (NM_STATIC_ASSERT_EXPR_1(cond) ? (value) : (value))

This is itself a compile time constant if value is a compile
time constant. Also, it does the compile time check that "cond"
is true.

2 files changed, 30 insertions, 5 deletions

diff --git a/src/libnm-glib-aux/tests/test-shared-general.c b/src/libnm-glib-aux/tests/test-shared-general.c
index 6c7ab4882f..593b8c42a1 100644
--- a/src/libnm-glib-aux/tests/test-shared-general.c
+++ b/src/libnm-glib-aux/tests/test-shared-general.c
@@ -31,6 +31,31 @@ G_STATIC_ASSERT(4 == _nm_alignof(NMIPAddr));
 /*****************************************************************************/
 
 static void
+test_nm_static_assert(void)
+{
+    int                                v1[NM_STATIC_ASSERT_EXPR_1(1)];
+    typeof(NM_STATIC_ASSERT_EXPR_1(1)) v_int;
+    int *                              p_int;
+
+    G_STATIC_ASSERT(sizeof(v1) == sizeof(int));
+    G_STATIC_ASSERT(NM_STATIC_ASSERT_EXPR_1(1) == 1);
+    G_STATIC_ASSERT(NM_STATIC_ASSERT_EXPR_1(NM_STATIC_ASSERT_EXPR_1(1)) == 1);
+    G_STATIC_ASSERT(NM_STATIC_ASSERT_EXPR_1(NM_STATIC_ASSERT_EXPR_1(NM_STATIC_ASSERT_EXPR_1(1)))
+                    == 1);
+
+    g_assert(NM_STATIC_ASSERT_EXPR_1(2) == 1);
+
+    p_int = &v_int;
+    g_assert(&v_int == p_int);
+
+    (void) NM_STATIC_ASSERT_EXPR_1(2 > 1);
+
+    NM_STATIC_ASSERT_EXPR_VOID(2 > 1);
+}
+
+/*****************************************************************************/
+
+static void
 test_gpid(void)
 {
     const int *int_ptr;
@@ -1352,6 +1377,7 @@ main(int argc, char **argv)
 {
     nmtst_init(&argc, &argv, TRUE);
 
+    g_test_add_func("/general/test_nm_static_assert", test_nm_static_assert);
     g_test_add_func("/general/test_gpid", test_gpid);
     g_test_add_func("/general/test_monotonic_timestamp", test_monotonic_timestamp);
     g_test_add_func("/general/test_nmhash", test_nmhash);
diff --git a/src/libnm-std-aux/nm-std-aux.h b/src/libnm-std-aux/nm-std-aux.h
index 1e341157fc..49ec8aab0c 100644
--- a/src/libnm-std-aux/nm-std-aux.h
+++ b/src/libnm-std-aux/nm-std-aux.h
@@ -182,11 +182,10 @@ typedef uint64_t _nm_bitwise nm_be64_t;
     })
 
 #define NM_STATIC_ASSERT(cond) static_assert(cond, "")
-#define NM_STATIC_ASSERT_EXPR(cond) \
-    ({                              \
-        NM_STATIC_ASSERT(cond);     \
-        1;                          \
-    })
+#define NM_STATIC_ASSERT_EXPR_1(cond) \
+    (sizeof(struct { char __static_assert_expr_1[(cond) ? 1 : -1]; }) == 1)
+#define NM_STATIC_ASSERT_EXPR_VOID(cond) \
+    ((void) (sizeof(struct { char __static_assert_expr_void[(cond) ? 1 : -1]; }) == 1))
 
 /*****************************************************************************/