Warn on type punning that toggles scalar storage order

As documented in the manual, we do not support type punning that toggles
the scalar storage order, so this adds a warning for the case of unions.

gcc/c/
	PR c/100653
	* c-decl.c (finish_struct): Warn for a union containing an aggregate
	field with a differing scalar storage order.
gcc/testsuite/
	* gcc.dg/sso-13.c: New test.

1 files changed, 24 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.dg/sso-13.c b/gcc/testsuite/gcc.dg/sso-13.c
new file mode 100644
index 0000000..ddfde00
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/sso-13.c
@@ -0,0 +1,24 @@
+/* Test support of scalar_storage_order attribute */
+
+/* { dg-do compile } */
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define REV_ENDIANNESS __attribute__((scalar_storage_order("big-endian")))
+#else
+#define REV_ENDIANNESS __attribute__((scalar_storage_order("little-endian")))
+#endif
+
+typedef struct tIp6Addr
+{
+    unsigned int s6_addr32[4];
+} tIp6Addr;
+
+struct _tBeTimNetAddr
+{
+    unsigned char isIPv4;
+    union
+    {
+        unsigned int addr;
+        tIp6Addr addr6;   /* { dg-warning "type punning toggles" } */
+    } REV_ENDIANNESS u;
+} REV_ENDIANNESS;