GCC supports -> in offsetof when forming a member designator

This is undocumented, but available as of GCC > 4.4:
gcc-mirror/gcc@f0d0d84

Author: tautschnig

regression/ansi-c/gcc_builtins6/main.c

@@ -0,0 +1,23 @@
+#include <stddef.h>
+
+struct S
+{
+  int x;
+  union {
+    int y;
+    struct S2
+    {
+      int z;
+    } s[1];
+  } u[2];
+};
+
+int main()
+{
+  int A[offsetof(struct S, u[0].y)==sizeof(int)?1:-1];
+#if defined(__GNUC__) && !defined(__clang__)
+  int B[offsetof(struct S, u->y)==sizeof(int)?1:-1];
+  int C[offsetof(struct S, u->s[0].z)==sizeof(int)?1:-1];
+#endif
+  return 0;
+}

regression/ansi-c/gcc_builtins6/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/parser.y

@@ -22,6 +22,8 @@ extern char *yyansi_ctext;
 
 #include "parser_static.inc"
 
+#include "literals/convert_integer_literal.h"
+
 #include "ansi_c_y.tab.h"
 
 // statements have right recursion, deep nesting of statements thus
@@ -425,6 +427,17 @@ offsetof_member_designator:
           mto($2, $3);
           mto($$, $2);
         }
+        | offsetof_member_designator TOK_ARROW member_name
+        {
+          $$=$1;
+          set($2, ID_index);
+          exprt tmp=convert_integer_literal("0");
+          stack($2).move_to_operands(tmp);
+          mto($$, $2);
+          set($2, ID_member);
+          stack($2).set(ID_component_name, stack($3).get(ID_C_base_name));
+          mto($$, $2);
+        }
         ;
 
 quantifier_expression: