From 7ba8a506a215846d14d71487807f5e525e9e5c16 Mon Sep 17 00:00:00 2001
From: nsfisis <nsfisis@gmail.com>
Date: Sat, 7 Feb 2026 19:02:38 +0900
Subject: feat: allow calling offsetof() in compile-time expression

---
 include/stddef.h |  2 +-
 src/parse.c      |  4 ++++
 tests/stddef.c   | 23 +++++++++++++++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 tests/stddef.c

diff --git a/include/stddef.h b/include/stddef.h
index ffcf225..2689311 100644
--- a/include/stddef.h
+++ b/include/stddef.h
@@ -7,7 +7,7 @@ typedef unsigned long size_t;
 typedef int wchar_t;
 typedef long ptrdiff_t;
 
-#define offsetof(T, m) (ptrdiff_t)((void*)((T*)0)->m)
+#define offsetof(T, m) ((ptrdiff_t)(void*)(&((T*)0)->m))
 
 // TODO:
 // - max_align_t (C11)
diff --git a/src/parse.c b/src/parse.c
index 55cac2b..3f55a51 100644
--- a/src/parse.c
+++ b/src/parse.c
@@ -2733,6 +2733,10 @@ static int eval(AstNode* e) {
         }
     } else if (e->kind == AstNodeKind_cast_expr) {
         return eval(e->as.cast_expr->operand);
+    } else if (e->kind == AstNodeKind_deref_expr) {
+        return eval(e->as.deref_expr->operand);
+    } else if (e->kind == AstNodeKind_ref_expr) {
+        return eval(e->as.ref_expr->operand);
     } else {
         unimplemented();
     }
diff --git a/tests/stddef.c b/tests/stddef.c
new file mode 100644
index 0000000..f63e57c
--- /dev/null
+++ b/tests/stddef.c
@@ -0,0 +1,23 @@
+#include <helpers.h>
+#include <stddef.h>
+
+struct S {
+    int x;
+    int y;
+};
+struct T {
+    char a;
+    int b;
+    char c;
+};
+
+int main() {
+    ASSERT_EQ(0, offsetof(struct S, x));
+    ASSERT_EQ(4, offsetof(struct S, y));
+    ASSERT_EQ(0, offsetof(struct T, a));
+    ASSERT_EQ(4, offsetof(struct T, b));
+    ASSERT_EQ(8, offsetof(struct T, c));
+
+    char buf[offsetof(struct T, b)];
+    ASSERT_EQ(4, sizeof(buf));
+}
-- 
cgit v1.3-1-g0d28