From 34ecea37ceabcbdd214a0413b6491cd39efdb2e6 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Fri, 29 May 2026 04:37:34 +0300
Subject: [PATCH] markused: keep structs used by __offsetof

---
 vlib/v/markused/walker.v                      |  4 ++-
 vlib/v/tests/pointers/offsetof_test.v         | 20 +++++++++++
 .../generic_fn_instantiation_pruning_test.v   | 33 +++++++++++++++++++
 3 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/vlib/v/markused/walker.v b/vlib/v/markused/walker.v
index 64a16746e..d1ade5980 100644
--- a/vlib/v/markused/walker.v
+++ b/vlib/v/markused/walker.v
@@ -1518,7 +1518,9 @@ fn (mut w Walker) expr(node_ ast.Expr) {
 			w.uses_lock = true
 			w.stmts(node.stmts)
 		}
-		ast.OffsetOf {}
+		ast.OffsetOf {
+			w.mark_by_type(w.resolve_current_specialized_type(node.struct_type))
+		}
 		ast.OrExpr {
 			w.or_block(node)
 		}
diff --git a/vlib/v/tests/pointers/offsetof_test.v b/vlib/v/tests/pointers/offsetof_test.v
index 3d9d66349..1a317665f 100644
--- a/vlib/v/tests/pointers/offsetof_test.v
+++ b/vlib/v/tests/pointers/offsetof_test.v
@@ -6,8 +6,20 @@ struct Cat {
 	age   int
 }
 
+struct OffsetInlineOnly {
+	a int
+	b int
+}
+
+struct OffsetConstOnly {
+	a int
+	b int
+}
+
 type Feline = Cat
 
+const offset_const_only_b = __offsetof(OffsetConstOnly, b)
+
 fn test_offsetof() {
 	cat := Cat{
 		name:  'Cthulhu'
@@ -21,6 +33,14 @@ fn test_offsetof() {
 	}
 }
 
+fn test_offsetof_inline_only_marks_struct_as_used() {
+	assert __offsetof(OffsetInlineOnly, b) == 4
+}
+
+fn test_offsetof_const_marks_struct_as_used() {
+	assert offset_const_only_b == 4
+}
+
 fn test_offsetof_struct_from_another_module() {
 	num := complex.Complex{1.0, 1.0}
 	unsafe {
diff --git a/vlib/v/tests/skip_unused/generic_fn_instantiation_pruning_test.v b/vlib/v/tests/skip_unused/generic_fn_instantiation_pruning_test.v
index 976fee43f..545787ced 100644
--- a/vlib/v/tests/skip_unused/generic_fn_instantiation_pruning_test.v
+++ b/vlib/v/tests/skip_unused/generic_fn_instantiation_pruning_test.v
@@ -36,6 +36,39 @@ fn test_skip_unused_prunes_unused_generic_fn_instantiations() {
 	assert !res.output.contains('VV_LOC Map_u8_int main__new_T_int(void)')
 }
 
+fn test_skip_unused_keeps_generic_offsetof_struct_instantiations() {
+	tmp_dir := os.join_path(os.vtmp_dir(), 'v_issue_27283_generic_offsetof')
+	os.mkdir_all(tmp_dir) or { panic(err) }
+	defer {
+		os.rmdir_all(tmp_dir) or {}
+	}
+	source_path := os.join_path(tmp_dir, 'issue_27283_generic_offsetof.v')
+	binary_path := os.join_path(tmp_dir, 'issue_27283_generic_offsetof')
+	source := [
+		'module main',
+		'',
+		'struct Box[T] {',
+		'\ta int',
+		'\tb T',
+		'}',
+		'',
+		'fn off[T]() u32 {',
+		'\treturn __offsetof(Box[T], b)',
+		'}',
+		'',
+		'fn main() {',
+		'\tprintln(off[int]())',
+		'\tprintln(off[string]())',
+		'}',
+	].join('\n')
+	os.write_file(source_path, source) or { panic(err) }
+	res :=
+		os.execute('${os.quoted_path(vexe)} -skip-unused -o ${os.quoted_path(binary_path)} ${os.quoted_path(source_path)}')
+	if res.exit_code != 0 {
+		panic(res.output)
+	}
+}
+
 fn test_skip_unused_does_not_emit_impl_methods_for_interface_extensions() {
 	tmp_dir := os.join_path(os.vtmp_dir(), 'v_skip_unused_interface_extension_collision')
 	os.mkdir_all(tmp_dir) or { panic(err) }
-- 
2.39.5