From cb11cdf4f40a9c78826161aa30c016af7fedcb1f Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Thu, 23 Apr 2026 23:35:42 +0300
Subject: [PATCH] checker: fix cgen error when using `sdl.Window` as non-ref
 type for struct field (fixes #23042)

---
 vlib/v/checker/struct.v                       | 23 +++++++++++++++++++
 .../opaque_c_typedef_struct_field_err.out     |  7 ++++++
 .../opaque_c_typedef_struct_field_err.vv      | 10 ++++++++
 3 files changed, 40 insertions(+)
 create mode 100644 vlib/v/checker/tests/opaque_c_typedef_struct_field_err.out
 create mode 100644 vlib/v/checker/tests/opaque_c_typedef_struct_field_err.vv

diff --git a/vlib/v/checker/struct.v b/vlib/v/checker/struct.v
index e6a49850c..b10acfa14 100644
--- a/vlib/v/checker/struct.v
+++ b/vlib/v/checker/struct.v
@@ -10,6 +10,21 @@ fn (c &Checker) can_be_embedded_in_struct(typ ast.Type) bool {
 	return c.table.final_sym(typ).kind in [.struct, .function]
 }
 
+fn (c &Checker) is_opaque_c_typedef_struct_alias(typ ast.Type) bool {
+	sym := c.table.sym(typ)
+	if sym.kind != .alias {
+		return false
+	}
+	final_sym := c.table.final_sym(typ)
+	if final_sym.language != .c || final_sym.kind != .struct {
+		return false
+	}
+	if final_sym.info is ast.Struct {
+		return final_sym.info.is_typedef && final_sym.info.is_empty_struct()
+	}
+	return false
+}
+
 fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 	util.timing_start(@METHOD)
 	defer {
@@ -195,6 +210,14 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 			if !c.ensure_type_exists(field.typ, field.type_pos) {
 				continue
 			}
+			if node.language == .v && !field.typ.is_ptr()
+				&& c.is_opaque_c_typedef_struct_alias(field.typ) {
+				field_typ := c.table.type_to_str(field.typ)
+				ref_typ := c.table.type_to_str(field.typ.clear_option_and_result().set_nr_muls(1))
+				c.error('cannot use opaque C struct `${field_typ}` as a non-reference struct field; use `${ref_typ}` instead',
+					field.type_pos)
+				continue
+			}
 			// gotodef for struct field types
 			if c.pref.is_vls && c.pref.linfo.method == .definition {
 				if c.vls_is_the_node(field.type_pos) {
diff --git a/vlib/v/checker/tests/opaque_c_typedef_struct_field_err.out b/vlib/v/checker/tests/opaque_c_typedef_struct_field_err.out
new file mode 100644
index 000000000..cd1997fb9
--- /dev/null
+++ b/vlib/v/checker/tests/opaque_c_typedef_struct_field_err.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/opaque_c_typedef_struct_field_err.vv:7:6: error: cannot use opaque C struct `Foo` as a non-reference struct field; use `&Foo` instead
+    5 | 
+    6 | struct Bar {
+    7 |     foo Foo
+      |         ~~~
+    8 | }
+    9 | 
diff --git a/vlib/v/checker/tests/opaque_c_typedef_struct_field_err.vv b/vlib/v/checker/tests/opaque_c_typedef_struct_field_err.vv
new file mode 100644
index 000000000..db92843fe
--- /dev/null
+++ b/vlib/v/checker/tests/opaque_c_typedef_struct_field_err.vv
@@ -0,0 +1,10 @@
+@[typedef]
+struct C.Foo {}
+
+type Foo = C.Foo
+
+struct Bar {
+	foo Foo
+}
+
+fn main() {}
-- 
2.39.5