From bc582027e55e8f43058d6851460cd13939e6c73a Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Thu, 23 Apr 2026 18:15:37 +0300
Subject: [PATCH] cgen: fix "builder error / C error. This should never
 happen.", when calling C.xcb_setup_roots_iterator() from XCB library (fixes
 #16906)

---
 .gitignore                                    |  1 +
 .../headerless_c_struct_return.c.must_have    |  2 ++
 .../c/testdata/headerless_c_struct_return.out |  1 +
 .../c/testdata/headerless_c_struct_return.vv  | 35 +++++++++++++++++++
 .../headerless_c_struct_return_impl.c         | 31 ++++++++++++++++
 5 files changed, 70 insertions(+)
 create mode 100644 vlib/v/gen/c/testdata/headerless_c_struct_return.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/headerless_c_struct_return.out
 create mode 100644 vlib/v/gen/c/testdata/headerless_c_struct_return.vv
 create mode 100644 vlib/v/gen/c/testdata/headerless_c_struct_return_impl.c

diff --git a/.gitignore b/.gitignore
index 6b439b551..2e39368c1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@
 # unignore c codegen testdata files
 !vlib/v/gen/c/testdata/*.vv
 !vlib/v/gen/c/testdata/*.out
+!vlib/v/gen/c/testdata/*.c
 !vlib/v/gen/c/testdata/*.c.must_have
 
 # unignore vlib/x/markdown
diff --git a/vlib/v/gen/c/testdata/headerless_c_struct_return.c.must_have b/vlib/v/gen/c/testdata/headerless_c_struct_return.c.must_have
new file mode 100644
index 000000000..e0d41efee
--- /dev/null
+++ b/vlib/v/gen/c/testdata/headerless_c_struct_return.c.must_have
@@ -0,0 +1,2 @@
+extern main__ScreenIter setup_roots_iterator(main__Setup* r);
+main__ScreenIter iter = setup_roots_iterator(&setup);
diff --git a/vlib/v/gen/c/testdata/headerless_c_struct_return.out b/vlib/v/gen/c/testdata/headerless_c_struct_return.out
new file mode 100644
index 000000000..f432d7ff4
--- /dev/null
+++ b/vlib/v/gen/c/testdata/headerless_c_struct_return.out
@@ -0,0 +1 @@
+5|7|641
diff --git a/vlib/v/gen/c/testdata/headerless_c_struct_return.vv b/vlib/v/gen/c/testdata/headerless_c_struct_return.vv
new file mode 100644
index 000000000..17dc8d7e1
--- /dev/null
+++ b/vlib/v/gen/c/testdata/headerless_c_struct_return.vv
@@ -0,0 +1,35 @@
+module main
+
+#flag @VMODROOT/vlib/v/gen/c/testdata/headerless_c_struct_return_impl.c
+
+type Keycode = u8
+
+struct Setup {
+	status      u8
+	min_keycode Keycode
+	max_keycode Keycode
+	pad1        [4]u8
+}
+
+struct Screen {
+	width u16
+}
+
+struct ScreenIter {
+	data  &Screen
+	rem   int
+	index int
+}
+
+fn C.setup_roots_iterator(r &Setup) ScreenIter
+
+fn main() {
+	setup := Setup{
+		status:      3
+		min_keycode: 4
+		max_keycode: 9
+		pad1:        [u8(1), 2, 3, 4]!
+	}
+	iter := C.setup_roots_iterator(&setup)
+	println('${iter.rem}|${iter.index}|${iter.data.width}')
+}
diff --git a/vlib/v/gen/c/testdata/headerless_c_struct_return_impl.c b/vlib/v/gen/c/testdata/headerless_c_struct_return_impl.c
new file mode 100644
index 000000000..4ef4e1ab7
--- /dev/null
+++ b/vlib/v/gen/c/testdata/headerless_c_struct_return_impl.c
@@ -0,0 +1,31 @@
+typedef unsigned char u8;
+typedef unsigned short u16;
+typedef u8 Array_fixed_u8_4[4];
+
+typedef struct main__Setup {
+	u8 status;
+	u8 min_keycode;
+	u8 max_keycode;
+	Array_fixed_u8_4 pad1;
+} main__Setup;
+
+typedef struct main__Screen {
+	u16 width;
+} main__Screen;
+
+typedef struct main__ScreenIter {
+	main__Screen* data;
+	int rem;
+	int index;
+} main__ScreenIter;
+
+main__ScreenIter setup_roots_iterator(main__Setup* setup) {
+	static main__Screen screen;
+	screen.width = (u16)(640 + setup->pad1[0]);
+	main__ScreenIter iter = {
+		.data = &screen,
+		.rem = (int)(setup->max_keycode - setup->min_keycode),
+		.index = (int)(setup->status + setup->pad1[3]),
+	};
+	return iter;
+}
-- 
2.39.5