From 1e86b653d34c41c51a4c2edfc6b3ad19a8bcc4bc Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 06:29:47 +0300
Subject: [PATCH] cgen: fix v-ui binding issues with sumtype ptr cast payload
 (fixes #22886)

---
 vlib/v/gen/c/cgen.v                           | 13 ++++++++-
 .../sumtype_ptr_cast_uses_payload.c.must_have |  1 +
 .../testdata/sumtype_ptr_cast_uses_payload.vv | 28 +++++++++++++++++++
 3 files changed, 41 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/gen/c/testdata/sumtype_ptr_cast_uses_payload.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/sumtype_ptr_cast_uses_payload.vv

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 4d12ffd64..a0b22dd23 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -8611,6 +8611,9 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 			g.write(')')
 			return
 		}
+		use_dynamic_ptr_payload := node_typ.is_ptr() && expr_type.is_ptr()
+			&& final_expr_sym.kind == .sum_type
+			&& final_sym.kind !in [.sum_type, .interface] && final_sym.language == .c
 		if (g.pref.translated || g.file.is_translated) && sym.kind == .function {
 			// TODO: handle the type in fn casts, not just exprs
 			/*
@@ -8710,7 +8713,15 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 					expr_styp := g.styp(node.expr_type)
 					g.write('(${expr_styp})')
 				}
-				g.expr(node.expr)
+				if use_dynamic_ptr_payload {
+					// C pointer casts from sum types/interfaces must target the stored payload pointer,
+					// not the wrapper address itself.
+					g.write('(*((void**)(')
+					g.expr(node.expr)
+					g.write(')))')
+				} else {
+					g.expr(node.expr)
+				}
 				g.inside_assign_fn_var = old_inside_assign_fn_var
 				g.inside_cast = old_inside_cast
 				g.write('))')
diff --git a/vlib/v/gen/c/testdata/sumtype_ptr_cast_uses_payload.c.must_have b/vlib/v/gen/c/testdata/sumtype_ptr_cast_uses_payload.c.must_have
new file mode 100644
index 000000000..025d08faa
--- /dev/null
+++ b/vlib/v/gen/c/testdata/sumtype_ptr_cast_uses_payload.c.must_have
@@ -0,0 +1 @@
+uiConsume(((uiControl*)((*((void**)(c))))));
diff --git a/vlib/v/gen/c/testdata/sumtype_ptr_cast_uses_payload.vv b/vlib/v/gen/c/testdata/sumtype_ptr_cast_uses_payload.vv
new file mode 100644
index 000000000..677bd209e
--- /dev/null
+++ b/vlib/v/gen/c/testdata/sumtype_ptr_cast_uses_payload.vv
@@ -0,0 +1,28 @@
+module main
+
+@[typedef]
+struct C.uiControl {}
+
+@[typedef]
+struct C.uiLabel {}
+
+@[typedef]
+struct C.uiBox {}
+
+fn C.uiNewBox() &C.uiBox
+
+fn C.uiConsume(x &C.uiControl)
+
+type Control = C.uiLabel | C.uiBox
+
+@[unsafe]
+fn use_control(c &Control) {
+	C.uiConsume(&C.uiControl(c))
+}
+
+fn main() {
+	box := C.uiNewBox()
+	unsafe {
+		use_control(box)
+	}
+}
-- 
2.39.5