From 593fdc5a9f00b13b76cc81a0f2d8fc71c13abf96 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 14 Apr 2026 12:45:31 +0300
Subject: [PATCH] checker: cgen: passing interface to generic function result
 in C error. (fixes #18320)

---
 .../generic_interface_callback_arg_err.out    |  7 +++++
 .../generic_interface_callback_arg_err.vv     | 26 +++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 vlib/v/checker/tests/generic_interface_callback_arg_err.out
 create mode 100644 vlib/v/checker/tests/generic_interface_callback_arg_err.vv

diff --git a/vlib/v/checker/tests/generic_interface_callback_arg_err.out b/vlib/v/checker/tests/generic_interface_callback_arg_err.out
new file mode 100644
index 000000000..76d47f993
--- /dev/null
+++ b/vlib/v/checker/tests/generic_interface_callback_arg_err.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generic_interface_callback_arg_err.vv:14:6: error: cannot use `BaseInterface` as `CustomStruct` in argument 1 to `cb`
+   12 | fn callback[T](ctx BaseInterface, cb fn (T)) {
+   13 |     $if T is BaseInterface {
+   14 |         cb(ctx)
+      |            ~~~
+   15 |     }
+   16 | }
diff --git a/vlib/v/checker/tests/generic_interface_callback_arg_err.vv b/vlib/v/checker/tests/generic_interface_callback_arg_err.vv
new file mode 100644
index 000000000..e240065c3
--- /dev/null
+++ b/vlib/v/checker/tests/generic_interface_callback_arg_err.vv
@@ -0,0 +1,26 @@
+// Regression test for issue #18320.
+struct CustomStruct {
+pub:
+	a int
+	b string
+}
+
+interface BaseInterface {
+	a int
+}
+
+fn callback[T](ctx BaseInterface, cb fn (T)) {
+	$if T is BaseInterface {
+		cb(ctx)
+	}
+}
+
+fn main() {
+	custom := CustomStruct{
+		a: 5
+		b: 'test'
+	}
+	callback(custom, fn (ctx CustomStruct) {
+		println(ctx)
+	})
+}
-- 
2.39.5