From 48e8099aacbfaac3fdf14d56f94e25c50dc1721e Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:23 +0300
Subject: [PATCH] checker: fix builder error when using function types from a
 module (fixes #18853)

---
 .../function_types_from_module_test.v         | 24 +++++++++++++++++++
 .../function_types_from_module/mymod/mymod.v  | 22 +++++++++++++++++
 2 files changed, 46 insertions(+)
 create mode 100644 vlib/v/tests/modules/function_types_from_module/function_types_from_module_test.v
 create mode 100644 vlib/v/tests/modules/function_types_from_module/mymod/mymod.v

diff --git a/vlib/v/tests/modules/function_types_from_module/function_types_from_module_test.v b/vlib/v/tests/modules/function_types_from_module/function_types_from_module_test.v
new file mode 100644
index 000000000..ad8232a26
--- /dev/null
+++ b/vlib/v/tests/modules/function_types_from_module/function_types_from_module_test.v
@@ -0,0 +1,24 @@
+module main
+
+import mymod
+
+struct RequestContext {}
+
+fn request_id() mymod.MiddlewareFn {
+	return fn (next mymod.HandlerFn) mymod.HandlerFn {
+		return fn [next] (c mymod.Context) Error {
+			return next(c)
+		}
+	}
+}
+
+fn test_function_types_imported_from_a_module_can_return_each_other() {
+	middleware := request_id()
+	mut app := mymod.new()
+	app.use(middleware)
+	handler := middleware(fn (_ mymod.Context) Error {
+		return Error{}
+	})
+	err := handler(RequestContext{})
+	assert err.code() == 0
+}
diff --git a/vlib/v/tests/modules/function_types_from_module/mymod/mymod.v b/vlib/v/tests/modules/function_types_from_module/mymod/mymod.v
new file mode 100644
index 000000000..06c665c78
--- /dev/null
+++ b/vlib/v/tests/modules/function_types_from_module/mymod/mymod.v
@@ -0,0 +1,22 @@
+module mymod
+
+pub interface Context {}
+
+pub type HandlerFn = fn (c Context) Error
+
+pub type MiddlewareFn = fn (next HandlerFn) HandlerFn
+
+pub struct App {
+mut:
+	middlewares []MiddlewareFn
+}
+
+// use appends middleware handlers in registration order.
+pub fn (mut app App) use(middleware ...MiddlewareFn) {
+	app.middlewares << middleware
+}
+
+// new returns an empty application.
+pub fn new() App {
+	return App{}
+}
-- 
2.39.5