From 6762c9a68958d1367c86d541969b57fe464eca36 Mon Sep 17 00:00:00 2001
From: CreeperFace <165158232+dy-tea@users.noreply.github.com>
Date: Fri, 26 Sep 2025 06:59:40 +0100
Subject: [PATCH] checker: disallow function implementing interface (fix
 #24846) (#25389)

---
 vlib/v/checker/checker.v                                   | 4 ++++
 vlib/v/checker/tests/interface_implemented_by_function.out | 6 ++++++
 vlib/v/checker/tests/interface_implemented_by_function.vv  | 7 +++++++
 3 files changed, 17 insertions(+)
 create mode 100644 vlib/v/checker/tests/interface_implemented_by_function.out
 create mode 100644 vlib/v/checker/tests/interface_implemented_by_function.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 151fe02c5..3b7377fd6 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -1179,6 +1179,10 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 	if typ_sym.name == 'JS.Any' {
 		return true
 	}
+	if typ_sym.kind == .function && inter_sym.name != 'JS.Any' {
+		c.error('cannot implement interface `${inter_sym.name}` using function', pos)
+		return false
+	}
 	if mut inter_sym.info is ast.Interface {
 		mut generic_type := interface_type
 		mut generic_info := inter_sym.info
diff --git a/vlib/v/checker/tests/interface_implemented_by_function.out b/vlib/v/checker/tests/interface_implemented_by_function.out
new file mode 100644
index 000000000..268d489fc
--- /dev/null
+++ b/vlib/v/checker/tests/interface_implemented_by_function.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/interface_implemented_by_function.vv:6:6: error: cannot implement interface `Ch` using function
+    4 |
+    5 | fn main() {
+    6 |   id(id)
+      |      ~~
+    7 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/interface_implemented_by_function.vv b/vlib/v/checker/tests/interface_implemented_by_function.vv
new file mode 100644
index 000000000..f9e6bfa31
--- /dev/null
+++ b/vlib/v/checker/tests/interface_implemented_by_function.vv
@@ -0,0 +1,7 @@
+interface Ch {}
+
+fn id(x Ch) Ch { return x }
+
+fn main() {
+  id(id)
+}
-- 
2.39.5