checker: disallow function implementing interface (fix #24846) (#25389)

Author: dy-tea

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

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 | }

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)
+}