checker: fix for iterator method .next(), not marked as used (fix v…

…lang#23312) (vlang#23321)

vlib/v/checker/for.v

@@ -173,6 +173,8 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 			unwrapped_typ := c.unwrap_generic(typ)
 			unwrapped_sym := c.table.sym(unwrapped_typ)
 
+			c.table.used_features.comptime_calls['${int(unwrapped_typ)}.next'] = true
+
 			if node.key_var.len > 0 {
 				key_type := match unwrapped_sym.kind {
 					.map { unwrapped_sym.map_info().key_type }

vlib/v/tests/skip_unused/generic_iterator_loop.vv

@@ -0,0 +1,23 @@
+module main
+
+struct Foo {}
+
+fn (f Foo) next() ?Foo {
+	return none
+}
+
+struct Bar {}
+
+fn (f Bar) next() ?Bar {
+	return none
+}
+
+fn loop[T](iter T) {
+	for _ in iter {
+	}
+}
+
+fn main() {
+	loop(Foo{})
+	loop(Bar{})
+}