ast: fix for in iterator of generic struct (fix #13579) (#13585)

vlib/v/ast/types.v

@@ -1266,10 +1266,16 @@ pub fn (t &TypeSymbol) find_method_with_generic_parent(name string) ?Fn {
 						Struct, Interface, SumType {
 							mut method := x
 							generic_names := parent_sym.info.generic_types.map(table.sym(it).name)
-							if rt := table.resolve_generic_to_concrete(method.return_type,
-								generic_names, t.info.concrete_types)
-							{
-								method.return_type = rt
+							return_sym := table.sym(method.return_type)
+							if return_sym.kind == .struct_ {
+								method.return_type = table.unwrap_generic_type(method.return_type,
+									generic_names, t.info.concrete_types)
+							} else {
+								if rt := table.resolve_generic_to_concrete(method.return_type,
+									generic_names, t.info.concrete_types)
+								{
+									method.return_type = rt
+								}
 							}
 							method.params = method.params.clone()
 							for mut param in method.params {

vlib/v/tests/for_in_iterator_of_generic_struct_2_test.v

@@ -0,0 +1,35 @@
+pub struct Item<V> {
+pub:
+	value V
+}
+
+pub struct Iter<V> {
+	arr []V
+mut:
+	ix int
+}
+
+pub fn (mut iter Iter<V>) next() ?Item<V> {
+	if iter.ix >= iter.arr.len {
+		return none
+	}
+
+	val := iter.arr[iter.ix]
+	iter.ix += 1
+	return Item<V>{val}
+}
+
+fn iterator<V>(arr []V) Iter<V> {
+	return Iter<V>{arr, 0}
+}
+
+fn test_for_in_iterator_of_generic_struct() {
+	mut ret := []int{}
+	mut x := iterator<int>([1, 2, 3, 4, 5])
+	for item in x {
+		println(item.value)
+		ret << item.value
+	}
+	println(ret)
+	assert ret == [1, 2, 3, 4, 5]
+}