checker, cgen: fix for in iterator of generic struct (#12441)

vlib/v/checker/checker.v

@@ -4816,7 +4816,7 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 		sym := c.table.get_final_type_symbol(typ)
 		if sym.kind == .struct_ {
 			// iterators
-			next_fn := sym.find_method('next') or {
+			next_fn := sym.find_method_with_generic_parent('next') or {
 				c.error('a struct must have a `next()` method to be an iterator', node.cond.position())
 				return
 			}

vlib/v/gen/c/cgen.v

@@ -2145,7 +2145,7 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 		}
 	} else if node.kind == .struct_ {
 		cond_type_sym := g.table.get_type_symbol(node.cond_type)
-		next_fn := cond_type_sym.find_method('next') or {
+		next_fn := cond_type_sym.find_method_with_generic_parent('next') or {
 			verror('`next` method not found')
 			return
 		}
@@ -2162,7 +2162,14 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 		t_var := g.new_tmp_var()
 		receiver_typ := next_fn.params[0].typ
 		receiver_styp := g.typ(receiver_typ)
-		fn_name := receiver_styp.replace_each(['*', '', '.', '__']) + '_next'
+		mut fn_name := receiver_styp.replace_each(['*', '', '.', '__']) + '_next'
+		receiver_sym := g.table.get_type_symbol(receiver_typ)
+		if receiver_sym.info is ast.Struct {
+			if receiver_sym.info.concrete_types.len > 0 {
+				fn_name = g.generic_fn_name(receiver_sym.info.concrete_types, fn_name,
+					false)
+			}
+		}
 		g.write('\t${g.typ(ret_typ)} $t_var = ${fn_name}(')
 		if !node.cond_type.is_ptr() && receiver_typ.is_ptr() {
 			g.write('&')

vlib/v/tests/for_in_iterator_of_generic_struct_test.v

@@ -0,0 +1,39 @@
+struct Split<T> {
+	arr  []T
+	pred fn (T) bool
+mut:
+	idx int
+}
+
+fn (mut iter Split<T>) next() ?[]T {
+	start := iter.idx
+	for iter.idx < iter.arr.len {
+		if iter.pred(iter.arr[iter.idx]) {
+			iter.idx++
+			return iter.arr[start..iter.idx]
+		}
+		iter.idx++
+	}
+	return none
+}
+
+fn split<T>(arr []T, pred fn (T) bool) Split<T> {
+	return Split<T>{arr, pred, 0}
+}
+
+fn test_for_in_iterator_of_generic_struct() {
+	items := [0, 1, 2, 3, 4, 5, 6, 7, 8]
+	mut iter := split<int>(items, fn (item int) bool {
+		return item % 3 == 0
+	})
+	iter.next() or {}
+	mut ret_list := [][]int{}
+	for item in iter {
+		dump(item)
+		ret_list << item
+	}
+	println(ret_list)
+	assert ret_list.len == 2
+	assert ret_list[0] == [1, 2, 3]
+	assert ret_list[1] == [4, 5, 6]
+}