From 30b8d0091c9b9e571f3324e128ba32567c2fdbf5 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 14 Apr 2026 12:45:23 +0300
Subject: [PATCH] checker: Unknown type for json.decode with nested generics
 (fixes #22220)

---
 .../tests/json_decode_with_generic_test.v     | 25 +++++++++++++++++++
 vlib/v/checker/fn.v                           |  4 ++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/vlib/json/tests/json_decode_with_generic_test.v b/vlib/json/tests/json_decode_with_generic_test.v
index 19ac5378c..efde9c2af 100644
--- a/vlib/json/tests/json_decode_with_generic_test.v
+++ b/vlib/json/tests/json_decode_with_generic_test.v
@@ -10,15 +10,40 @@ struct User {
 	username string
 }
 
+struct Box[T] {
+	items []T
+}
+
 fn func[T]() !T {
 	text := '{"ok": true, "result":{"id":37467243, "username": "ciao"}}'
 	a := json.decode(Result[T], text)!
 	return a.result
 }
 
+fn string_decode[T](text string) !T {
+	return json.decode(T, text)
+}
+
+fn open_box[T](text string) ![]T {
+	box := string_decode[Box[T]](text)!
+	return box.items
+}
+
 fn test_decode_with_generic_struct() {
 	ret := func[User]()!
 	println(ret)
 	assert ret.id == 37467243
 	assert ret.username == 'ciao'
 }
+
+fn test_decode_with_nested_generic_struct() {
+	text := '{"items":[{},{"id":37467243,"username":"ciao"},{}]}'
+	items := open_box[User](text)!
+	assert items.len == 3
+	assert items[0].id == 0
+	assert items[0].username == ''
+	assert items[1].id == 37467243
+	assert items[1].username == 'ciao'
+	assert items[2].id == 0
+	assert items[2].username == ''
+}
diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index e6afbce17..8d5f1b647 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -1302,7 +1302,9 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 	mut concrete_types := []ast.Type{}
 	node.concrete_types = node.raw_concrete_types
 	for concrete_type in node.concrete_types {
-		if concrete_type.has_flag(.generic) {
+		if concrete_type.has_flag(.generic)
+			|| (c.type_has_unresolved_generic_parts(concrete_type)
+			&& c.table.sym(concrete_type).kind != .placeholder) {
 			has_generic = true
 			concrete_types << c.unwrap_generic(concrete_type)
 		} else {
-- 
2.39.5