json: fix codegen for struct default expr (fix #23216) (#23233)

vlib/json/tests/json_decode_struct_with_default_test.v

@@ -0,0 +1,34 @@
+import json
+
+pub struct Response {
+pub:
+	results []Result = []Result{len: 0} @[json: list]
+	tags    []string = []string{len: 0} @[json: tags]
+	kind    string   @[json: result_type]
+}
+
+pub struct Result {
+pub:
+	id            int      @[json: defid]
+	author        string   @[json: author]
+	definition    string   @[json: definition]
+	link          string   @[json: permalink]
+	thumbs_down   int      @[json: thumbs_down]
+	thumbs_up     int      @[json: thumbs_up]
+	word          string   @[json: word]
+	date          string   @[json: written_on]
+	audio_samples []string @[json: sound_urls]
+	example       string   @[json: example]
+}
+
+pub fn define(word string) !&Response {
+	resp := '{"list":[{"defid":3439287}]}'
+	response := json.decode(Response, resp) or { return err }
+	return &response
+}
+
+fn test_main() {
+	response := define('')!
+	assert response.results.len > 0
+	assert response.results[0].id == 3439287
+}