checker: fix json decode with generic array of struct (fix #18300) (#18308)

vlib/json/json_decode_with_generic_array_test.v

@@ -0,0 +1,44 @@
+import json
+
+struct Response[T] {
+	time string
+	cars []T
+}
+
+struct Car {
+	brand string
+	power int
+}
+
+fn string_to_response[T](text string) {
+	result := json.decode([]Response[T], text) or { panic(err) }
+	println(result)
+	assert result.len == 2
+	assert result[0].time == '12ms'
+	assert result[0].cars[0].brand == 'Ferrari'
+	assert result[0].cars[0].power == 232
+	assert result[0].cars[1].brand == 'Lamborghini'
+	assert result[0].cars[1].power == 345
+	assert result[1].time == '12ms'
+	assert result[1].cars[0].brand == 'Ferrari'
+	assert result[1].cars[0].power == 232
+	assert result[1].cars[1].brand == 'Lamborghini'
+	assert result[1].cars[1].power == 345
+}
+
+fn test_json_decode_with_generic_array() {
+	text := '[
+{ "time":"12ms",
+  "cars":[
+	{"brand":"Ferrari","power":232},
+	{"brand":"Lamborghini", "power":345}
+  ]},
+{ "time":"12ms",
+  "cars":[
+	{"brand":"Ferrari","power":232},
+	{"brand":"Lamborghini", "power":345}
+  ]}
+]'
+
+	string_to_response[Car](text)
+}