vlib/v/parser/tests/for_in_mut_key_of_map.vv:3:6: error: index of array or key of map cannot be mutated
    1 | fn main() {
    2 |     mut m := {'foo': 1, 'bar': 2}
    3 |     for mut k, _ in m {
      |         ~~~
    4 |         println(k)
    5 |     }
