vlib/v/checker/tests/option_selector_fn_unwrap_err.vv:7:17: error: to propagate the call, `callback` must return an Option type
    5 | 
    6 | fn callback(foo &Foo) bool {
    7 |     return foo.func? == callback
      |                    ^
    8 | }
    9 |
Details: vlib/v/checker/tests/option_selector_fn_unwrap_err.vv:6:23: details: prepend ? before the declaration of the return type of `callback`
    4 | }
    5 | 
    6 | fn callback(foo &Foo) bool {
      |                       ~~~~
    7 |     return foo.func? == callback
    8 | }