$ v run bug.v
bug.v:2:16: warning: you should declare it as Option instead
    1 | 
    2 | type MapFunc = fn () 
      |                ~~~~~~
    3 | 
    4 | struct Test {
bug.v:5:4: warning: you should declare it as Option instead (?fn ...)
    3 | 
    4 | struct Test {
    5 |     a fn (int) // warn
      |       ~~~~~~~~
    6 |     b ?fn () // ok
    7 |     c MapFunc // warn
bug.v:7:4: warning: this is not recommended, you should use Option function instead
    5 |     a fn (int) // warn
    6 |     b ?fn () // ok
    7 |     c MapFunc // warn
      |       ~~~~~~~
    8 |     d ?MapFunc // ok
    9 | }
cool
[bug.v:17] t: Test{
    a: fn (int)
    b: Option(fn ())
    c: fn ()
    d: Option(none)
}