F# member constraints + ^a byref parameters

Question

After some playing around F# member constraints feature and writing function like this:

let inline parse< ^a when ^a : (static member Parse: string ->          


        

Answer 1

I think it is a bug too, something with member constraints and byref types. I can make a slightly less ugly reflection version by changing the signature of the member constraint:

let inline tryParse<'a when 'a : (static member TryParse : string -> 'a byref -> bool)>  s  =
    let args = [| s ; null |]
    if typeof<'a>
        .GetMethod("TryParse", [| typeof; typeof< ^a>.MakeByRefType() |])
        .Invoke(null, args) = box true 
        then Some (args.[1] :?> 'a) 
        else None

This one is very close:

let inline tryParse< ^a when ^a: (static member TryParse: string -> ^a byref -> bool)> s =
    let mutable x = Unchecked.defaultof<'a>
    if (^a: (static member TryParse: string -> ^a byref -> bool) (s, &x))
        then Some x else None

but I get a error FS0421: The address of the variable 'x' cannot be used at this point when I try to compile it.