Weird value returned after (maybe failed) overload resolution

[gusty]

This bug is not about taking the wrong overload, which is an ongoing problem not easy to solve, but about returning a value not corresponding to any of the overloads.

I'm under the impression that overload resolution fails and then some buggy code returns a no sense value instead of a compile error.

Here's a repro

open System

type Default6 = class end
type Default5 = class inherit Default6 end
type Default4 = class inherit Default5 end
type Default3 = class inherit Default4 end
type Default2 = class inherit Default3 end
type Default1 = class inherit Default2 end

type [<Struct>]Result2<'T, 'E> = Ok2 of OkField:'T | Error2 of ErrorField:'E

type Bind =
    static member (>>=) (source: Lazy<'T>   , f: 'T -> Lazy<'U>    ) = lazy (f source.Value).Value    : Lazy<'U>
    static member (>>=) (source: seq<'T>    , f: 'T -> seq<'U>     ) = Seq.collect f source           : seq<'U>
    static member (>>=) (source             , f: 'T -> _           ) = Option.bind   f source         : option<'U>
    static member (>>=) (source             , f: 'T -> _           ) = List.collect  f source         : list<'U>
    static member (>>=) (source             , f: 'T -> _           ) = Array.collect f source         : 'U []
    static member (>>=) (source             , f: 'T -> _           ) = async.Bind (source, f)         : Async<'U>
    static member (>>=) (source             , k: 'T -> _           ) = Result.bind k source           : Result<'U,'E>
    static member (>>=) (source: Result2<'T,'E>   , k: 'T -> Result2<'U,'E>) = Unchecked.defaultof<_> : Result2<'U,'E>

    static member inline Invoke (source: '``Monad<'T>``) (binder: 'T -> '``Monad<'U>``) : '``Monad<'U>`` =
        let inline call (_mthd: 'M, input: 'I, _output: 'R, f) = ((^M or ^I or ^R) : (static member (>>=) : _*_ -> _) input, f)
        call (Unchecked.defaultof<Bind>, source, Unchecked.defaultof<'``Monad<'U>``>, binder)

type Return =
    inherit Default1

    static member inline InvokeOnInstance (x: 'T) = (^``Applicative<'T>`` : (static member Return : ^T -> ^``Applicative<'T>``) x)

    static member inline Invoke (x: 'T) : '``Applicative<'T>`` =
        let inline call (mthd: ^M, output: ^R) = ((^M or ^R) : (static member Return : _*_ -> _) output, mthd)
        call (Unchecked.defaultof<Return>, Unchecked.defaultof<'``Applicative<'T>``>) x    

    static member        Return (_: seq<'a>        , _: Default2) = fun  x      -> Seq.singleton x : seq<'a>
    static member inline Return (_: 'R             , _: Default1) = fun (x: 'T) -> Return.InvokeOnInstance x      : 'R
    static member        Return (_: Lazy<'a>       , _: Return  ) = fun x -> Lazy<_>.CreateFromValue x : Lazy<'a>
    static member        Return (_: option<'a>     , _: Return  ) = fun x -> Some x                               : option<'a>
    static member        Return (_: list<'a>       , _: Return  ) = fun x -> [ x ]                                : list<'a>
    static member        Return (_: 'a []          , _: Return  ) = fun x -> [|x|]                                : 'a []
    static member        Return (_: 'a Async       , _: Return  ) = fun (x: 'a) -> async.Return x
    static member        Return (_: Result<'a,'e>  , _: Return  ) = fun x -> Ok x                                 : Result<'a,'e> 
    static member        Return (_: ResizeArray<'a>, _: Return  ) = fun x -> ResizeArray<'a> (Seq.singleton x)

type Delay =
    inherit Default1

    static member inline Delay (_mthd: Default3, x: unit-> ^``Monad<'T>`` when ^``Monad<'T>`` : struct     , _: Default2) = Bind.Invoke (Return.Invoke ()) x : ^``Monad<'T>``
    static member inline Delay (_mthd: Default4, x: unit-> ^``Monad<'T>`` when ^``Monad<'T>`` : not struct , _: Default1) = Bind.Invoke (Return.Invoke ()) x : ^``Monad<'T>``

    static member        Delay (_mthd: Default2, x: unit-> _                                              , _          ) = Seq.delay x      : seq<'T>
    static member        Delay (_mthd: Default2, x: unit-> 'R -> _                                        , _          ) = (fun s -> x () s): 'R -> _

    static member inline Invoke (source : unit -> '``Monad<'T>``) : '``Monad<'T>`` =
        let inline call (mthd: ^M, input: unit -> ^I) = ((^M or ^I) : (static member Delay : _*_*_ -> _) mthd, input, Unchecked.defaultof<Delay>)
        call (Unchecked.defaultof<Delay>, source)

let res = Delay.Invoke (fun () -> [5] )
printfn "res is %A" res

Running this prints res is [object Object], but if we comment out the first overload of Delay, we get res is [5] which is the expected result.

Not sure, but it seems to be a problem when there are constraints in the overload, like not struct.

[alfonsogarciacaro]

Hi @gusty! Thanks a lot for the report and providing the code to reproduce. This is interesting, when debugging the sample I can see the F# compiler does provide witnesses for resolution so Fable's own mechanism doesn't come into play here. It could be an issue with Fable when assigning the witness, but right now it looks as if FCS resolves the witness differently for .NET compilation and when creating the AST 😕

If you change the first argument in the second Delay method to Default3 (or Default2 or Default1) then it works.

    static member inline Delay (_mthd: Default3, x: unit-> ^``Monad<'T>`` when ^``Monad<'T>`` : struct     , _: Default2) = Bind.Invoke (Return.Invoke ()) x : ^``Monad<'T>``
    static member inline Delay (_mthd: Default3, x: unit-> ^``Monad<'T>`` when ^``Monad<'T>`` : not struct , _: Default1) = Bind.Invoke (Return.Invoke ()) x : ^``Monad<'T>``

[gusty]

Thanks for the workaround and interesting trace into the overload resolution problem, however the goal of this repro was to understand why Fable returns this object value. Where is it coming from? None of these overloads would return such value.

[alfonsogarciacaro]

Ah, sorry, didn't read well the description. The returned object is the lazy value, it looks like indeed is picking the wrong overload. The problem here is we haven't implemented ToString for lazy objects so you just see the default stringification for JS objects. We should fix that.