Incorrect overload resolution when using interface

[francotiveron]

type IApi<'t> = 
    (*1*)abstract member Then: ('t -> 'msg) * (exn -> 'msg) -> Cmd<'msg>
    (*2*)abstract member Then: ('t -> unit) * (exn -> unit) -> unit

type Api<'t>(operation: unit -> Async<'t>) = 
    (*3*)member __.Then(msgConstructor: 't -> 'msg, errConstructor: exn -> 'msg) = Cmd.OfAsync.either operation () msgConstructor errConstructor
    (*4*)member __.Then(onSuccess: 't -> unit, onError: exn -> unit) = operation() |> Async.StartAsPromise |> Promise.eitherEnd onSuccess onError
    interface IApi<'t> with
        (*5*)member this.Then(msgConstructor: 't -> 'msg, errConstructor: exn -> 'msg) : Cmd<'msg> = this.Then(msgConstructor, errConstructor)
        (*6*)member this.Then(onSuccess: 't -> unit, onError: exn -> unit): unit = this.Then(onSuccess, onError)

type T = ...

type Msg = 
  | Msg1 of T
  | Msg2 of exn

//f of type unit -> Async<T>
let create1() = Api(f)
let create2(): IApi<_> = Api(f)

let api1 = create1()
api1.Then(Msg1, Msg2) //No problem: Ctrl+Click (design time VS Code) jumps to (*3*); at runtime (*3*) is called

let api2 = create2()
api2.Then(Msg1, Msg2) //PROBLEM: Ctrl+Click (design time VS Code) jumps to (*1*) as expected; HOWEVER, at runtime (*6->4*) is called, instead of the expected (*5->3*)

[alfonsogarciacaro]

Unfortunately, this is "by design". This is because we use interfaces to interop with JS so interface members are not mangled by default (we tried to change that in Fable 3 but it broke many existing projects). This means the two Then implementing the interface get compiled with the same so the second version overwrites the first. In that case, you should see a warning similar to this:

Member Then is duplicated, use Mangle attribute to prevent conflicts with interfaces

If you decorate Api interface with the [<Mangle>] attribute, Fable will mangle the member names and the problem should go away.

[MangelMaxime]

Doing some clean up