Missing syntax for trait call with concrete type

[NinoFloris]

Today it's possible to express SRTP constraints that involve a concrete type like

type Foo =
    static member inline Invoke< ^N, 'a, 'b when (Foo or ^N): (static member Bar: 'a -> 'b)>  source = ...

Yet there's no way to then use this constraint in a trait call

type Foo =
    static member inline Invoke< ^N, 'a, 'b when (Foo or ^N): (static member Bar: 'a -> 'b)>  source = 
        ((Foo or ^N): (static member Bar: _ -> _)(source)) // doesn't compile

Now I know it's advised not to use explicit signatures, especially for SRTP, though I'm unsure why the concrete type in the signature is allowed then.

I'd say it should be removed from signatures or get trait call syntax to match.

Though if you're willing to tolerate some compiler warnings it can be worked around like so

type Foo =
    static member inline Invoke< ^N, 'a, 'b when (Foo or ^N): (static member Bar: 'a -> 'b)>  source = 
        let inline call (_: ^M, a) =
            ((^M or ^N): (static member Bar: _ -> _)(a))

        call (Unchecked.defaultof<Foo>, source)

The compiler seemingly unifies ^M with Foo and allows that trait call to piggyback on the outer function's constraint. Only surfacing a warning, not an error, that (^M or ^N): (static member Bar: 'a -> 'b) is a missing constraint at the trait call location. This may or may not be a compiler bug ;)

[dsyme]

Making this a language suggestion, see fsharp/fslang-suggestions#908