Consider this code:

type MyId =
    | IdA of int
    | IdB of string

    member this.IdA =
        match this with
        | IdA x -> Some x
        | _ -> None


let onlyIdA (ids: MyId list) = ids |> List.choose _.IdA

The last line fails to compile with an error on _.IdA:

Error FS0812 : The syntax 'expr.id' may only be used with record labels, properties and fields

I would expect this to compile, since IdA is indeed a property.