The static coercion from type EventInfoProxy to 'a involves an indeterminate type based on information prior to this program point. Static coercions are not allowed on some types. Further type annotations are needed.

upcast.fsx

let annotateMember data (memb:'t :> #MemberInfo) =
    match box memb with
    | :? EventInfo as event ->
        { new EventInfoProxy(event) with
            override __.GetCustomAttributesData() = data } :> 't
    | :? MethodInfo as meth ->
        { new MethodInfoProxy(meth) with
            override __.GetCustomAttributesData() = data } :> 't
    | :? PropertyInfo as prop ->
        { new PropertyInfoProxy(prop) with
            override __.GetCustomAttributesData() = data } :> 't
    | _ ->
        memb :> 't

If I do :?> then it says can't convert 't to 't ... ugh.