F# generic enumeration helper

diaenum.fs

// helpers to iterate IDiaEnum* objects
let inline toSeq (o: ^T) =
    let e = { new IEnumerable with member this.GetEnumerator () = (^T: (member GetEnumerator: unit -> _) (o)) }
    if true then
        Seq.cast e
    else
        // Dummy expression to constrain return type to that of o.Item
        seq [| (^T: (member Item: _ -> _) (o, Unchecked.defaultof<_>)) |]

In FsControl there is a more generic version, which use a default-case which is equivalent to your code. If you extract that overload to a single function you get this:

open System
let inline toSeq (x:'S when 'S :> Collections.IEnumerable) = 
    let f i x :'T = ( ^S : (member get_Item : int -> 'T) x, i) // this is a dummy function
    Seq.cast<'T> x : seq<'T>

Although it's better to use the even more generic toSeq function defined there.