Seq.cast's non-generic IEnumerable implementation doesn't throw even when the cast is to the wrong type #650

let strings = [1] |> Seq.cast<string>
for s in strings do ()

let strings = [1] |> Seq.cast<string> :> System.Collections.IEnumerable
for s in strings do ()

let sum = [1] |> Seq.cast<string> |> Seq.cast<int> |> Seq.sum // doesn't throw!