Optional types in Tuple about runtypes

[yuhr]

Optional runtype (which is generated by .optional() shorthand) doesn't work like that. As described in the error message, your tuple corresponds to [string, string | undefined] in TS's notation, that obviously doesn't accept ['Hello']. In such case you must use Union around two Tuples:

const tuple = t.Union(t.Tuple(t.String), t.Tuple(t.String, t.String))

We've noticed recently that the naming of Optional is somewhat misleading, so we're planning to have it reporting a static error to the user when misused.