Promise.dmd21804workaround

Work-around for DMD bug 21804: https://issues.dlang.org/show_bug.cgi?id=21804

If your then callback argument is a tuple, insert this call before the then call. (Needs to be done only once per Promise!T instance.)

class Promise(T, E : Throwable = Exception)
typeof(this)
dmd21804workaround
()

Meta