msg[0] or msg.field[0] ?

[bearophile]

SK:
> What is the explanation?

The syntax shown in TDPL can't be supported yet, because the DMD compiler doesn't yet allow it. It will be added. In the meantime you have to use the .field[] syntax, or the "secret" ._0 ._1 ecc syntax. But keep in mind that this second syntax will be removed once DMD allows to perfom an "alias this" over a TypeTuple, so if you use it a lot you will have to change lot of your code.

Bye,
bearophile