[Issue 19917] unions should require that all members are `= void` initialised

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19917

--- Comment #4 from Manu <turkeyman at gmail.com> ---
Wow.
Seriously, the only sensible thing to do is issue a compile error if more than
one item doesn't have `= void`. That would be very sane.

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