[Issue 19903] postblit called for uninitialised elements of unions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19903

--- Comment #1 from Manu <turkeyman at gmail.com> ---
Oh my, I just noticed that the DESTRUCTOR is also called for each element in
the union!
That's completely wrong, since only one item in the union may be valid at a
time.

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