[Issue 17440] Nullable.nullify() resets referenced object

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17440

--- Comment #16 from hsteoh at quickfur.ath.cx ---
Rebooted: https://github.com/dlang/phobos/pull/6043

Due to design issues with the current implementation of Nullable, it's not
possible to fix the problem of having two distinct null states without
extensive changes and breaking backward-compatibility.  So for now, we'll just
make it so that .nullify doesn't call .destroy on class references.

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