A static analyzer identified that this operator was unsafe in the case
of self-assignment.
In the placement new statement, StringValue's copy constructor was being
implicitly called, which received a reference to "itself". In fact, it
was being passed an old StringValue at the same address - one whose
lifetime had already ended. The copy constructor was thus copying fields
from a dead object.
We need to be careful when switching active union members, and calling
the destructor on the old StringValue will avoid memory leaks which I
believe the old code exhibited.
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