[Issue 20624] [REG2.088] AA access gives wrong deprecation message.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20624

--- Comment #1 from johanengelen at weka.io ---
Instead of opAssign, the better fix is `aa[s] = Nullable!ulong(value);`.

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