[Issue 18672] Error in @safe transitive propagation with associative arrays

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18672

--- Comment #4 from Seb <greensunny12 at gmail.com> ---
@RazvanN how about setting the opAssign to @trusted instead when we now that
the dtor is @safe?
The problem is that sd.dtor.isSafe() returns 0 :/
Maybe we need to go through semantic first before looking at this?
Also there seems to be an existing check for this already - it just doesn't get
triggered

```
stc = mergeFuncAttrs(stc, sd.dtor);
if (stc & STC.safe)
```

Well at least the high-level isSafe traits infers the safety correctly:

https://run.dlang.io/is/6ePqDY

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