https://issues.dlang.org/show_bug.cgi?id=22237

          Issue ID: 22237
           Summary: AA.update is underspecified
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: dlang.org
          Assignee: nobody@puremagic.com
          Reporter: dkorpel@live.nl

The spec gives this signature for .update on associative arrays:

> .update(Key key, Value delegate() create, Value delegate(Value) update)

https://dlang.org/spec/hash-map.html#properties

But the actual implementation in druntime also allows returning void:
```
void main() {
    int[int] aa;
    aa.update(3, () => 3, (int x) {});
}
```

The idea being that the delegate can take the existing value by `ref` and update that way as well. This is how the update function is described:

> The update function provides a means to construct a new value via the create delegate or update an existing value via the update delegate.

> Looks up key; if it exists applies the update delegate else evaluates the create delegate and adds it to the associative array

This doesn't specify how the update delegate is actually applied, and whether you need to modify with ref, return a new value, or both.

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