[Issue 22237] New: AA.update is underspecified

[d-bugmail at puremagic.com]

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 at puremagic.com
          Reporter: dkorpel at 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.

--