[Issue 23531] New: [DIP1000] scope variable can be assigned as AA key

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23531

          Issue ID: 23531
           Summary: [DIP1000] scope variable can be assigned as AA key
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

```
void main () @safe
{
    auto r = bar();
    assert(r["foo!"] == "oi");
}

string[string] bar () @safe
{
    immutable(char)[4] s = "foo!";
    return foo(s);
}

string[string] foo (scope string f) @safe
{
    string[string] aa;
    aa[f] = "oi";
    return aa;
}
```

Results in RangeError because the key is garbage. Funny thing is that assigning
`f` as the value of the AA (e.g. `aa[f] = f;`) is correctly detected.

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