[Issue 21731] New: SumType should provide convenient access to the type index

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21731

          Issue ID: 21731
           Summary: SumType should provide convenient access to the type
                    index
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: snarwin+bugzilla at gmail.com

This feature was requested on the D forums [1].

It is possible to compute the type index in user code, but the implementation
is a bit verbose, and easy to get wrong:

```d
size_t typeIndex(Val)(Val val)
    if (isSumType!Val)
{
    alias QualifiedTypes = CopyTypeQualifiers!(Val, Val.Types);
    return val.match!(v => staticIndexOf!(typeof(v), QualifiedTypes));
}
```

For both convenience and correctness, it would be better if this functionality
were included in the `sumtype` module.

[1] https://forum.dlang.org/post/ouqgmoxzlcyrfqxsrtnm@forum.dlang.org

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