[Issue 24684] New: Bad diagnostic for an enum member in an enum that use another enum as base type

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24684

          Issue ID: 24684
           Summary: Bad diagnostic for an enum member in an enum that use
                    another enum as base type
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: diagnostic
          Severity: minor
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: b2.temp at gmx.com

For

```
enum A1 : int
{
    a,
    b,
}

enum A2 : A1
{
    c, // does not require +1
    d, // requires + 1
}  
```

The definition of A2.d causes the error

> /tmp/temp_7F4942E9C090.d(10,5): Error: comparison between different enumeration types `A2` and `A1`; If this behavior is intended consider using `std.conv.asOriginalType`

which makes no sense.

In fact the real problem is that `A2.d` violates the specification 18.1.5
(https://dlang.org/spec/enum.html#named_enums), specifically _"it is given the
value of the previous EnumMember + 1_".

In consequence a better message would be that `d` value cannot be automatically
defined because the base type does not support increment.

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