[Issue 24850] New: Named enum construction from base type is inconsistent

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24850

          Issue ID: 24850
           Summary: Named enum construction from base type is inconsistent
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: nick at geany.org

The following is an error:
```
enum E : int;
E e = E(4);
```

enumbase.d(2): Error: cannot implicitly convert expression `4` of type `int` to
`E`

Which is consistent with uniform construction because a base type instance does
not implicitly convert to the enum type. But this compiles:

```
struct S {
    this(int) {}
}

enum E : S;
E e = E(2);
```

However, an instance of S does not implicitly convert to E, so this is
inconsistent.

The spec does not seem to mention enum construction with an argument list.
There is a test in dmd for issue 16346 to allow E(E.member), presumably for
generic code.

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