Named enum construction from base type is inconsistent

[dlangBugzillaToGithub]

Nick Treleaven (@ntrel) reported this on 2024-11-09T16:35:12Z

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

Description

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.

[dlangBugzillaToGithub]

nick (@ntrel) commented on 2024-11-09T17:28:53Z

There is also a dmd test case for issue 23778 using construction from base struct. So it would need to be deprecated.