Why is it not allowed to have empty UDAs for classes?

Let's say we have an UDA like this:
struct Exclude { }

Then we want to put it on a class like:

@Exclude class Foo
{
    ...
}

This will fail with the following error:
Error: type Exclude has no value

But on everything else we can place an empty UDA like that ex. on a function:

This is okay:

@Exclude void foo()
{
    ...
}

Can someone explain to me why it's not possible with classes?

On Thursday, October 26, 2017 15:09:48 bauss via Digitalmars-d-learn wrote:
> Why is it not allowed to have empty UDAs for classes?
>
> Let's say we have an UDA like this:
> struct Exclude { }
>
> Then we want to put it on a class like:
>
> @Exclude class Foo
> {
>      ...
> }
>
> This will fail with the following error:
> Error: type Exclude has no value
>
> But on everything else we can place an empty UDA like that ex. on a function:
>
> This is okay:
>
> @Exclude void foo()
> {
>      ...
> }
>
> Can someone explain to me why it's not possible with classes?

It worked just fine when I just tried it on my machine - both with dmd master and with 2.076.1. Are you using an older version of the compiler than 2.076.1? It's possible that there was a bug that was fixed.

- Jonathan M Davis

On Thursday, 26 October 2017 at 21:24:43 UTC, Jonathan M Davis wrote:
> On Thursday, October 26, 2017 15:09:48 bauss via Digitalmars-d-learn wrote:
>> [...]
>
> It worked just fine when I just tried it on my machine - both with dmd master and with 2.076.1. Are you using an older version of the compiler than 2.076.1? It's possible that there was a bug that was fixed.
>
> - Jonathan M Davis

Yeah I can't reproduce it right now, so I'm not even sure.

On Thursday, 26 October 2017 at 15:09:48 UTC, bauss wrote:
> Why is it not allowed to have empty UDAs for classes?
>
> Let's say we have an UDA like this:
> struct Exclude { }
>
> Then we want to put it on a class like:
>
> @Exclude class Foo
> {
>     ...
> }
>
> This will fail with the following error:
> Error: type Exclude has no value
>
> But on everything else we can place an empty UDA like that ex. on a function:
>
> This is okay:
>
> @Exclude void foo()
> {
>     ...
> }
>
> Can someone explain to me why it's not possible with classes?

FWIW all parameterless UDA in LDC use a struct literal e.g.
https://github.com/ldc-developers/druntime/blob/ldc/src/ldc/attributes.d#L237