Hi all,

just wandering if this is a bug, I certainly didn't expect the output:

```d
alias AliasSeq(T...) = T;
alias Nothing = AliasSeq!();

template MyTemplate(S, Args...)
{
  pragma(msg, "Args: ", Args);
}

void main()
{
  alias types = AliasSeq!(bool, string, ubyte, short, ushort);
  alias x = MyTemplate!(Nothing, types);
}
```

Output (first item is gone):

```terminal
Args: (string, ubyte, short, ushort)
```

If I do:

```d
alias y = AliasSeq!(Nothing, types);
pragma(msg, "y: ", y);
```

I'd get expected behaviour:

```terminal
y: (bool, string, ubyte, short, ushort)
```

Any added insights as to why this happens would be be much appreciated.

Thanks in advance.

On Saturday, 22 August 2020 at 21:45:35 UTC, data pulverizer wrote:
> Hi all,
>
> just wandering if this is a bug, I certainly didn't expect the output:
>
> ```d
> alias AliasSeq(T...) = T;
> alias Nothing = AliasSeq!();
>
> template MyTemplate(S, Args...)
> {
>   pragma(msg, "Args: ", Args);
> }
>
> void main()
> {
>   alias types = AliasSeq!(bool, string, ubyte, short, ushort);
>   alias x = MyTemplate!(Nothing, types);
> }
> ```
>
> Output (first item is gone):
>
> ```terminal
> Args: (string, ubyte, short, ushort)
> ```

AliasSeq's don't nest and automatically expand when you use them, so when you write

    MyTemplate!(Nothing, types)

it gets expanded to

   MyTemplate!(bool, string, ubyte, short, ushort)

So `bool` gets bound to the first parameter, `S`, and the rest of the arguments get bound to `Args`.

If you want a non-expanding version of AliasSeq, you can write one like this:

    template AliasTuple(Args...) {
        alias expand = Args;
    }

On Saturday, 22 August 2020 at 21:52:53 UTC, Paul Backus wrote:
> On Saturday, 22 August 2020 at 21:45:35 UTC, data pulverizer wrote:
>
> AliasSeq's don't nest and automatically expand when you use them, so when you write
>
>     MyTemplate!(Nothing, types)
>
> it gets expanded to
>
>    MyTemplate!(bool, string, ubyte, short, ushort)
>
> So `bool` gets bound to the first parameter, `S`, and the rest of the arguments get bound to `Args`.
>
That totally makes sense, it was in code where S could be something some times but Nothing at other times, and I was getting issues with the Nothing situation, but when I look at your explanation I should have totally expected it.

Thanks again!