I dun a DIP, possibly the best DIP ever

[Stefan Koch]

On Thursday, 23 April 2020 at 18:57:04 UTC, Steven Schveighoffer 
wrote:
> On 4/23/20 2:18 PM, Stefan Koch wrote:
>> [...]
>
> The question is whether the expanded inner expression is 
> considered a tuple for expansion later or not.
>
> alias tuple1 = AliasSeq!(1, 2, 3);
> alias tuple2 = AliasSeq!(4, 5, 6);
> pragma(msg, [AliasSeq!(tuple1..., tuple2)...]);
>
> If it expands tuple1 first, then expands tuple 2 without 
> re-expanding the result of tuple1..., then it should be:
>
> [1, 2, 3, 4, 1, 2, 3, 5, 1, 2, 3, 6]
>
> If it expands both tuple1 and tuple2 together, it should be:
>
> [1, 4, 2, 5, 3, 6]
>
> -Steve

It's the latter.
you cannot re-expand.