Why: error("multiple ! arguments are not allowed");

[Adam D. Ruppe via Digitalmars-d]

On Sunday, 21 May 2017 at 13:08:18 UTC, Adam D. Ruppe wrote:
> If I comment that line and use the hacked dmd, it seems to work.

Well, not exactly work, it didn't actually instantiate the inner 
template like it probably should have.

So is it just not implemented correctly and became an error 
because y'all never got around to fixing the bugs, or is it 
prohibited by design? If so, why? Is it impossible to implement 
correctly?