[dmd-internals] Small bug in TemplateInstance.genIdent and question about Tuple in tiargs

[Elie Morisse via dmd-internals]

Just a small bug found while trying to generalize template 
parameter packs for C++11 support in Calypso (this is more 
intrusive that I would like, but I can't see any simple 
alternative to implementing it in DMD):

   
https://github.com/dlang/dmd/blob/7e60772afac6401b934981df5e2b33b33487514a/src/dtemplate.d#L8120

> if (i < nparams && (*tempdecl.parameters)[i].specialization())
>     buf.writeByte('H'); // 
> https://issues.dlang.org/show_bug.cgi?id=6574

isn't correct when i and args get switched by:

   
https://github.com/dlang/dmd/blob/7e60772afac6401b934981df5e2b33b33487514a/src/dtemplate.d#L8205

> // Tuple va = isTuple(o);
> else if (va)
> {
>     assert(i + 1 == args.dim); // must be last one
>     args = &va.objects;
>     i = -cast(size_t)1;
> }

But this begs a question: why and how does some template 
instances end up with Tuple in their tiargs?

The presence of Tuple doesn't seem consistent with findBestMatch 
which expands Tuple tdtypes into multiple tiargs:

   
https://github.com/dlang/dmd/blob/7e60772afac6401b934981df5e2b33b33487514a/src/dtemplate.d#L7706

Having to deal with the two possible cases, one where Tuple are 
expanded, the other where they're aren't, makes adding support 
for multiple parameter packs much harder.

Any good reason for not wanting a Tuple-free tiargs?