PackedAliasSeq?

[Andrei Alexandrescu]

After coding https://github.com/dlang/phobos/pull/6192 with AliasSeq, 
the experience has been quite pleasurable. However, in places the 
AliasSeq tends to expand too eagerly, leading to a need to "keep it 
together" e.g. when you need to pass two of those to a template.

I worked around the issue by nesting templates like this:

template Merge(T...)
{
     template With(U...)
     {
         static if (T.length == 0)
             alias With = U;
         else static if (U.length == 0)
             alias With = T;
         else static if (T[0] < U[0]
                  || T[0] == U[0] && T[1].stringof <= U[1].stringof)
             alias With =
                 AliasSeq!(T[0], T[1], Merge!(T[2 .. $]).With!U);
        else
             alias With =
                 AliasSeq!(U[0], U[1], Merge!T.With!(U[2 .. $]));
     }
}

So instead of the unworkable Merge!(AliasSeq!(...), AliasSeq!(...)), one 
would write Merge!(AliasSeq!(...)).With!(AliasSeq!(...)).

The problem remains for other use cases, so I was thinking of adding to 
std.meta this simple artifact:

template PackedAliasSeq!(T...)
{
     alias expand = AliasSeq!T;
}

That way, everything stays together and can be expanded on demand.


Andrei