Maybe the problems are caused by overloading void_. I haven't looked at MPL
recently, but as a general observation I have identified at least three uses
of a void_-like entity.
1. A type parameter used to emulate a variable argument template. I use
'_missing' for this purpose (leading underscore for implementation details.)
template<class A1 = _missing, class A2 = _missing, ...> struct F;
2. An optional parameter that, when not supplied, has a reasonable (dependent) default. I use 'unspecified'.
template<class R = unspecified, class F> ... bind(F f);
3. A type that is guaranteed to be distinct from all other useful types.
'nil' is what Lisp calls it; void_ is fine, too.
Another possible spelling for this animal is:
class nat {nat();};
Inspired from nan. In this case means Not A Type. It is nice and short which comes in handy for when there are a lot of template parameters to default. It is easily pronounceable, and won't be confused with any other type when discussed verbally.
-Howard
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