The way Phobos currently does it is (essentially) by storing a void* and a TypeInfo...

No, each type would have to be enumerated in the declaration.

The usual way to...

Informally Any would be the sum of all types. Would your proposal encompass this?

Isn't that what a sumtype is?

Variant is important to some of us because of its universality. If all conversions are...

...qsort_r()'s order of last two parameters was
 * changed, and the comparator function is...

We've considered it many times, and have decided against it due to the consistently...

...F)
{
    static if (is(typeof(F) P == __parameters))
        alias FunctionParameters = P;
}

template numConstructorOverloads(Overloads...)
{
    static...

Overloading is the distinguishing reason.

At one point something related was viewed with approval. Probably got nixed behind the scenes...