>281
ダサくてよければ

Variant[Variant] constructor(T...)(T plist){
Variant[Variant] result;
foreach(ref p; plist) result[new Variant(p.keys[0])] = new Variant(p[p.keys[0]]);
return result;
}
Variant[Variant] moge = constructor(["A": "B"], ["C": 3], [4: "E"]);

試してないけど