import std.conv;

void main() {
int number_normal = 10;
auto a = to!(string)(number_normal);

const number_const = 10;
auto b = to!(string)(number_const); // だめ

immutable number_immutable = 10;
auto c = to!(string)(number_immutable); // だめ
}

----------------------------------------

C:\dmd\dmd\src\phobos\std\conv.d(2580): Error: variable std.conv.to!(immutable(char)[],const(int)).to.u cannot modify const
C:\dmd\dmd\src\phobos\std\conv.d(8): Error: template instance std.conv.to!(immutable(char)[],const(int)) error instantiating

使い方が悪いんだろうか？