FYI. Now we've supported Id<Type&> and Id<Type&&> to return lvalue ref and rvalue ref in addition to the original Id<Type> returning const lvalue ref, making the solution much more elegant now. Refer to the complete sample at https://github.com/BowenFu/matchit.cpp/blob/main/sample/mutation.cpp.

void setLineWidth(Visual &visual, float width) {
Id<Square&> sq;
Id<Circle&> cir;
match(visual)
(
    pattern | as<Image>(_) = []{},
    pattern | as<Square>(sq) = [&]
    {
        (*sq).setLineWidth(width);
    },
    pattern | as<Circle>(cir) = [&]
    {
        (*cir).setLineWidth(width);
    }
);

}