isNarrowMatrix

static struct S(size_t R, size_t C)
{
    enum rows = R;
    enum cols = C;

    auto opIndex(size_t i, size_t j) const
    {
        return i + j;
    }

    alias at = opIndex;
}

static assert(isNarrowMatrix!(S!(0, 0)));
static assert(isMatrix!(S!(1, 1)));