This code doesn't compile:

struct R
{
    bool empty();
    ref int front();
    void popFront();
}

ref int popNext(ref R fwdRange)
{
    auto result = & fwdRange.front();
    fwdRange.popFront;
    return *result;
}

void main()
{
    R r;
    int x = r.popNext;
}

The code should work: popNext should be looked up in R's outer scope if not a member.