std.range.repeat does not define a popBack method and as such cannot be used as a bidirectional range.

Here is the code with the (almost non-existent...) missing function:

struct Repeat(T)
{
    private T _value;
    ref T front() { return _value; }
    ref T back() { return _value; }
    enum bool empty = false;
    void popFront() {}
    void popBack() {}
    ref T opIndex(uint) { return _value; }
}