last

import std.algorithm : filter;
import std.range : take, takeExactly;

struct PowersOfTwo(bool shouldSave)
{
    size_t i = 1;

    void popFront() pure nothrow
    {
        i *= 2;
    }

    @property size_t front() const pure nothrow
    {
        return i + 0;
    }

    @property bool empty() const pure nothrow
    {
        return false;
    }

    static if (shouldSave)
    {
        @property PowersOfTwo save() const pure nothrow
        {
            return cast(typeof(return)) this;
        }
    }
}

assert(last([1, 2, 3]) == 3);
assert(last(PowersOfTwo!true(1).takeExactly(5)) == 16);
assert(last(PowersOfTwo!true(1).take(5)) == 16);
assert(last(PowersOfTwo!false(1).take(5)) == 16);