import std.stdio;

​

struct KeyRange {

    size_t max;

    bool opBinaryRight(string op : "in")(size_t v) {

        return v >= 0 && v < max; // the < 0 never passes but meh clearer code

    }

}

​

KeyRange keys(T)(T[] t) { return KeyRange(t.length); }

alias keys = object.keys; // also bring in the runtime library's keys for AA; this line lets both work together in the same module without name conflicts

​

​

void main()

{

    int[] x = [0, 2];

    int y = 1;

    writeln(y in x.keys); // works!

    writeln(y+5 in x.keys); // this too

}