In a struct, opIn_r is not detected by the compiler as beeing the method implementing the operator 'in'. Example:

struct S {
    int i;
    void show() { writeln(i); }
    const bool opEquals (ref const(S) s) {
        writeln("==");
        return (i == s.i);
    }
    bool opIn_r (int j) { return (i==j); }
}
unittest {
    S* sp = &(S(1));
    writeln(sp.i);
    sp.show();
    
    S s2 = S(1);
    writeln(sp == s2);
    writeln(1 in s2);
    
    // error:
    writeln(1 in sp);
}
==>
    Error: rvalue of in expression must be an associative array, not S*

This concerns opIn_r on pointed struct only; the other struct members of the structn and the not-pointed one are only here to contrast:
* Data members, regular methods and even "language methods" like opEquals are correctly taken into account on a struct, even via implicite deref.
* opIn_r is correctly detected as implementing 'in' on a non-pointed or explicitely dereferenced struct.

Thus, the code works fine if one comments out the very last line.

Waiting for a fix, the error message should be corrected to eg:
    Error: right operand of 'in' operation must be an associative array
           or implement the operator 'in' via method opIn_r

Denis