oof. I see what you say about the LHS defining how the RHS is evaluated but that attaches expression evaluation to assignment. You can't do one without the other. Also the target type may be obscure to the programmer. What's the behaviour with

  string s="This is a testing case";
  println("i" in s);

But separately your example now looks even worse. It's quite reasonable to expect 'in' to behave the same way for both cases; that they both return [2,5,14] - now you've added another complication.

No offence but without some overriding principle, and a justification for that principle (that "things are demonstrably easier if you do it this way"), you've just dumped and extra load on the programmer which I do not need!.