My recent experience with this question taught me this:
I completely agree with you regarding current salary being irrelevant to the future salary. The current salary is as per the skills analysis by the current company and the new company cannot carry forward that.
But the real challenge is convincing this to recruiters and HRs especially in India from where I am who have corrupted mindsets following this strategies for salary discussion even before the interview. Also, almost all the candidates reveal their salary details and accept the percentage hike as given by the recruiter. So if any one candidate who does not follow this will be odd man out and will be rejected in the application stage itself even before the interview.
Irony is that they ask candidates both current and expected salary without specifying the salary range or other compensations details or perks for the position. When asked about the salary range, they say they provide 30-40% hike on the current salary. So, I once asked recruiter then what's the point of asking the expected salary when that is fixed by the specified percentage without the interview. He replied this is to check the expectation is not beyond their percentage hike.
Your suggestion of giving range for current salary sounds cool and will try it.
Surely, in that sort of situation, the best approach is to give yourself a pay rise: when asked, increase your salary by some believable percentage.
If someone happens to know your actual salary and calls you on it, you could say that you were including the value of the various extra benefits provided by the organisation.
I like bouncing the question back - it sounds like the best way to handle this. I found some of the suggestions in this guide helpful also, especially pushing back salary conversation until the very end. Last time I negotiated, didn't do that and I'm sure it cost be tens of thousands. Here if anyone else is negotiating: candor.co/guides/salary-negotiation
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