It's less an issue of changing semantics, and more an issue of just being potentially confusing. Before I played around with the example I posted in the thread I didn't actually know what the result would be, because I hadn't internalised all the rules around mixing whitespace and prefix/infix operators (and I doubt I'm the only one). I'm not suggesting that the inability to provide good warnings in confusing situations is fatal to the proposal, just that there is a tradeoff here and people will reasonably disagree. Personally, I haven't made my mind up. I do agree with @xwu here, though: