> I'd love to have a flag "-fno-surprizing-ub" or "-fhighlevel-assembler" combined with the above `assume` function or some other syntax to let me help the comp

> I'd love to have a flag "-fno-surprizing-ub" or "-fhighlevel-assembler" combined with the above `assume` function or some other syntax to let me help the compiler, so that I can write C like in the 90s - close to metal but with less surprizes. The problem, which you may realise with some more introspection is that "surprising" is actually a property of you, not of the compiler, so you're asking for mind-reading and that's not one of the options. You want not to experience surprise. You can of course still get 1990s compilers and you're welcome to them. I cannot promise you won't still feel surprised despite your compiler nostalgia, but I can pretty much guarantee that the 1990s compiler results in slower and buggier software, so that's nice, remember only to charge 1990s rates for the work.