I think you have illustrated the point quite well actually: >And trying to defeat that by modifying buffer to be 1023, the compiler realises the assertions are

I think you have illustrated the point quite well actually: >And trying to defeat that by modifying buffer to be 1023, the compiler realises the assertions are always going to crash so it creates a weird-ass "bad" version (which it calls), only implements the slicing (and its error) in the base and doesn't even bother outputting code for the simpler version. With optimizations off, we can see what the compiler is doing before optimization passes potentially obfuscate it. In practice when the call is nested behind many layers it may not be able to elide the bounds check in the loop in all cases; the opt-level may be hiding the truth because it is defeating the test case. Whether that's true in this case or not is irrelevant, of course; I'd say it's almost always preferable to have better code as early as possible in the code generation process.